Tailoring the frictional properties of granular media
  A method of modifying the roughness of soda-lime glass spheres is presented, with the purpose of tuning inter-particle friction. The effect of chemical etching on the surface topography and the bulk frictional properties of grains is systematically investigated. The surface roughness of the grains is measured using white light interferometry and characterised by the lateral and vertical roughness length scales. The underwater angle of repose is measured to characterise the bulk frictional behaviour. We observe that the co-efficient of friction depends on the vertical roughness length scale. We also demonstrate a bulk surface roughness measurement using a carbonated soft drink. 
Sonia Utermann, Philipp Aurin, Markus Benderoth, Cornelius Fischer and   Matthias Schr\"oter
cond-mat.stat-mech cond-mat.soft