Liquid marble coalescence via vertical collision

Abstract

The coalescence process of liquid marbles is vital to their promising roles as reactors or mixers in digital microfluidics. However, the underlying mechanisms and critical conditions of liquid marble coalescence are not well understood. This paper studies the coalescence process of two equally-sized liquid marbles via vertical collision aided by dielectrophoretic handling. A liquid marble was picked up using the dielectrophoretic force and then dropped vertically onto another liquid marble resting on a hydrophobic powder bed. The whole collision process was recorded by a high-speed camera and the recorded images were then analysed to derive the generalised conditions of liquid marble coalescence. By varying the marble volume, impact velocity and offset ratio in the experiments, we concluded that liquid marble coalescence may occur through the coating pore opening mechanism. We quantitatively measured the radius change versus time of the liquid neck formed between two coalescing marbles and estimated the maximum deformation of impacting marbles before rupture in rebound cases. We also qualitatively described the redistribution of coating particles at the impact area during coalescence as well as the consequent ejection of particles. Finally, we summarised the critical conditions for liquid marble coalescence, providing a frame for future applications involving liquid marbles as micromixers and microreactors.

Mapping outcomes of liquid marble collisions

“…That said, with care, some interesting observations can be made. Firstly, in both of the studies by Planchette et al 17 and Jin et al, 18 it was noticed that smaller LMs are harder to coalesce than larger LMs. This was also noted in our study.…”

“…The offset ratio, X*, is a dimensionless description of the amount of overlap that two colliding LMs have at impact (succinctly described by Jin et al 18 ). It is defined by X* = x/D; where x is the distance between the centres of the two LMs, perpendicular to the direction of motion, and D is the diameter of the LM.…”

“…15,16 The impact resulting from the vertical collision of two LMs has been reported on by Planchette et al, 17 and more recently by Jin et al in 2018. 18 In both cases, a LM was dropped from a known height onto a stationary LM. These impacts were monitored by high-speed camera, using individual frames to determine speed and impact behaviour.…”

“…The velocity required to initiate coalescence has been reported to vary from 0.29 m s À1 to 0.68 m s À1 . [17][18][19] Magnetic fields have proven to be a popular way to 'open' and 'close' LMs, but they have also been used to initiate coalescence. The magnetic properties of a LM can be enhanced by the use of either a magnetic coating, 20 or by inclusion of magnetic nanopowders suspended in the liquid core.…”

Influence of water evaporation/absorption on the stability of glycerol–water marbles

“…19,22 When two liquid marbles are placed in contact, coalescence may occur by different coalescence techniques such as impact, magnetic force, vertical collision and direct-current voltage. [23][24][25][26][27] Given the exibility in the choice of solid particles and liquids to fabricate liquid marbles, liquid marbles are emerging as a versatile platform for a wide range of applications in miniature reactors, microuidics, drug transport, gas sensing, and biomedical and genetic analyses. [2][3][4][5][6] As a versatile and cost-effective miniature reactor providing a well-conned microenvironment, [28][29][30][31][32] progress in miniaturized chemical and biological processes using liquid marbles has been reported recently.…”