Through The Sandglass

Reposing differently on Mars

Sand, Mars, and a challenge to conventional wisdom. What better ingredients for a post could I wish for?

The two images above show sedimentary fan deposits, the great  - yes, fan-shaped – aprons of detritus that accumulate at the foot of a gully, driven by gravity, often, on earth at least, assisted by periodic torrents of water. Water is an interesting factor: on Earth these things are often called “alluvial fans” because the occasional flood clearly plays a role.

These images are an opportunity for a quiz, since one is from Earth and the other is from Mars – which is which? Sorry, but I’ll maintain the suspense for a while. Are the fans on Mars alluvial – is water involved? The debate about the role of water in Martian surface processes continues, and it’s fascinating and the evidence for,against, ambiguous, is diverse. One of the lines of reasoning involves the slopes of fan deposits – they can be characterised by lower angles than expected and this is cited as a result of the lubricating effect of water. But this makes one critical assumption: that the slope of a deposit of granular materials (the angle of repose) is not affected by the low gravity of Mars – the angle of repose is independent of gravity. But is this assumption correct? Oddly enough, this question had not been seriously addressed until Maarten Kleinhans of Utrecht University together with Sebastiaan de Vet of the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the UvA and colleagues from Delft University of Technology decided to do so. How? Well, as the University press release describes, they took a plane:

The research team used a parabolic flight campaign to mimic Mars’s lower gravity and test the effect on angles of repose of different materials. As the plane followed its roller coaster style path, slowly rotating cylinders containing different materials experienced one tenth of Earth's gravity, Martian gravity and the Earth's normal pull.

And here, from their paper in the Journal of Geophysical Research, is the gravity-reducing plane.

 
But, before we look at what they found, let’s back up for a minute and remind ourselves about angle of repose. In its simplest sense it’s the slope that sand, or any granular material, naturally settles at after it has been poured. It varies substantially with the nature of the material – grain size and shape and so on. The paper included this helpful illustration of “A pile of rounded gravel at the angle of repose, built by Gijsbert F. Kleinhans.”

Because it’s important in many contexts, I have written several posts on the angle of repose, for example in kitchen physics, and “From Bogart to Bugs,” And it’s one of many principles of physics that can be observed simply by watching the flow of sand in this blog’s icon, the sandglass. But the sandglass, simple though it may appear, illustrates the complexity of many things, among them the angle of repose. For there is not one simple angle of repose for a given material – there is a static one and a dynamic one. Essentially, the static angle of repose is demonstrated when a pile of grains is at rest, but, once they start avalanching down a slope, the constant angle of the avalanche is different, and that’s the dynamic angle of repose, always lower than the static.

There are different ways of studying all this (should you be inclined – and many people are), but a classic is the rotating cylinder. This is basically like watching clothes in the drier – a transparent cylinder is partially filled with granular material and rotated – the sand, or whatever stuff you chose, cascades down-slope constantly at it’s dynamic angle of repose. When it comes to rest, it does so at its static angle. This is exactly the equipment that the Dutch researchers took up with them in their gravity-reducing plane: nine cylinders half-filled with sand, gravel, glass beads, some in air, some in water, their behaviours captured by HD video cameras:

All kinds of clever corrections for the noise and accelerations of the aircraft itself were applied. Each stomach-churning parabola allowed for around 20 seconds of experiment at gravities of down to one tenth the normal earthly value – the gravity on Mars is just over a third of Earth’s.

As the authors write in their paper, “Our main result is surprising.” They found that, with decreasing gravity, the static angle typically increases by around 5 degrees, whereas the dynamic angle decreases by about 10 degrees. And this is true of all the materials and regardless of whether they were in air or water – the angle of repose is not independent of gravity. The difference between the two angles is critical to the natural behaviours of granular materials, and in these experiments it increased by roughly an order of magnitude. Why is this important? If the static angle is higher, then a slope can build more steeply, but, once stability is lost, then avalanches cascade down the slope at the dynamic angle – if this is low, then the avalanches keep going and involve more material.

So, bigger avalanches, lower slopes on Mars are possible without the lubricating effect of water. And, as the authors point out, this gravity dependency of the angle of repose has implications for a wide variety of phenomena on Mars and elsewhere.

Ah yes, the answer to the quiz: it was great fun juxtaposing these images (well, to me at least) because of their remarkable similarities. The image on the left is of gullies and fans in a severely eroded 100 kilometer impact crater in the southern polar region of Mars (courtesy NASA/JPL/University of Arizona). On the right is a Digital Elevation Model (DEM) image of gullies and fans on Svalbard (a.k.a. Spitsbergen) from the Europlanet HRSC-AX flight campaign. The laws of physics are compellingly universal.

 

[Many thanks to Brian Romans of Clastic Detritus for catching this paper for me. Paper citation: Kleinhans, M. G., H. Markies, S. J. de Vet, A. C. in 't Veld, and F. N. Postema (2011), Static and dynamic angles of repose in loose granular materials under reduced gravity, J. Geophys. Res., 116, E11004, doi:10.1029/2011JE003865.]

Zen Faulkes, sand crabs, and RocketHub

 
We don’t much think about sand as an environment, a home for critters that can be more or less welcoming, its inhabitants more or less thriving. But of course sand is home to a mind-boggling menagerie of critters on a mind-boggling range of scales; I have written, for example, about bubbler crabs, sandfish, the beast of Bali, the Southern Stargazer, and our good beach companions, the meiofauna (whom we should appreciate even more given the recent report on the hazards of building a sandcastle).

But today, it’s back to crabs, the variety of sand crab known as Lepidopa benedicti to be precise. Back in January, Zen Faulkes, otherwise known as “Dr. Zen,” a researcher at  University of Texas-Pan American, wrote a fascinating guest post here on these retiring and unheralded critters. He talked about the mysteries of colour differences amongst his sand crabs, but now his focus is on size. It seems that Lepidopa benedicti on the Atlantic coast of Florida are substantially bigger than their relatives along the Gulf Coast, and Dr. Zen wants to know why.

To help him with the fieldwork, he has this project up for (very modest) funding from the innovative “crowdfunding” initiative, RocketHub. Here’s his description from that site – which is well worth visiting to watch a video of him talking about his “Beach of the Goliath Crabs” project, and, should you wish, to make a contribution.

They say, "Everything is bigger in Texas!" But that's not always true.
Sand crabs are hipster crustaceans: you probably haven't heard of them. They live submerged in sand and leave no traces where they are that you can see from the surface.

Lepidopa benedicti live in the Gulf of Mexico, and on the Atlantic side of Florida. Museum records claim this sand crab is 50% longer in Florida than in Texas. This would be like finding a group of people who are 8-9 feet tall!
Why are Atlantic sand crabs bigger than those from the Gulf of Mexico?
One possibility is that the Atlantic environment is much better for sand crabs. If so, sand crabs might be a good species to monitor beach health.
Another possibility is that no animals move between the Atlantic and the Gulf. They might be drifting apart genetically and starting to form new species. This can be tested using DNA to see how similar the populations are genetically. Sand crabs might be a good species to study a big question in marine ecology: how do small, slow-moving animals spread from place to place?

Your donation will buy supplies (sample jars, alcohol to preserve samples, DNA sequencing supplies) pay travel for a student to come with me to gain research experience!

I am already going to Florida later this year as part of my first SciFund project (Doctor Zen and the Amazon Crayfish Civilization). Your contribution will allow me to tackle a second project on the same trip, giving me more bang for your buck!

Granular flow: the icon continues to surprise

Anyone who reads this blog (and/or the book) will know that I have developed a deep fascination with the bizarre behaviours of granular materials: sand, a seemingly simple material, does stuff that hundreds of leading physicists around the world just don’t understand. Listing links to articles on this blog that explore the wondrous diversity of granular research would be too lengthy a task – just put “granular” into the search box and have a browse; new developments (and new mysteries) arise on a regular basis, and recent work at MIT is a fine example (all reports of which are illustrated – as, naturally, is this one – by a sandglass).

Here’s the news release from MIT. I hope that you enjoy as much as I did, the idea of “a million applications” from listening to chattering sand grains, and resist, as I did, any comments on size mattering in the chattering. Oh, and please, please, be sure to actually follow the link above and watch the movie of the researcher, Ken Kamrin, and his very beautiful sandglass – it’s essentially a manifesto for this blog.

Shifting sands

New model predicts how sand and other granular materials flow.

Jennifer Chu, MIT News Office

Photo: Lucy Lindsey

April 6, 2012

Sand in an hourglass might seem simple and straightforward, but such granular materials are actually tricky to model. From far away, flowing sand resembles a liquid, streaming down the center of an hourglass like water from a faucet. But up close, one can make out individual grains that slide against each other, forming a mound at the base that holds its shape, much like a solid.  
Sand’s curious behavior — part fluid, part solid — has made it difficult for researchers to predict how it and other granular materials flow under various conditions. A precise model for granular flow would be particularly useful in optimizing processes such as pharmaceutical manufacturing and grain production, where tiny pills and grains pour through industrial chutes and silos in mass quantities. When they aren’t well-controlled, such large-scale flows can cause blockages that are costly and sometimes dangerous to clear.
Now Ken Kamrin of MIT’s Department of Mechanical Engineering has  come up with a model that predicts the flow of granular materials under a variety of conditions. The model improves on existing models by taking into account one important factor: how the size of a grain affects the entire flow. Kamrin and Georg Koval, assistant professor of civil engineering at the National Institute of Applied Sciences in Strasbourg, France, used the new model to predict sand flow in several configurations — including a chute and a circular trough — and found that the model’s predictions were a near-perfect match with actual results. A paper detailing the new model will appear in the journal Physical Review Letters.
“The basic equations governing water flow have been known for over a century,” says Kamrin, the Class of ’56 Career Development Assistant Professor of Mechanical Engineering. “There hasn’t been something similar for sand, where I can give you a cupful of sand, and tell you which equations will be necessary to predict how it will squish around if I squeeze the cup.”
Blurring the lines
Kamrin explains that developing a flow model — also known as a continuum model — essentially means “blurring out” individual grains or molecules. While a computer may be programmed to predict the behavior of every single molecule in, say, a cup of flowing water, Kamrin says this exercise would take years. Instead, researchers have developed continuum models. They imagine dividing the cup into a patchwork of tiny cubes of water, each cube small compared to the size of the entire flow environment, yet large enough to contain many molecules and molecular collisions. Researchers can perform basic lab experiments on a single cube of water, analyzing how the cube deforms under different stresses. To efficiently predict how water flows in the cup, they solve a differential equation that applies the behavior of a single cube to every cube in the cup’s grid.

Such models work well for fluids like water, which is easily divisible into particles that are almost infinitesimally small. However, grains of sand are much larger than water molecules — and Kamrin found that the size of an individual grain can significantly affect the accuracy of a continuum model.
For example, a model can precisely estimate how water molecules flow in a cup, mainly because the size of a molecule is so much smaller than the cup itself. For the same relative scale in the flow of sand grains, Kamrin says, the sand’s container would have to be the size of San Francisco.
Neighboring chatter
But why exactly does size matter? Kamrin reasons that when modeling water flow, molecules are so small that their effects stay within their respective cubes. As a result, a model that averages the behavior of every cube in a grid, and assumes each cube is a separate entity, gives a fairly accurate flow estimate. However, Kamrin says in granular flow, much larger grains such as sand can cause “bleed over” into neighboring cubes, creating cascade effects that are not accounted for in existing models.
“There’s more chatter between neighbors,” Kamrin says. “It’s like the basic mechanical properties of a cube of grains become influenced by the movement of neighboring cubes.”
Kamrin modified equations for an existing continuum model to factor in grain size, and tested his model on several configurations, including sand flowing through a chute and rotating in a circular trough. The new model not only predicted areas of fast-flowing grains, but also where grains would be slow moving, at the very edges of each configuration — areas traditional models assumed would be completely static. The new model’s predictions matched very closely with particle-by-particle simulations in the same configurations.
Lyderic Bocquet, a professor of physics at the University of Lyon in France, sees Kamrin’s results as a big step toward a granular flow model with “trustable, predictive power.”
“There have been huge efforts over the last years to propose laws to describe granular flows, with the ultimate aim of designing devices or processes involving granular
materials,” Bocquet says. “The generalization to a broader system was still lacking. This is where Ken’s model could make the connection.”
The model, run on a computer, can produce accurate flow fields in minutes, and could benefit engineers designing manufacturing processes for pharmaceuticals and agricultural products. For example, Kamrin says, engineers could test various shapes of chutes and troughs in the model to find a geometry that maximizes flow, or mitigates potentially dangerous wall pressure, before ever actually designing or building equipment to process granular materials.
Kamrin says understanding how granular materials flow could also help predict geological phenomena such as landslides and avalanches and help engineers come up with new ways to generate better traction in sand.
“Granular material is the second-most-handled material in industry, second only to water,” Kamrin says. “I’m convinced there are a million applications.”

 

The costs of ignoring science

 
Cue sounds of timpani, stage left and stage right. A couple of weeks ago, I wrote a drum-banging post about coastal management myopia; now, recent reports compel me to continue my instrumental activity. So let’s think a little about the implications of rising sea levels (emphasis on the word think).

It is a fact (which I choose to define as something supported by observation, measurement, and analysis – in other words, science) that sea levels have been rising and it is reasonable to infer that this will continue. The world is, in general, warming, and the laws of physics (if you choose to accept them) require that warming oceans expand in volume; the cause of the warming, and any so-called debate over human influence, is irrelevant. Expanding ocean volumes have to go somewhere, and that somewhere is, inevitably, low-lying coastal land. OK so far? Anything really to challenge here? Those questions are, of course, essentially rhetorical since I have good reason to believe that readers of this blog are capable of independent, rational thought. Now here’s the tricky bit: predicting the magnitude of future sea level rise at any place on the planet is not easy – there is (oh dear) some uncertainty associated with this. But perhaps we can, nevertheless, attempt to come to grips with this uncertainty, and plan accordingly? Unfortunately, the answer seems to be “no.”

In that previous post, I mentioned NC20, the lobbying organisation of twenty North Carolina counties, who declare that they “concentrate primarily on actions to prevent regulation and rule making not based on science.” That statement would imply that, in turn, they would support regulation that is based on science, yes? No. When presented with a report by the North Carolina Coastal Resources Commission (CRC) in which a group of nineteen scientists presented an assessment of sea level rise, the response by the NC20 Board Members (credentials) was as follows:

Perhaps the most impressive accomplishment to date was the complete reversal of the draft policy of CRC regarding Sea Level Rise (SLR). The draft policy presented by the Science Panel of the CRC mandated a planning component of 39 inches of SLR across the CAMA area. NC 20 Board members met in a private meeting with the Chair of the CRC and carefully laid out the unscientific assumptions used in the draft report, showing the numerous errors and inconsistencies. In the final presentation, all “mandates” were removed and the final document showed a sensitivity to the needs of the counties, the role of the CRC, and the need for a relationship characterized by scientific advice and assistance on the part of the CRC rather than ironfisted dictates. The “redlined” version of that final document is attached for your review.

Follow the link and you will see the original policy wording (deleted) and its replacement (I shall resist commenting):

And here, just for a laugh, are the "unscientific assumptions" on North Carolina sea-level rise (taken from the CRC report):

So this really is about noise: the decibels from the drumbeats and grinding axes of vested interest lobbying easily drown out the voices of nineteen scientists.

But for those who heroically maintain an interest in science and how it can inform planning and economics, let’s turn to the couple of recent reports that I mentioned, that address exactly this topic. First, there has been a fair level of justified (but not particularly cacophonous) buzz in the media about a report from Climate Central titled Surging Seas: Sea level rise, storms, and global warming’s threat to the US coast, published on the 14th March, simultaneously with the research being published in the peer-reviewed journal Environmental Research Letters. Two pieces from the New York Times (grounds, I sadly realise, for them to be dismissed), one by Andy Revkin, the other by Justin Gillis, summarise the findings from the report. As Revkin quotes from the report, which makes it clear that it is not simple sea level rise itself, but associated storm surges that represent the threat:

Rising seas dramatically increase the odds of damaging floods from storm surges. For over two-thirds of the locations analyzed (and for 85% of sites outside the Gulf of Mexico), past and future global warming more than doubles the estimated odds of “century” or worse floods occurring within the next 18 years — meaning floods so high they would historically be expected just once per century. For over half the locations analyzed, warming at least triples the odds of century-plus floods over the same period. And for two-thirds the locations, sea level rise from warming has already more than doubled the odds of such a flood even this year….

Sea level rise has already cost governments and private landowners billions of dollars as they have pumped sand onto eroding beaches and repaired the damage from storm surges.

Insurance companies got out of the business of writing flood insurance decades ago, so much of the risk from sea level rise is expected to fall on the financially troubled National Flood Insurance Program, set up by Congress, or on state insurance pools. Federal taxpayers also heavily subsidize coastal development when the government pays to rebuild infrastructure destroyed in storm surges and picks up much of the bill for private losses not covered by insurance.

For decades, coastal scientists have argued that these policies are foolhardy, and that the nation must begin planning an orderly retreat from large portions of its coasts, but few politicians have been willing to embrace that message or to warn the public of the rising risks.

Some further selected key points:

• By 2030, storm surges combined with rising seas could raise waters to 4 feet (1.2 metres) or more above high tide lines at many locations; 4.9 million people live in 2.6 million homes in this vulnerable zone between the observed high tide and the top of expected flood waters.
• In 285 coastal cities and towns, more than half the population lives below the 4-foot mark, the Climate Central report found. Florida has 106 of these at-risk municipalities; Louisiana has 65, New Jersey and North Carolina have 22 each, Maryland has 14, New York has 13 and Virginia has 10.
• For two-thirds of the locations, sea level rise from warming has already at least doubled the annual risk of century-plus floods.
• The population and homes exposed are just part of the story. Flooding to four feet would reach higher than a huge amount of dry land, covering some 3.0 million acres of roads, bridges, commercial buildings, military bases, agricultural lands, toxic waste dumps, schools, hospitals, and more.

And a couple of quotes from Ben Strauss, one of the lead scientists authoring the report:

Sea level rise is like an invisible tsunami, building force while we do almost nothing. We have a closing window of time to prevent the worst by preparing for higher seas….

A lot of the state [of Florida] is built on porous bedrock, bedrock that's like Swiss cheese. You can't practically build a wall to keep the sea out. The water will come up through the ground….

Right now the projection for the end of the century is two to seven feet of sea level rise and seven feet would put a lot of Lower Manhattan under water if we didn’t build sea walls to protect it. 

So, if the Board of NC20 were to read this report, what would they find about their state? Well, the report’s website has an excellent interactive map, and a series of fact sheets on each state; here are some headlines for North Carolina:

• Odds of a 100‐year flood or worse by 2030, with sea level rise from global warming: 24% (odds without global warming: 9%)
• Historic local sea level rise rate: 0.8 inches/decade; projected new sea level rise by 2050: 11 inches 
• Population at risk: 76,000
• Homes at risk: 56,000 
• Land area at risk: 1.2 million acres
• Towns and cities where at least half the population is at risk: 22 
• Counties where at least 10% of the population is at risk: 9
  

Perhaps grounds for a little planning effort? Supported by facts? Incorporating some science?

And then there’s the recently published study whose focus is on California; the study "Estimating the Potential Economic Impacts of Climate Change on Southern California Beaches," is featured in a special edition of the peer-reviewed journal Climatic Change, and is the work of researchers at Duke University. The study again looks at the effects of sea level rise in the long term, and the short term effects of storms. As reported in Science Daily  last month:

While some beaches may shrink or possibly disappear, others are poised to remain relatively large -- leaving an uneven distribution of economic gains and losses for coastal beach towns, according to a study by researchers at Duke University and five other institutions.

"Some beaches actually stand to benefit economically from sea level rise, creating winners and losers among California beach towns," said Linwood Pendleton, director of ocean and coastal policy at Duke's Nicholas Institute for Environmental Policy Solutions. "We found, as relatively small beaches shrink more due to sea level rise, people will stop visiting them, opting for wider beaches."

The same topic was addressed last year in a report by economists at San Francisco State University that examined the economic effects of sea level rise and storms on five California coastal communities:

The results suggest that visitor hotspots like Venice Beach could lose up to $440 million in tourism revenue between now and 2100 if sea levels rise by 4.6 feet (1.4 meters), a projection specific to the California coast, based on recent scientific studies. At San Francisco's Ocean Beach, accelerated erosion could cause up to $540 million worth of damage.

"Sea level rise will send reverberations throughout local and state economies," said Philip King, associate professor of economics at San Francisco State University. "We also found that the economic risks and responses to a changing coastline will vary greatly over time and from beach to beach."

Here, as an example, are the estimated impacts on San Francisco’s Ocean Beach: 

Ocean Beach (north of Sloat Boulevard), San Francisco County

Based on a sea level rise estimate of 4.6 feet (1.4 meters) by 2100, Ocean Beach could lose:

• $19.6 million in damages caused by a 100-year coastal flood damaging homes and contents. This is an increase of 200 percent from the present day risk of a 100-year flood, which is $6.5 million
• $82 million in tourism spending and local and state tax revenue losses (accumulated between now and 2100) caused by a narrower, eroded beach attracting fewer visitors
• $16.5 million in habitat and recreation losses, caused by erosion reducing the beach area by 92 percent (53 acres lost). Ocean Beach provides a habitat for native species such as the Western Snowy Plover, a bird that is federally listed as a threatened species
• $540 million caused by land, buildings and infrastructure being lost or damaged by erosion and subsidence

The facts, the predictions, the costs, the social implications, are there to be found, whether the result of work by universities, government agencies, or – as I happened upon – even the real estate “community.” The property site, World Property Channel, published the results of a “Special report: Long Island, Miami Have Highest Property Exposure to Storm Surge Damage as Hurricane Season Approaches” that was accompanied by dramatic graphics and the following figures:

Of the metro areas studied in the report, Long Island was found to have the highest exposure to risk, valued at $99 billion, followed by the Miami-Palm Beach region and Virginia Beach. Projected exposure to storm surge damage for the ten geographies is as follows:

Long Island, NY - $99 billion, Miami-Dade, FL - $44.9 billion, Virginia Beach, VA - $44.6 billion, New Orleans, LA - $39 billion, Tampa, FL - $27 billion, Houston, TX - $20 billion, Jacksonville, FL - $19.6 billion, Charleston, SC - $17.7 billion, Corpus Christi, TX - $4.7 billion, Mobile, AL - $3 billion

But, silly me, I’m forgetting that all this work, all these “facts," are flawed by “unscientific assumptions… showing the numerous errors and inconsistencies.”

 

[Photographs: the surge before the storm swamps Galveston Island, Texas, and a fire destroys homes along the beach as Hurricane Ike approaches. (AP Photo/David J. Phillip). Flooding over access road 523 to Surfside beach, caused by Hurricane Ike forming in the Gulf of Mexico, is seen near Surfside Beach, Texas. (REUTERS/Carlos Barria). And http://myhydros.org/more-about-water/storm-surge/]

 

 

Critters versus Sand, Round 2: awesome Androctonus

 
This time, imagine that you're a scorpion, a fattail scorpion to be precise (sorry, but I never said anything about this role-playing being about cuddly and loveable, handsome or cute, but it’s often critters from the less celebrated realms of the animal kingdom that demonstrate the most remarkable skills). You are going about your desert business, scurrying across the sand in search of something to slaughter, when a sandstorm blows up. What should you do? Quickly seek shelter, bury yourself in the sand? Absolutely no need if you’re Androctonus australis, because you, unlike much of the world’s mechanised military, have developed an extraordinarily clever resistance to windblown sand. Blasting that can frost a windshield, clog up a filter, render many moving parts unmoving, never mind erode rocks, simply blows by you – sand off a scorpion’s back.

The yellow fattail scorpion (Androctonus australis) is an unpleasant creature, growing up to 10 cms in length, and capable of an envenomation that is dangerously and mortally toxic. But on their backs, they carry a bionic shield against sand abrasion, an exoskeleton that has bumps and grooves, the design of which cunningly deflects flying sand grains (illustration above, right).

All this has been recently revealed by a group of researchers at the Key Laboratory of Bionic Engineering, Jilin University, China, and published by the American Chemical Society. Erosion Resistance of Bionic Functional Surfaces Inspired from Desert Scorpions describes how these scorpions use microtextures to achieve erosion resistance, and how the researchers used Computational Fluid Dynamics to understand how these patterns work. The results have significant implications for the mechanised military (not to mention, perhaps, Toyota and Land Rover). Here’s the account from Science Daily:

Taking inspiration from the yellow fattail scorpion, which uses a bionic shield to protect itself against scratches from desert sandstorms, scientists have developed a new way to protect the moving parts of machinery from wear and tear.

A report on the research appears in ACS' journal Langmuir.

Zhiwu Han, Junqiu Zhang, Wen Li and colleagues explain that "solid particle erosion" is one of the important reasons for material damage or equipment failure. It causes millions of dollars of damage each year to helicopter rotors, rocket motor nozzles, turbine blades, pipes and other mechanical parts. The damage occurs when particles of dirt, grit and other hard material in the air, water or other fluids strike the surfaces of those parts. Filters can help remove the particles but must be replaced or cleaned, while harder, erosion-resistant materials cost more to develop and make. In an effort to develop better erosion-resistant surfaces, Han and Li's group sought the secrets of the yellow fattail scorpion for the first time. The scorpion evolved to survive the abrasive power of harsh sandstorms.

They studied the bumps and grooves on the scorpions' backs, scanning the creatures with a 3-D laser device and developing a computer program that modeled the flow of sand-laden air over the scorpions. The team used the model in computer simulations to develop actual patterned surfaces to test which patterns perform best. At the same time, the erosion tests were conducted in the simple erosion wind tunnel for groove surface bionic samples at various impact conditions. Their results showed that a series of small grooves at a 30-degree angle to the flowing gas or liquid give steel surfaces the best protection from erosion.

So just orient yourself appropriately to the wind, and scuttle off on your evil way.

 

[See also the Physorg.com report. Oh, and Richard, “envenomation” was especially for you…]

Critters versus Sand, Round 1: the fabulous fungiids

Imagine for a moment, if you will – or can – that you’re a coral. You find yourself in the lap of luxury, ensconced on a cosy bed of sand in a carefully maintained aquarium, far from the vicissitudes of life on the sea floor. And then, suddenly, this wretched scientist comes along and throws a pile of sand in, completely burying you. You’re entombed alive - what are you to do? Well, if you’re a fungiid, you simply shrug it off.

Of all the amazing things that I come across in this amazing world, the most startling are nearly always from new revelations about life on this planet. Whether it’s the constant discovery of new species, the seemingly daily expansion of the known limits for extremophiles, or simply how critters do stuff that we never realised they could, the news continues. And it is, of course, a couple of recent discoveries about how critters deal with sand problems that have caught my eye, and I will begin with a report that I just happened to see yesterday on the BBC News website, Corals inflate to escape being buried alive in sand. I have long ceased to be astounded by the frequency with which my favourite topic appears prominently in the mainstream media, but this is quite extraordinary.

For a start, if you think (like I did) of corals as static, colourful and sculptural installations on the sea floor, to be wondered at while snorkelling and protected from human depredations, then you’d be wrong. Some corals move. They may do it slowly and sedately, but they move – and the fungiidae, the so-called “mushroom corals” are the prime exponents of this skill.

So that’s a start – having been buried alive, you’re relieved to find that you’re a fungiid, capable of movement. But even so, just any old movement will hardly be sufficient to rid yourself of this burden of sand. So you flex yourself a little – and wow! You can do pulsed inflation! A few grains cascade from the pile, the burden is slightly relieved and you have even managed to get a couple of your cilia protruding above the sand. You pulse inflate again….

“Enough!” I can distinctly hear you exclaiming, dear reader, and you are quite right, enough of this anthropomorphised coral role-playing. Rather, first of all, just go to the BBC link above and watch the time-lapse animation there – it is absolutely extraordinary. It’s the work of Dr. Pim Bongaerts and his colleagues in Australia and the Netherlands, the evil scientists who buried the poor coral, and you can read - and reproduce - the authors’ personal copy of the paper here. It begins as follows:

Sedimentation represents a major stressor for scleractinian corals. Although many coral species exhibit the capacity of active sediment rejection (Stafford-Smith and Ormond 1992), only few are capable of freeing themselves after becoming completely buried. Fungiid corals appear to be an exception, as they can remove sediments through substantial polyp inflation (up to five times their normal size) in addition to mucus entanglement and ciliary action… Using time-lapse photography (speeding up time 300x), we observed that this inflation occurs in rhythmic pulses, allowing corals to completely exhume themselves after becoming covered in sand.

Ok, it’s speeded up (although really not that much) but, if you’ve already watched it, I hope that you share my astonishment. The complexity of what these apparently simple and passive critters are doing is mind-boggling - “mucus entanglement” and “ciliary action” don’t really convey what’s going on (although they are consistent with the science fiction image that comes to mind of this happening at full speed and on a large scale….).

It really is a wonderful world.

 

[The fungiid at the head of this post is Lobactis scutaria, the one below Herpolitha limax, and the images are screenshots from large movie files downloadable here.]

Tom, Dick, Harry, and George - sand and The Great Escape

 

The plan to dig down to the workshop at the starting point of the tunnel known as Harry, where the wartime fliers began their effort to tunnel beneath the barbed wire, was aborted when engineers failed repeatedly to prevent tons of sand from collapsing their own access tunnel.

“The tunnel known as Harry” was one of the incredible constructions by the prisoners of Stalag Luft III, the escape tunnels, laboriously and ingeniously built, that would be immortalised in the movie, The Great Escape. But this story did not need a movie for immortality – it was extraordinary enough in reality. And now a geoarchaeological project, led by Hugh Hunt, a Cambridge University engineering professor, has shed further light on just how extraordinary it was.

The prisoner of war camp was built, intentionally, on the sandy soils of the forests of today’s western Poland, along the banks of the Bóbr river. Intentionally, because the river valley is filled with sandy sediments deposited from melt waters of the Ice Age glaciers and carried by the ancestral Bóbr. And sand is difficult to tunnel through. Very difficult.

An earlier geoarchaeological expedition to the site of Stalag Luft III was reported in a fascinating article by Peter Doyle and his colleagues that includes a geological assessment of these soils. The following illustration of the sands is taken from their work:

A thin grey organic soil is underlain by glacio-fluvial cross-bedded sands, yellow and reddish-yellow in colour. As the authors point out, this in itself was a problem for the would-be escapers – the excavated sand from the tunnels was immediately visible if deposited against the darker topsoil.

There were numerous tunnelling attempts made at Stalag Luft III, each tunnel given a name, but the events captured in The Great Escape related to Harry. Harry was 330ft long, made from 4,000 bed boards and dug with thousands of pieces of cutlery. The ingenuity and engineering were astonishing – and, as Hunt discovered, impossible to reproduce without unacceptable danger. The movie was based on the book of the same name by Paul Brickhill, whose illustration of the project was reproduced in the article by Peter Doyle and his colleagues:

 
The escape would end in tragedy for all but three of the 76 prisoners who emerged from the tunnel on that March night in 1944. “Following the escape, it is estimated that 5 million Germans were mobilized to recapture them. Of the 76 that escaped, 3 eventually reached Allied territory, but the remainder were recaptured, and 50 of them were executed by the Gestapo.”

One of the remarkable details of Hugh Hunt’s recent project was that one of the team was Frank Stone. Now 89, Frank was waiting his turn to enter the tunnel that night when the escape was discovered. As reported in the Daily Mail, he demonstrated the use of the trolleys made to move men quickly down the tunnels:

Memorials now mark the sites of the entrance and exit to Harry:

 
Hunt’s work was the subject of a recent documentary on British TV, and compellingly described in the following piece from The New York Times:

Latter-Day Dig of ‘Great Escape’ Tunnels Humbles Modern Engineers

By JOHN F. BURNS

CAMBRIDGE, England — For scale, they were no match for the Great Pyramids of Giza or the Panama Canal. The labor took months rather than years and a work force of barely 100 men. As for materials, there were none, beyond what the captured Royal Air Force fliers who built them could scavenge, scrounge or improvise.

But by the measures of ingenuity, courage and persistence, the tunnels built almost 70 years ago in sandy scrubland near the small town of Zagan, 130 miles southeast of Berlin in what was then Hitler’s Germany and is today western Poland, were a legendary feat of engineering, although on a miniature scale.

Chronicled by the 1963 movie “The Great Escape,” the tunnel building is one of World War II’s great stories. In the decades since, the legend of the allied fliers’ mass breakout on the night of March 24, 1944, together with the ingenious planning and the Nazi retribution that followed — 73 of the 76 escapers recaptured, and 50 of them summarily executed on Hitler’s orders — has, in a way, eclipsed reality.

In an effort to establish more clearly how the escape was accomplished — and, in a sense, to reclaim the narrative of the breakout — British-based engineers, battlefield archaeologists and historians traveled into the pine forest outside Zagan last summer to unearth the secrets buried there for a television documentary by Wildfire Television in London that was broadcast in late 2011 in Britain. They were accompanied by modern-day Royal Air Force pilots, as well as veterans of wartime bombing raids, now in their 80s, who helped build the tunnels at the encampment known as Stalag Luft III.

The team’s task was to employ “reverse engineering” by uncovering the tunnels and what remained of the tunnelers’ jury-rigged equipment to replicate the wartime fliers’ ingenuity. Ultimately, the team members were stunned that, even without the menace of the ever-watchful Nazi camp guards, they were unable to match their wartime counterparts fully, particularly in the most crucial skill, digging a tunnel 30 feet below the camp surface without repeated collapses of the sandy soil above.

For years, veterans and others have pored over the camp’s ruins, laying memorial stones amid the outcroppings of broken brick and concrete scattered among the pine trees, all that remains of the 60-acre site built by the Germans to house 10,000 captured fliers. But no group matched the expertise of the 2011 team, which went determined to lay bare what Hugh Hunt, a Cambridge University engineering professor, described as “the final secrets of a remarkable story.”

A maverick Australian affectionately nicknamed Dr. Screwloose by his colleagues, Dr. Hunt went to Poland as a consultant to the current R.A.F. pilots, including some with combat experience over Iraq and Afghanistan. Their task was to use insights gleaned from the digs at the sites of wartime tunnels known as Harry and George to build a new 35-foot tunnel they called Roger, after Squadron Leader Roger Bushell, the principal organizer of the 1944 escape and one of those executed by the Nazis.

“What those men did at Stalag Luft III was an astonishing feat of improvisational engineering,” Dr. Hunt, 50, said in an interview at Cambridge’s Trinity College. “Their resourcefulness was beyond belief. It wasn’t a case of one man’s genius, more the accomplishment of a team, one man’s skills complementing another’s. And they had one precious resource, time. If you have time, somebody will eventually come up with something, and the others will say, ‘Let’s give it a go.’ ”

Like others who joined in the expedition to Zagan, Dr. Hunt has little quarrel with the escape story as told in the film version, which was based on Paul Brickhill’s book of the same name, partly because the tunnelers’ real-life drama required little embellishment. The wartime camp, for one thing, did not have a Virgil Hilts, the irrepressible American flier played by Steve McQueen, and there was no climactic sequence like his flight from German troops on a stolen BMW motorcycle and entanglement in a border fence, one of cinema’s great chase sequences.

The plan to dig down to the workshop at the starting point of the tunnel known as Harry, where the wartime fliers began their effort to tunnel beneath the barbed wire, was aborted when engineers failed repeatedly to prevent tons of sand from collapsing their own access tunnel. But Dr. Hunt and his team struck gold in the excavation of George, a tunnel built under the camp auditorium after the escape and designed to give inmates a place to hide as Nazi control east of Berlin collapsed before advancing Soviet troops.

Their dig yielded a set of rusting trolley wheels, the metal scavenged from remnants of a campsite stove and a coil spring taken from prison gramophones; wood paneling for the tunnel’s roof and sidewalls, fashioned from the prisoners’ bed boards; and a ventilation pump with a bellows and piping made from a prisoner’s kitbag, ice hockey sticks and tins of powdered milk. The pièce de résistance was a rusting radio made from a biscuit box, the wiring stolen from the prisoners’ huts and batteries scrounged from German guards.

The contemporary Royal Air Force fliers built Roger, the replica tunnel, but in a trench just beneath the surface; anything deeper was deemed too dangerous. With Dr. Hunt, they fashioned a trolley system, given its first run in the new tunnel by Frank Stone, 89, a camp veteran involved in preparing the 1944 escape plan. The 2011 team also built a replica of the original tunnelers’ ventilation system, with facsimiles of wartime milk tins and a World War II kitbag.

Mr. Stone, maker of the biscuit-tin radio found in the excavated remains of tunnel George, was one of those still waiting for his turn to escape when a German guard spotted one flier scrambling from the tunnel exit into the pine trees. He remained a prisoner until the camp was liberated in 1945, with a lifetime to grieve for the men who made it, if only briefly, to freedom. “People say to me, ‘How unlucky you were,’ ” he told the documentary makers. “But I say, no, I was lucky to have taken part in it at all.”

 

[Memorials images Lech Muszynski/European Press Photo Agency and http://www.raf.mod.uk/project104/gallery/stalagluftiiigallery.cfm?viewmedia=3]

Sand critter guest post: the South Texas sand crab

By Zen Faulkes

South Padre Island in Texas has a world-class beach, with lovely fine-grained sand that is wonderful to walk on. The sand grains close up seem to be mostly grains of light or transparent quartz, with just the occasional bit of shell grit. At least, that’s what it looks like to my completely untrained eye.

 
I personally am not so much interested in the sand, however, as some of the creatures living in it. I have been spending the last few years digging around in the beaches there, studying a small local sand crab, Lepidopa benedicti. Because they are so little known, the species doesn’t have a common name; the entire superfamily is generally called “sand crabs” or “mole crabs.” I got started working with sand crabs for my doctoral research because I was interested in how they are able to dig so quickly and efficiently into the sand, and why they couldn’t walk like other crustaceans (Faulkes & Paul, 1997a, b).

  

For the last couple of years, I have been studying these crabs more from an ecological viewpoint. During my regular collections, I eventually made a conscious mental note of something that is incredibly obvious when you say it out loud. Some of these crabs are white, while others are almost a battleship gray.

 
When I looked in the scientific descriptions of these species, the colour of this particular species was never mentioned, and other Lepidopa species were usually described as white. Why are they different colors? We often think of colours as being important signals for animals; think of warning colours, for instance. Sand crabs spend almost all of their time buried in sand. In ten years of digging on this beach, I have seen these sand crabs above the sand twice (weirdly, on the same day!), and even then only for a couple of seconds. The crabs have small, poorly developed eyes. It seemed unlikely that they are using these different colors to signal each other.

Although the eyes of some sand crabs are small, they sometimes take note of their surroundings. Some very nice experiments with sand crabs showed how environmental factors can affect colour. For instance, mole crabs (Hippa) are found on either white coral beaches or black volcanic beaches, and they tend to match their background. What’s even cooler is that you take an animal from a white beach, and put it into black sand, and wait, it will tend to darken up after it molts (picture from Bauchau & Passelecq-Gérìn 1987; see also Wenner 1972).

This doesn't seem to explain the color differences I'm seeing in the local sand crabs, however, because they’re all being collected from one beach, which is a light tan as far as the eye can see.

Another idea that came to mind was that this was related to whether an animal was a male or female. That also turned out not to be the case, as each colour had an even split of sexes.

But two pieces of evidence suggest that these colours are not just incidental. There are more gray sand crabs than white ones. And the gray ones tend to be larger than the white ones, although there is overlap (data from Nasir & Faulkes 2010; carapace drawing by Boyko 2002).

Something that might help sort out this puzzling little question would be to look at other populations. The animals I have collected over the last few years are nowhere near the largest on record for the species. The largest individuals in this species have been found on the Atlantic coast of Florida (Boyko 2002). The average size of sand crabs there is about the size of the biggest individuals I’ve found on South Padre Island. And it makes me wonder, if the Atlantic L. benedicti are that much bigger, are they more likely to be that dark battleship gray?

References

Bauchau AG, Passelecq-Gérìn E. 1987. Morphological color changes in anomuran decapods of the genus Hippa. Indo-Malayan Zoology 4(1): 135-144.

Boyko CB. 2002. A worldwide revision of the recent and fossil sand crabs of the Albuneidae Stimpson and Blepharipodidae, new family (Crustacea, Decapoda, Anomura, Hippoidea). Bulletin of the American Museum of Natural History 272(1): 1-396. http://dx.doi.org/10.1206/0003-0090(2002)272<0001:AWROTR>2.0.CO;2

Faulkes Z, Paul DH. 1997a. Coordination between the legs and tail during digging and swimming in sand crabs. Journal of Comparative Physiology A 180(2): 161-169. http://dx.doi.org/10.1007/s003590050037

Faulkes Z, Paul DH. 1997b. Digging in sand crabs (Decapoda, Anomura, Hippoidea): interleg coordination. The Journal of Experimental Biology 200(4): 793-805. http://jeb.biologists.org/cgi/content/abstract/200/4/793

Nasir U, Faulkes Z. 2011. Color polymorphism of sand crabs, Lepidopa benedicti (Decapoda, Albuneidae). The Journal of Crustacean Biology 32(2): 240-245. http://dx.doi.org/10.1651/10-3356.1

Wenner AM. 1972. Incremental color change in an anomuran decapod Hippa pacifica Dana. Pacific Science 26: 346-353.

 

[“Doctor Zen” first got in touch with me in the spring of last year to kindly point me to some research on bubbles behaving like granular materials. I had a look at his personal web page at the University of Texas-Pan American, and discovered his research interest in decapod crustacean behaviour, in particular the unusual crayfish, Marmorkrebs; I was particularly interested in his work on crustaceans digging in sand – the slipper lobster video on his site is strangely compelling. I asked if he would contribute a guest post and he kindly agreed. I hope that readers find this as fascinating as I do – I’m reminded of some of the intriguing evolutionary questions raised by beach mice and the bleached earless lizard. Thanks, Zen, for this – well worth waiting for!].

Lying vehicles

It has become a sarcastic proverb that a thing must be true if you saw it in a newspaper. That is the opinion intelligent people have of that lying vehicle in a nutshell. But the trouble is that the stupid people -- who constitute the grand overwhelming majority of this and all other nations -- do believe and are moulded and convinced by what they get out of a newspaper, and there is where the harm lies. (Mark Twain, 1873)

The laurels on which I was complacently resting after my fervent New Year’s wish for Earth Science Literacy have instantly withered, and my understanding of the heroic – if not futile and stupid – nature of my wish cruelly underlined. I reproduce for you below, essentially in its entirety, a piece published on Tuesday by one John Trumbo of the Tri-City Herald in the State of Washington. This was taken up by a number of newspapers, both online (the sadly misnamed “Seattle Post Intelligencer”) and hardcopy. And it appeared as news. Read this and weep – scarcely a sentence goes by that does not include utter, mind-boggling-numbing, lying, rubbish. I despair.

Richland man sees sandstone proof of global flood

WEST RICHLAND, Wash. (AP) — A swirling, twisting sandstone formation in northern Arizona is evidence of Noah's flood, says a West Richland man who recently visited the unusual geologic phenomenon.

Greg Morgan, a nuclear safety engineer at Hanford, said he was amazed to see sandstone resembling waves, whirlpools and reversing currents that appear to have been frozen in place.

Morgan's photographs of The Wave and his article, "Flood Currents Frozen in Stone," are in the latest issue of Answers magazine, a quarterly publication of Answers in Genesis, a Christian creation research organization based in Petersburg, Ky. The nonprofit organization's 70,000-square-foot facility also houses the Creation Museum.

Mike Matthews, editor of Answers, said the way the layers of sandstone came to rest at Paria Canyon "fits with the viewpoint that these are flood layers."

But the strongest evidence of a global flood is in the sandstone itself, he said.

"It's that the layers themselves have been traced out worldwide, even to Europe and the Mideast," Mathews said.

The Wave at Paria provides a little more -- what Matthews called important "signature details."

Morgan, who is a mechanical engineer and worked in the aviation industry before coming to Hanford, said he was shocked when he first saw a picture of The Wave because it contradicted his original thinking about an ancient Earth and evolution.

Morgan, who became a Christian as an adult and takes the Bible literally, said the convoluted formations at Paria Canyon forced him to consider there must be another explanation.

"There are no broken rocks. All of this happened when it was still mud," Morgan said.

Morgan first visited The Wave at Paria Canyon in 2008 and again in September, each time taking many photographs he later could study.

The whirlpools suggest the stone was flowing, as if it was a slurry of sand that suddenly froze in place, he said.

"This is excellent evidence for Noah's flood. It is far better than what anyone believes for an ancient Earth," Morgan said.

The formation itself is classified as Jurassic Navajo Sandstone, and according to conventional "old Earth" geology, was formed 200 million years ago when seasonal winds laid down the layers to create the dramatic land forms.

Morgan explains it differently in his article for Answers magazine.

"Creation geologists believe huge sand waves were piled up and laid down deep under the ocean water. The fast currents then created tell-tale features, known as cross beds or 'striations,' with the steep cut-offs we see today," Morgan wrote in support of a global flood event.

The article also mentions that the sand at The Wave has been identified as having come from the Appalachians, about 1,800 miles to the east.

Moses' account in Genesis of Noah's flood describes a worldwide inundation that covered even the tallest mountains.

Morgan, who initially thought The Wave was just an example of water and wind erosion cutting through many layers of sandstone, says the evidence at Paria Canyon shows "what Moses wrote was true."

"I may be the first among creation geologists to openly promote this as evidence of Noah's flood," he said.

If such a cataclysmic flood event deposited the twisted layers of sand and whirlpools that later were to turn to stone, then how did the flood currents frozen in stone become exposed in a waterless desert?

Morgan believes a second flood catastrophe -- perhaps similar to the Ice Age floods that scoured Eastern Washington thousands of years ago -- unleashed icy waters that ravaged the Southwest, as well.

That would account for the rapid erosion in Paria Canyon and in the Grand Canyon, which is about 70 miles south of The Wave, he said.

"About half the people I've talked to say, 'Yeah, that's proof the Earth is young,' " Morgan said.

Andrew Snelling, who has a doctorate in geology and is a content editor for Answers magazine, said two items of evidence at Paria Canyon point to a massive flood event.

One concerns analysis of grinds in the sandstone at Paria, which match mineral sources in the Appalachians. It would take a lot of wave action to move sand that far, he said.

And the sheer size of the Paria sandstone cross beds, at more than 100 feet thick, would need a massive wave -- a tsunami-sized wall of water -- in order to be laid down as they are. Snelling said such a wave would have to be twice as high as the bed of sand it deposits, which is on the scale of a global flood.

"The volume of sand we see out there is enormous. We don't see anything like that happening today," Snelling noted.

Mr. Trumbo, you are welcome to get in touch with me – or any other professional geologist - should the bizarre notion of adding a counter-argument disturb your intellectual complacency.

Playing in the sand box – continuing connections

 
What do all these images have in common? Well, immediately, it’s quite obvious: they are all 3D models of landscapes. But the thinking, the execution, and the technology behind them is fascinatingly different. And yet all are enticing in some way or another – the still images are not sufficient, and the viewer yearns to touch, to interact, to play with them.

I will readily confess to a couple of fascinations: three-dimensional models of how our planet works, and connections, those odd intersections between the seemingly different. I wrote a little about the first fascination in celebrating the early personal influence of the models such as the one in the centre, above, for an Accretionary Wedge event a couple of years ago with the theme Inspiration - the beginnings of a geologist.

...the book which I now appreciate fired my fascination with geology was The Earth’s Crust—A New Approach to Physical Geography and Geology. Published in 1951, it was called “a new approach” because Dudley Stamp commissioned huge, detailed models of different landscapes—and what lies beneath them—to be made in exquisite detail by Tom Bayley, a lecturer in sculpture, and photographed in colour for the book.

Fascination number one in many respects lay behind my previous post on the brilliant linkage between sand box modelling and digital technology, Simtable (bottom left, above). I was delighted that Simtable’s inventor, Stephen Guerin, immediately commented, and included a link to Solid Terrain Modeling, who print high quality graphical data on to 3D landscape models (top left, above). All of this started to stimulate fascination number two, and I found myself visualising some the wonderfully dynamic stream table modelling capabilities that Steve Gough’s Little River Research and Design produces (top right, above). I enjoy following Steve’s activities, developments and applications (entertainingly chronicled on his blog), and I began to wonder if there wasn’t some interesting intersection potential between the Simtable technology and his. I dropped Steve an email and, in spite of being caught up in the challenges of exhibiting – in the end, successfully (see his blog) - at the AGU conference in San Francisco, he replied immediately and commented on the blog post. He provided a link to a description of the SandyStation, an invention by two students at the University of West Bohemia in Pilsen in the Czech Republic, that cleverly combines a physical sand box and the digital technology of the Xbox Kinect (lower right, above).

And then, in browsing around for images, I happened upon this description of what seems like essentially the same conspiracy of sand and the Xbox, but invented by game developers in the Netherlands:

In a refreshingly different take, the world of the game Mimicry is the “ultimate sandbox game” – set in a literal sandbox. Participants manipulate piles of real sand, as Kinect-powered cameras track their work and project imagery onto the sand from a rendered analog version of the same world. The player mimics the virtual, the virtual mimics the player, and the stuff of each fuse in a real/virtual hybrid in sand.

I’m a little confused by this, but that’s not my point. It simply seems to me that here are multiple possibilities for connections, for educational and other applications that might generate examples of the whole being greater than the sum of the parts. And one thing that’s compelling about much of the imagery from all of these activities? They all feature interaction, the satisfaction of being able to touch – and to play.

[Images from Simtable, Solid Terrain Modeling, SandyStation, and Little River Research and Design]