Sound waves meet a droplet of liquid

Sound waves in the form of surface acoustic waves are utilized to manipulate a sessile droplet of liquid and suspended micro-objects.

Article  |  Spring 2017

Microfluidics is an emerging technology that deals with liquid flows on the microscale. The efficient actuation of liquid and the manipulation of microscale objects are fundamental to the field of microfluidics. Among a variety of methods to manipulate liquid droplets and suspended micro-objects, the acoustofluidic method based on sound waves has attracted much attention since it is energy-efficient, label-free, contactless, and versatile.

Recently, the acoustofluidic actuation of liquid using surface acoustic waves (SAWs) has been used in various biological assays such as diagnostics, nebulization, and drug delivery. SAWs are sound waves that propagate only along the surface of piezoelectric substrates. This acoustofluidic approach is particularly advantageous for the efficient manipulation of micro-objects (cells or particles) suspended in the liquid using the SAW-induced acoustic streaming flow (ASF) and the acoustic radiation force (ARF). The liquid droplet can be actuated (mixed, nebulized, translocated, merged, etc.) by using SAWs, and suspended micro-objects can be manipulated on demand.

Prof. Hyung Jin Sung’s group (Mechanical Engineering, KAIST) has succeeded in investigating the underlying physical mechanisms of the motion of polystyrene microparticles inside a sessile droplet of water actuated by SAWs, which produce an ASF and impart an ARF on the particles. As a result of extensive experiments with in-depth theoretical study, Prof. Sung’s group categorized four distinct regimes (R1–R4) of particle aggregation that depend on the particle diameter, the SAW frequency, the acoustic wave field (travelling or standing), the acoustic waves’ attenuation length, and the droplet volume, respectively. The particles are respectively concentrated at the center of the droplet in the form of a bead (R1), around the periphery of the droplet in the form of a ring (R2), at the side of the droplet in the form of an isolated island (R3), and close to the center of the droplet in the form of a smaller ring (R4). The ASF-based drag force, the travelling or standing SAW-based ARF, and the centrifugal force are utilized in various combinations to produce these distinct regimes.

An article on this research entitled “Acoustofluidic particle manipulation inside a sessile droplet: Four distinct regimes of particle concentration” was published on February 21^st, 2016 in Lab on a Chip (front cover), DOI: 10.1039/c5lc01104c.