Coalescence of liquid drops

Coalescence has become a hot-topic recently due to the use of both high-speed imaging and, in particular, an electrical method developed by Sid Nagel’s Group, which has resulted in the first sub-micron scale measurements of this phenomenon, published in [Paulsen et al, 2011]. This is described in the video below:

Simulating singularities

Classical fluid mechanics models predict that the speed at which the neck between two drops starts to open is infinite. Depending on the liquid and the size of the drops, the coalescence process can then go through a variety of different asymptotic regimes, where scaling laws hold, – namely ones in which viscosity and inertia are dominant.

We have used multiscale finite element simulations to capture the dynamics of both the initial stages of coalescence, on sub-micron scales, right through to the global motion, on the scale of millimetres. This allows us simultaneously to compare to experimental measurements on scales at which they are possible as well as to investigate the early time behaviour of coalescence.

Specifically, we have been able to establish the accuracy of ‘scaling laws’ proposed for different stages of the process, and in [Sprittles & Shikhmurzaev, 2014] we outline a number of inconsistencies in previous works.

This motivated our work in [Sprittles & Shikhmurzaev, 2014c], where we have provided a simple analytic improvement of the scaling law for the inertial regime.

Coalescence induced jumping

It has been observed that small microfluidic water drops which condense onto a super-hydrophobic solid can spontaneously jump off the surface. It has been discovered that the physical mechanism for this process is the coalescence of adjacent drops on the solid, which is sufficiently violent and asymmetric to launch the drops away from the solid – see video below.

Using computational simulations for guidance and insight, we have developed a simple model for the jumping event that agrees well with experiments which measure the jumping speed dependence on drop size; work which has been published in ACS Nano [Enright et al 14]. This approach allows us to study the reasons for the relatively low conversion of surface energy into jumping energy and quantify the level of viscous dissipation.

>