October 28, 2011
Building Granular Towers Drop By Drop And How To See Out From Under An Invisibility Cloak
An unprecedented variety of smooth symmetric, corrugated, zig-zag shaped slender structures can be observed by simply dripping a mixture of sand and water on a liquid absorbing surface such as a dry bed of sand or blotting paper. The various shapes are in contrast with the liquid drops which can splash, spread or bounce upon hitting a surface. Successive drops freeze rapidly upon impact due to the drainage of a small fraction of liquid, literally stacking on top of each other into surprisingly slender structures know as granular towers. In addition, twisted pagoda dome-like structures result upon increasing the flow rate of the damp granular mixture. Experiments show that the towers are held together because of capillary and friction forces, and the shape of the towers depends on a subtle balance between dripping frequency, density of grains, and impact speed. Besides applications in surface patterning, this tower building technique may be a new and easy way to probe the flow properties of dense granular suspensions by observing the shapes of the towers they produce.