Acoustic Control of Directed Assembly in Microfluidic Devices

A combined top-down/bottom-up microfluidic assembly system is demonstrated in which microspheres undergo directed assembly within microchannels. The approach is adapted from macroscale templated assembly by selective removal (TASR), in which high-frequency acoustic excitations drive selective assembly into shape- and size-matched wells patterned in a planar surface. A PDMS layer patterned with microchannels is bonded to a PDMS layer molded to define assembly wells. A suspension of 10 $\mu \text{m}$ diameter polystyrene microspheres is flowed into the microchannels, and an external, high-frequency ultrasonic transducer circulates the microspheres within the channels and removes them from everywhere except the size-matched wells. Microspheres are successfully assembled in wells inside the microchannels with yields of up to 90%. Smaller channel dimensions reduce assembly yield in part because smaller internal channel volumes reduce the availability of microspheres relative to assembly wells. In addition, reducing channel width below about $500~\mu \text{m}$ is shown to be an independent predictor of low assembly yields; this is attributed to the physics of acoustic streaming flow under the acoustic excitation. Assembly yield is largely independent of the location of the assembly wells within the microchannels, and selectivity is confirmed by low assembly yield of $6~\mu \text{m}$ diameter microspheres into the larger wells. [2020-0072]