Erin R. Dauson

Reflections and standing waves for particle concentration in microfluidic channels

Standing waves in a microfluidic channel can be applied for the concentration and separation of particles and biological cells. In this paper we examine the formation of surface standing waves from pairs of interdigitated transducers with 100 μm and 200 μm periodicity on a lithium niobate substrate 500 μm thick. A 200 μm open-circuited transducer is a weak reflector while reflections from the 100 μm transducer are much stronger. Finite element simulations have been used to understand the nature of the waves formed when the substrate thickness is of the order of the wavelength. With both transducers driven, concentration of 6 μm particles has been observed and the time required to align has been measured as a function of drive voltage. We also discuss the impact of various channel materials on the electrical terminal characteristics.

Microparticle transport and concentration with surface acoustic waves

We describe lithium niobate SAW devices and PDMS microfluidic channels with which we study microparticle movement. We generate standing surface acoustic waves (with wavelengths of 200 micrometers) and show that microparticles (between 5 and 35 micrometers in diameter) move to nodes or antinodes. We report measurements of device response in the presence and absence of the microfluidic channel, which we combine with finite element simulation modeling to extract estimates of the PDMS damping.

Mechanically robust microfluidics and bulk wave acoustics to sort microparticles

Sorting microparticles (or cells, or bacteria) is significant for scientific, medical and industrial purposes. Research groups have used lithium niobate SAW devices to produce standing waves, and then to align microparticles at the node lines in polydimethylsiloxane (PDMS, silicone) microfluidic channels. The “tilted angle” (skewed) configuration is a recent breakthrough producing particle trajectories that cross multiple node lines, making it practical to sort particles. However, lithium niobate wafers and PDMS microfluidic channels are not mechanically robust. We demonstrate “tilted angle” microparticle sorting in novel devices that are robust, rapidly prototyped, and manufacturable. We form our microfluidic system in a rigid polymethyl methacrylate (PMMA, acrylic) prism, sandwiched by lead-zirconium-titanate (PZT) wafers, operating in through-thickness mode with inertial backing, that produce standing bulk waves. The overall configuration is compact and mechanically robust, and actuating PZT wafers in through-thickness mode is highly efficient. Moving to this novel configuration introduced new acoustics questions involving internal reflections, but we show experimental images confirming the intended nodal geometry. Microparticles in “tilted angle” devices display undulating trajectories, where deviation from the straight path increases with particle diameter and with excitation voltage to create the mechanism by which particles are sorted. We show a simplified analytical model by which a “phase space” is constructed to characterize effective particle sorting, and we compare our experimental data to the predictions from that simplified model; precise correlation is not expected and is not observed, but the important physical trends from the model are paralleled in the measured particle trajectories.

Particle separation using bulk acoustic waves in a tilted angle microfluidic channel

In prior research, separation of particles or biological cells in liquids has been accomplished using a balance of ultrasonic forces and viscous drag forces, which depend on particle size and acoustic contrast. Recent work by two groups has exploited surface acoustic waves launched into tilted microfluidic channels for particle or cell separation. Here we demonstrate a similar tilted-channel approach using machined channels in PMMA (poly (methyl methacrylate)) and PZT (lead zirconate titanate) wafer excitation. Experimental observations confirm the separation of particles. Simulations have been used to understand the observed particle motion and to identify operating parameters for particle separation.

Microparticle separation using a PMMA channel at an oblique angle to a SAW field

In systems using two opposing transducers to create standing waves in a microfluidic channel with flow, the acoustic force on microparticles in the channel is proportional to the volume of the particles, while the drag force is proportional to the radius of the microparticles. Consequently, larger particles migrate towards acoustic nodes faster than smaller particles. We have investigated particles flowing in channels at an oblique angle relative to the SAW field. By applying the drag force and the acoustic force to the particles at an angle, depending on the water velocity, larger particles migrate towards one side of the channel faster than smaller particles, enabling particle separation in a continuous flow system without dilution from sheath flow.

Surface acoustic wave action on microfluidic channels and microparticles

We describe lithium niobate SAW devices and their wave structure at different resonant frequencies, and we discuss the difference between PDMS and PMMA as the material for the microfluidic channel. We discuss the different wave structure for SAW devices operating at different resonant frequencies, showing simulation results and laboratory measurements. We discuss our recent studies to sort microparticles by size.

Ultrasonic alignment of microparticles in nozzle-like geometries

Additive manufacturing (3-D printing) is presently limited by the mechanical properties of the materials, such as polymer resins, that are otherwise efficient and economical for part-forming. Reinforcing the resin with microscale fibers and/or particles would be an effective mechanism to achieve desired mechanical properties such as strength and ductility. Our work combines standing wave ultrasonics and microfluidics to align microparticles in devices that can act as print nozzles, based in part on our prior work with cell sorting. In this paper three different approaches are presented illustrating different engineering tradeoffs, and demonstrating laboratory results of particle alignment. First acoustic resonators are discussed, in which the ultrasonic standing waves result mostly from the mechanical properties of the microfluidic structure, excited by a piezoceramic transducer. Next non-resonant microfluidic structures are discussed, in which ultrasonic standing waves are produced directly by symmetrical transducer deployment. Finally, devices that combine nozzle-like structures, which themselves are suitable acoustic resonators, subjected to symmetrical ultrasonic excitation are presented. We will show that all three configurations will align microparticles, and discuss the tradeoffs among them for subsequent configuration of a print nozzle.