Created by: Alexis Lunberg
Have you ever been walking along the beach as the tide was going out? When you step onto the wet sand, the sand around your foot turns white and appears to be dry. This phenomenon is usually explained that the whitening is caused from the water getting squeezed out by your weight. This is one of the many misconceptions that we have about physics, because sand does not act like a sponge like this explanation conludes. So, if this is not the explanation then what is?
The answer to this question is explained by Osborne Reynolds findings on the dilatancy of granular materials. He "explained the whitening of the beach sand in 1885 when he pointed out that the sand expanded when stepped on" (Aste & Weaire, 2000). Before you step on the sand, the sand grains are closely packed and the shearing from your footsteps results "in less efficient packing" (Aste & Weaire, 2000). Basically the sand was forced to occupy more volume and the sand level rose. The water in the sand can only change through capillary action. Capillary action takes more time than it does for the sand to occupy more volume, so the sand appears white and dry for a short period of time.
According to Warren Mead dilatancy is "the expansion of granular masses when deformed due to the rearrangement of the grains" (Mead, 1925). Spherical grains shaken in a container usually arrange "in a condition of maximum-density packing" (Mead, 1925). When the grain is like this it is impossible to change the shape without increasing the volume. Reynolds considered that he close packing of particles in a liquid medium was altered by an applied force so that the volume was increased and more liquid was drawn from periphery in to the dilatant mixture (Chapman).
"When granular material, such as sand or rice, is poured into a jar its density is relatively low and it flows rather like an ordinary fluid" (Aste & Weaire, 2000) If the granular material has water added to it and were shaken in the jar the person shaking it may be surprised to see that the granular material does not become less dense by occupying more volume in the jar, but it becomes more compact/dense. This has been known by vendors for a long time, especially when product was sold by measure. Of course, the vendors tried to give their costumer as little as possible while it still looked like they were giving them a fair measure. Vendors would often shake a material so that the volume of the measure what right but it wasn't as dense as if they would have just poured the measure in. For corn and rice vendors soon found out that you don't shake these products to make them less dense, because the more you shook them the more dense they became, like sand does.
This can all tie back to Reynolds findings on the footprint in the sand. Since sand is a granular material it acts in the same way as the corn and the rice, as stated above. The pressure of shaking (or pressure from the tide coming in and out) on these materials allows the material to compact if it is in a confined volume, and on a beach the wet sand is in a confined space, because it is at a maximum-density from the packing of the water and gravity. When sand is wet it becomes firm because all of the spaces in the sand are filled with the liquid. The sand isn't like a sponge that absorbs the water, but the water just sits in the spaces in-between the sand. When you step onto the sand it is forced to move creating a larger volume, but the water in the sand can not move so easily because it needs to move through capillary action. So the period of time between the sand being forced to move into a large volume and the water moving through capillary action is where the sand appears to be white and dry.
The property of granular masses of expanding in bulk with change of shape. It is due to the increase of space between the individually rigid particles as they change their relative positions.
Putting a strain in the structure of a substance produced by pressure, when its layers are laterally shifted in relation to each other
The property of a liquid to move into small spaces if it has the ability to wet these surfaces
Aste, T., & Weaire, D. L. (2000). The pursuit of perfect packing. 3.5 Osborne Reynolds: a footprint in the sand. Bristol: Institute of Physics Pub..
This chapter in the book explains Osborne Reynolds findings on granular materials. One of the main points is the explanation of why vendors didn't shake granular materials when measuring, because it makes them more dense. This chapter helped me to further explain why footprints in the sand act the way they do in my article. It also gave me other examples of granular materials besides sand that people would be able to relate to better. I trust this source because it is in a published book that is explaining several questions about physics.
Chapman, G. (n.d.) The Thixotropy and Dilatancy of a Marine Soil. The Department of Zoology, 123-140. (October 26, 2011.) Retrieved from http://sabella.mba.ac.uk/1347/01/The_thixotropy_and_dilatancy_of_a_marine_soil.pdf.
This article is discussing the studies done on the thixotropy and dilatancy of marine soils such as sand, clay, and silt. A majority of the article wasn't related to my topic, but there were a few parts that were relevant to the question I chose. The parts that I used helped me to further explain Osborne Reynolds findings on the property of dilatancy. I felt that this source was reliable because it was from the Department of Zoology at Queen Mary College in London. If the college is publishing the work then it must have some type of reliability. Also the article is supported by several resources that are listed.
Mead, W. J. (1925). The Geologic Role of Dilatancy. The Journal of Geology, 33(7), 685-698. Retrieved from EBSCOhost.
This article explained what dilatancy is and what role it plays. It was very helpful to me, because I wasn't sure what dilatancy was and that was the main concept of the answer to my question. I believe this to be a reliable source because it was published in The Journal of Geology. While this article is very old, the information that it contains was supported by the other sources that I found.
