Nathan Coombs : Colloidal Deposits From Evaporating Sessile Droplets

The coffee ring effect (CRE) refers to the accumulation of solute particles near the contact line of an evaporating sessile droplet and arises due to evaporation-induced capillary flow. Suppression of the CRE is desirable in many industrial applications which utilize colloidal deposition from an evaporating liquid, notably inkjet printing. It is therefore important that the influence of experimentally accessible physical parameters (ambient temperature, humidity, particle size/shape etc.) on the deposit morphology are well understood.

Of critical importance in CRE modelling is the inclusion of particle “jamming”: when solute reaches a threshold volume fraction (approximately 64% for mono-disperse spherical particles), a transition towards a porous solid is observed. Jammed particles have a semi-crystalline structure and can exhibit both ordered and disordered phases depending on the local advection speed. Since jammed solute is incompressible, it also influences the shape of the drop’s surface, ultimately leading to a reversal in surface curvature and meniscus touchdown at the late stages of evaporation.

Existing CRE models that include jamming are limited in scope to pre-touchdown dynamics and so are not able to describe the drying process in full. In this talk I will introduce a modelling framework that remedies this issue. Though much of the focus will be on axisymmetric drops, the model can be easily generalised to arbitrary drop shapes, allowing us to explore the influence of contact line curvature on the local CRE intensity.

Peter Lewin-Jones : Impacts of Liquid Drops: When Do Gas Microfilms Prevent Merging?

Collisions and impacts of drops are critical to numerous processes, including raindrop formation, inkjet printing and spray cooling. For drop-drop collisions, increasing the impact speed leads to multiple transitions: from merging to bouncing and back to merging - transitions recently discovered to be sensitive to the drops' radii as well as the ambient gas pressure. A drop impacting a liquid bath is even more complex: for a fixed speed, the result can go from merging to bouncing to merging and back to bouncing with increasing bath depth.

To provide new insight into the physical mechanisms involved and as an important predictive tool, we have developed a novel, open-source computational model for both drop-drop and drop-bath events, using the finite element package oomph-lib. This uses a lubrication framework for the gas film and incorporates fully, for the first time, the crucial micro- and nano-scale influences of gas kinetic effects and disjoining pressure.

Our simulations show strong agreement with experiments for the transitions between merging and bouncing, but can also go beyond these regimes to make new experimentally-verifiable predictions. We will show how our model enables us to explore the parameter space and discover the regimes of contact (that are inaccessible to experiments). Finally, we will overview potential extensions to the computational model, including impacts in Leidenfrost conditions.