(262f) SAW Induced Microfluidic Droplet Manipulation for Lab-On-a-Chip Systems

Abstract: Liquid droplet based microfluidic systems find extensive applicability in biological applications, such as carriers to target drug delivery to the intended diseased tissue, encapsulation agents for various biological entities, micro-reactors for rapid mixing and reaction of reagents, liquid reaction vessels for protein crystallization, biosensing, DNA analysis, sampling glucose concentrations in bodily fluids, to name a few. In droplet based microfluidics, discrete droplets are created via the use of two immiscible phases in microchannels. All applications require a great control on droplet size, shape, and distribution; mondispersed droplets are typically required which can be used as micro-reactors and for achieving hands-free contact of reagents in biological applications such as in Lab-On-a-Chip systems. Surface acoustic wave (SAW) devices, with hydrophobically modified surfaces, can be utilized for efficient microfluidic control and generation as well as actuation of droplets. In this work, we investigate droplet actuation using SAW devices as well as the acoustic streaming velocity fields generated in micro-liter size droplets as a result of interaction with SAW devices. A SAW device based, on YZ Lithium Niobate, in contact with a micro-liter sized droplet is modeled using a three dimensional bidirectionally coupled fluid-structure interaction model. The structure is simulated for a total of 100 nanoseconds (ns), with a time step of 1 ns. The excitation of the piezoelectric solid is provided by applying an AC voltage on the transmitter IDT fingers. The Rayleigh wave generated in the SAW device impinges on the fluid medium and imparts momentum generating fluid motion known as acoustic streaming. Our simulations indicate that when a droplet is placed on the SAW device, this acoustic streaming can actuate the droplet. Simulation derived fluid velocity profiles inside the droplet exhibit recirculation, thereby indicating an enhancement in mixing and agitation in droplets due to acoustic streaming, and thus underscoring their utility as micro-reactors. The influence of various parameters, namely fluid viscosity, applied voltage, and droplet size on the droplet actuation and acoustic streaming is studied using the developed fluid-structure finite element model. The role of acoustic wave focusing via the use of focused interdigital transducers, on droplet actuation as well as fluid velocity profiles within the droplet, is investigated as well. Results will be presented in detail.