Continuous Focusing of Microparticles in Horizontally Actuated Rectangular Channels

We present a device to continuously focus microparticles in a liquid-filled open channel subjected to lateral vibration at frequencies of the order of 10 Hz. The vibration generates a capillary wave at the liquid-air interface. This capillary wave leads to a net motion of microparticles over multiple cycles, causing collection under the displacement nodes of the capillary wave. These accumulated particles are observed as a concentrated stream in the presence of a continuous flow along the open channel, which means that the channel can be designed such that the focused particle stream exits through one outlet, while the filtrate is removed via interspaced outlets on each side of the particle stream. A numerical model is proposed, which superimposes the periodic flow field due to the capillary wave and the inlet-induced transverse flow field between the inlet and the outlet. The model is utilized to predict the smallest distance from the inlet at which the focused stream of particles is obtained, termed here the collection length. In addition, experiments are performed for different channel and inlet sizes, vibration actuation amplitudes, flow rates, and particle sizes. By considering the design factors extracted from the modeling data, we demonstrate that the resulting device is capable of continuous particle collection down to 1 μm diameter, at flow rates of up to 1.2 ml/min.