The Wet-Sand Effect

When wet sand along the water's edge at the beach is stepped on, it becomes partially dry in a small area surrounding one's foot. The dry patch is evident as the thin reflective film of water at the surface surrounding your foot is no longer there and has instead been drawn into the bulk of the sand beneath your foot.

This drying of the sand underfoot is referred to as the Wet-Sand Effect. The effect results from the dilatancy of the media. Here the volume of the sand increases. Shearing caused beneath each foot step causes the sand to move ever so slightly from a highly compact state
to one where the amount of spacing between adjacent grains of sand has
increased. Water from the surface flows under gravity into the newly created space and leads to the momentarily drying in the sand with each step.

The dilatancy effect of sand under an applied compressive force can be readily reproduced using a sealed plastic bottle filled with water
soaked sand. A small water filled tube with one end buried into the bottle is used to monitor for any water displacement occurring in the sand. On squeezing the bottle hard, one intuitively expects the level
of water in the tube to rise. To the surprise of many, the level of water actually drops and the harder one squeezes, the further the drop!

Category:

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Tags:

• wet
• sand
• effect
• dilatancy
• wet-sand
• Reynolds
• physics
• experiments
• demonstrations
• beach

[PaulNaoki]

ugly left foot

[pod0boq]

Man, whoever discovered this phenomenon was feeling original when they named it.

[Tartrus1]

@TheReasonWhyGuy Except that isn't what happens... as you see in the water bottle test. The effect of squeezing the sand allows more water to flow into the bottle.

[TheZse45678]

are you an ice skater

[habbobaby1]

Ein Fußabdruck in feuchtem Sand ist trocken, weil der Druck des Fußes die Sandkörner verschiebt und dadurch das Volumen vergrößert wird. In den nun vergrößerten Porenraum dringt das Wasser ein, wodurch die Oberfläche trocken erscheint.

[DravenX53]

@EngyElms Not quite. Watch the video again; squeezing the bottle forced water INTO the sand, not out of it. That's the counter-intuitive part here, it's like the opposite of squeezing a sponge.

[epSosDE]

Sand increases surface and empty spaces under pressure.

[EngyElms]

Its just consolidation!!!

Congratz, your weight forced the water outta the sand!

Now go squeeze a wet sponge :D

[95lillen95]

dude, this is freaking hardcore sciens... o.O

[VarykGerai]

@TheReasonWhyGuy no! look at the bottle, when he squeezes it water isnt pushed out, water is sucked in. Read the freakin description.

[sharpshard]

I'm surprised that cohesion between particles didn't come up. The water makes the sand more cohesive (a.k.a sticky) than just normal dry sand. The larger of the cohesion, the bigger the shear stress between adjacent particles. This is where dilatancy comes from for wet sand (and also why wet sand can be characterised as a dilatant fluid). There's a critical sand-to-water fraction for maximum cohesion.

[mrflyer107]

@PaulNaoki poor him