Particle Separation inside a Sessile Droplet with Variable Contact Angle Using Surface Acoustic Waves

Abstract

A sessile droplet of water carrying polystyrene microparticles of different diameters was uniformly exposed to high frequency surface acoustic waves (SAWs) produced by an interdigitated transducer (IDT). We investigated the concentration behavior of the microparticles as the SAWs generated a strong acoustic streaming flow (ASF) inside the water droplet and exerted a direct acoustic radiation force (ARF) on the suspended particles, the magnitude of which depended upon the particle diameter. As a result of the ARF, the microparticles were concentrated according to their diameters at different positions inside the sessile droplet placed in the path of the SAW, right in front of the IDT. The microparticle concentration behavior changed as the sessile droplet contact angle with the substrate was varied by adding surfactant to the water or by gradually evaporating the water. The positions at which the smaller and larger microparticles were concentrated remained distinguishable, even at very different experimental conditions. The long-term exposure of the droplets to the SAWs was accompanied by the gradual evaporation of the carrier fluid, which dynamically changed the droplet contact angle as well as the concentration of particles. Complete evaporation of the fluid left behind several concentrated yet separated clusters of particles on the substrate surface. The effect of the droplet contact angle on particles' concentration behavior and consequent separation of particles has been uniquely studied in this SAW-based report.

Volume and Frequency-Independent Spreading of Droplets Driven by Ultrasonic Surface Vibration

“…Manipulating droplet wetting can be useful for numerous tasks such as transporting or separating solid objects [1][2][3][4]. Most coating and bonding processes also depend on wetting conditions.…”

Additive manufacturing of three-dimensional (3D) microfluidic-based microelectromechanical systems (MEMS) for acoustofluidic applications

“…for droplet-based mixing and particle trapping) have received significant attention. 35, 38, 47, 48 Thus, as shown in Figure 4a, we examined the potential to manipulate suspended particles in droplet-based systems using the in-plane transducer to verify the acoustic coupling of the robotically embedded PZT materials with surrounding fluids. It is well established that propagating acoustic waves subject suspended particles to acoustic radiation and streaming forces that can result in particle trapping or dynamic mixing.…”

“…We note that frequency-dependent dynamic particle separation profiles in droplets have also been observed using SAW devices at frequencies ranging from 10 to 130 MHz. 38, 47…”

On‐Chip Generation of Vortical Flows for Microfluidic Centrifugation

“…To generate an azimuthal microfluidic centrifugation flow either in a liquid drop or chamber atop the piezoelectric substrate ( Figure a), on the other hand, it is necessary to break the symmetry of the planar SAW wave. This can be achieved, for example, by offsetting the interdigitated transducer(s) used to generate the SAW(s) with respect to the position of the drop, by introducing an asymmetric cut to the SAW chip such that the reflection of the SAW at its edges varies transversely across the drop, or, by absorbing part of the SAW prior to its reflection, as illustrated in Figure b . Given that the flow is driven primarily as a consequence of the attenuation of the sound wave in the liquid, the vortex dimension is naturally inversely proportional to the frequency.…”