Five glass columns (2.5 cm i.d., 30 cm bed depth, PTFE end
fittings, polyethylene bed supports with 20 mm pore size,
Kontes, Vineland, NJ) were filled with dry sand (30e40 mesh,
washed, BDH, VWR, West Chester, PA). Two centimeter-layers
of sand were tamped 25 times before the next sand layer was
added.