Coalescence of Surfactant-Stabilized Adjacent Droplets Using Surface Acoustic Waves

Abstract

A novel, on-demand microuidic droplet merging mechanism is presented in this paper. We demonstrate that a narrow beam surface acoustic wave (SAW), targeted at the oil buer, causes nearby surfactant-stabilised droplets to coalesce. The lack of direct exposure of the droplet to the excitation stimulus makes this method ideal for sensitive samples as harm will not occur. This powerful technique works on a straight channel with no special design, is not aected by surfactant concentration and droplet volume hence promises seamless integration into existing microuidic systems. It oers highthroughput, biologically safe, on-demand droplet merging for applications ranging from fast reaction kinetics to microuidic high throughput screening (µHTS). We thoroughly characterise the physical mechanism triggering droplet-droplet coalescence and observe a cut-o distance from the centre of the acoustic beam to the droplet-droplet interface after which the merging mechanism does not work any more. We establish that the most likely mechanism for merging is acoustic streaming induced droplet deformation.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

“…D) Acoustic‐based on‐demand splitting and steering of droplets. Reproduced with permission . Copyright 2018, Royal Society of Chemistry.…”

“…Acoustofluidics is capable to revolute the microfluids and can overcome the difficulties in long‐standing droplets manipulation by microfluidics. Neild and co‐workers have reported an on‐demand microfluidic droplet merging mechanism and successfully realized the coalescence of adjacent droplets by surface acoustic waves (Figure D) . The process was done in a straight channel without any special treatments, and provides plenty of advantages, such as high‐throughput, biologically compatibility, on‐demand droplet merging.…”

“…[77] In this work, they used a device which consists of two T-shaped junctions and a Y-junction. These two parts were used to form droplets and to mix the Small 2020, 16,1903940 Creation, mixing, incubation, sorting An integrated droplet-digital microfluidic system [103] Sorting Superhydrophobic interdigitated array chips [104] Floating Movable electrode biased with a high voltage [105] Mixing Open and closed digital microfluidic platforms [106] Coalescence, migrating Polypyrrole whelk-like arrays [107] Generation, transportation, splitting Ionic-surfactant-mediated electro-dewetting digital microfluidics [108] Magnetic Sorting Magnetophoretic sorting system [101] Splitting, dispensing, exchange, trapping, release, demulsification An integrated magnetic repulsion-actuated microfluidic system [111] Mixing, migrating, transport, release, coalescence Ferrofluid-containing liquid-infused porous surfaces [113] Transport, sorting, mixing A magnetic digital microfluidics platform [114] Shape evolution, splitting A device with hydrophobic surface under the magnetic field [115] Transport, splitting, motion Magnetic tubular microactuators [116] Motion, transport, fusion A deformable paramagnetic liquid substrate [117] Acoustic Coalescence A straight channel with a narrow beam surface acoustic wave [119] Splitting, steering A disposable parallel-type SAW-based acoustofluidic device [120] Gravity Transport, collision, fusion, mixing, stopping A gravity-actuated droplet microfluidics device [67] Movement, mixing Polydopamine microfluidic system [121] Trapping, exchange, transfer, fusion A microstructures device possesses microwell arrays and straight microchannel [122] Optical Motion A device with a single focused laser [124] Motion, patterned writing A device with photoelectric cooperative-responsive slippery surface [125] Sorting, dispensing Printed droplet microfluidics [126] Transportation, merging, splitting Light-driven flexible opto-electrowetting devices [127] Mixing, motion, coalescence A pyroelectrotrapping superhydrophobic surface platform [128] Mechanical Mixing, movement, Mechanical-activated digital microfluidics…”

“…Reproduced with permission. [119] Copyright 2018, Royal Society of Chemistry. E) Fluorescence-activated droplet manipulation in printed droplet microfluidics.…”

“…Neild and co-workers have reported an on-demand microfluidic droplet merging mechanism and successfully realized the coalescence of adjacent droplets by surface acoustic waves ( Figure 5D). [119] The process was done in a straight channel without any special treatments, and provides plenty of advantages, such as high-throughput, biologically compatibility, on-demand droplet merging. Sung and his colleagues reported the on-demand splitting and steering of droplets by a parallel-type, disposable microfluidic chip.…”