Coalescence Processes of Droplets and Liquid Marbles

Abstract

The coalescence process of droplets and, more recently, of liquid marbles, has become one of the most essential manipulation schemes in digital microfluidics. This process is indispensable for realising microfluidic functions such as mixing and reactions at microscale. This paper reviews previous studies on droplet coalescence, paying particular attention to the coalescence of liquid marbles. Four coalescence systems have been reviewed, namely, the coalescence of two droplets freely suspended in a fluid; the coalescence of two sessile droplets on a solid substrate; the coalescence of a falling droplet and a sessile droplet on a solid substrate; and liquid marble coalescence. The review is presented according to the dynamic behaviors, physical mechanisms and experimental parameters of the coalescence process. It also provides a systematic overview of how the coalescence process of droplets and liquid marbles could be induced and manipulated using external energy. In addition, the practical applications of liquid marble coalescence as a novel microreactor are highlighted. Finally, future perspectives on the investigation of the coalescence process of liquid marbles are proposed. This review aims to facilitate better understanding of the coalescence of droplets and of liquid marbles as well as to shed new insight on future studies.

Liquid marble coalescence via vertical collision

“…It is clear to see that the radius of liquid neck increased with time following a certain power scaling law with a power exponent ranging from 0.3815 to 0.4564, namely ∝ 0.3815~0.4564 , which is similar to the coalescence behaviour of bare droplets. 10,[33][34][35][36][37] After coalescence, the merged LM oscillated and slowly evolved to its equilibrium shape. However, the relaxation process of the merged LM will not be discussed in this paper.…”

“…[7][8][9] However, LM coalescence is one of the most essential manipulation schemes to realise microfluidic functions such as microreaction and micromixing in digital microfluidics. 10 A number of coalescence techniques for LMs have been reported in last two decades. Dorvee et al first reported that two liquid droplets encapsulated by magnetic amphiphilic particles can be manipulated using a magnet and coalesce to perform a reaction.…”

Dielectrophoretic Trapping of a Floating Liquid Marble

“…Moving the marble around induces internal flow and promotes mixing. Furthermore, two or more LMs containing different contents can be actuated to collide and coalesce to initiate mixing and reaction [20]. Besides, magnetic LMs [21][22][23][24] can be manipulated to implement multiple microfluidic functions such as reaction and detection.…”

“…We have successfully demonstrated the use of dielectrophoretic (DEP) force to move sessile [29,30] and floating LMs [31] in a nonuniform electric field. These manipulation schemes allow for dispensing, merging, trapping, and mixing droplets and LMs [20]. Among these tasks, trapping of droplets and LMs is one of the most important tasks in DMF.…”

Mapping outcomes of liquid marble collisions

“…Another recent technique for LM coalescence proceeds through the collision of floating LMs. 24 Floating LMs can be self-propelled or have their movement externally influenced. Selfpropelled LMs usually exploit the Marangoni effect by modifying the surface tension of the supporting medium near the LM via gaseous diffusion of the LMs core.…”