Sound Waves Make Filters Finer

Generally, filters that remove particulates from fluids are limited by their pore sizes. That is, a filter with millimeter-sized pores isn't likely to catch many micron-sized particles. On the other hand, a filter with tiny pores can trap small particles at the expense of inhibiting fluid flow.

Donald Feke (Case Western Reserve University, dlf4@po.cwru.edu, 216-368-2750), however, has found a way to reduce the effective pore size in highly porous media without significantly hindering fluid flow. By applying a low power acoustic signal to a filter, Feke can trap particles as much as a hundred times smaller than the nominal filter pore size. An acoustically aided filter provides relatively little resistance to the fluid that passes through it, and yet collects particles as efficiently as a much finer filter. And once the filter has done its job, the trapped particles can be released at the flip of a switch that cuts off the acoustical signal (see figure).

The trapping arises because acoustic signals traveling through a porous material create patterns of standing waves that focus particulate matter toward certain positions on the walls of the pores. Rather than wending their way through the filter, particles headed for the focal points line up to form intricate, stable filaments. In other locations, groups of particles collect in regions of stability within the pores, where they orbit for as long as the signal persists.

In addition to novel filter designs, Feke proposes that acoustic manipulation may lead to efficient material sorting technologies or methods that aid in assembling microscopic structures. Feke presented his work at the 73rd Annual Society of Rheology meeting in Bethesda, Maryland.