Continuous Focusing of Microparticles in Horizontally Actuated Rectangular Channels

Abstract

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.

Capillary-Wave Sieve: Continuous Particle Separation Using Millimeter-Scale Capillary Waves

“…A syringe pump (NE-1000, New Era Pump Systems) is connected to the channel's inlet and outlet ports to simultaneously infuse and extract particle suspensions. If an equal input and outlet flow is applied then the volume of the open liquid remains stable, excluding the evaporation rate 54,55 . Before starting the flow, the inlet and outlet ports are flooded with DI water and the channel is pre-filled with 200 µL of DI water, to avoid bubbles in any of the liquid connections 54 .…”

“…If an equal input and outlet flow is applied then the volume of the open liquid remains stable, excluding the evaporation rate 54,55 . Before starting the flow, the inlet and outlet ports are flooded with DI water and the channel is pre-filled with 200 µL of DI water, to avoid bubbles in any of the liquid connections 54 . In these experiments we did not measure the vibration amplitude of the ater-air interface because across this entire length, the curvature of the interface in each cross-section will change under a continuous flow due to the imposed pressure differential between the inlet and the outlet 54 .…”

“…Before starting the flow, the inlet and outlet ports are flooded with DI water and the channel is pre-filled with 200 µL of DI water, to avoid bubbles in any of the liquid connections 54 . In these experiments we did not measure the vibration amplitude of the ater-air interface because across this entire length, the curvature of the interface in each cross-section will change under a continuous flow due to the imposed pressure differential between the inlet and the outlet 54 . As a result, visualising a consistent displacement of the interface across the entire interface is challenging.…”

“…where, a represents the time-averaged value of the variable a, the subscripts represent the order of the respective pressure (p) or velocity (u) field. The flow field ensuing from these equations is depicted in Figure 2 (d) and is obtained from the simulation procedure by Agrawal et al 54 . Although the viscous drag force in x-direction due to these streaming fields enhances particle migration towards the collection positions, the drag force in z-direction can overcome the particle's weight and lift it away from the collection location such that the particle remains swirling in the liquid bulk (Figure 2 (d)).…”

“…The channel is vibrated at a frequency of 10 Hz, which corresponds to one capillary wavelength, and at an actuation amplitude of 225 µm. The actuation amplitude is chosen such that the polystyrene particles remain dispersed in the bulk while the silica particles collect stably 54 . As the particles enter the channel, the silica particles migrate towards the center of the channel to collect underneath the node of the capillary wave, whilst traversing in the y-direction 54 .…”