Sound waves make filters finer

Generally, the performance of filters that remove particulates from fluids is limited by their pore sizes: A filter with large pores isn’t likely to catch many tiny particles. By contrast, a filter with tiny pores will trap small particles but inhibit fluid flow. Now, Donald Feke (Case Western Reserve University) has trapped particles up to a hundred times smaller than the nominal pore size by applying a low-power acoustic signal to the filter. The sound field within a porous material creates patterns of standing waves associated with the pores. Rather than wending their way through the filter, particles headed for the focal points either form intricate, stable filaments or gather into groups that orbit in regions of stability for as long as the signal persists. Such an acoustically aided filter offers little resistance to the fluid that flows through it, yet collects particles as efficiently as a much finer filter does. And once the filter has done its job, the trapped particles can be released with the flip of a switch that cuts off the signal. Feke presented his work last October at the 73rd annual Society of Rheology meeting in Bethesda, Maryland. (http://www.rheology.org/sor01a/abstract.asp?PaperID=157 )