Deforming Ice with Drops.

A uniform solidification front undergoes nontrivial deformations when encountering an insoluble dispersed particle in a melt. For solid particles, the overall deformation characteristics are primarily dictated by heat transfer between the particle and the surrounding, remaining unaffected by the rate of approach of the solidification front. In this Letter we show that, conversely, when interacting with a droplet or a bubble, the deformation behavior exhibits entirely different and unexpected behavior.

Spreading of volatile droplets in a humidity-controlled environment.

When a pure ethanol droplet is deposited on a dry, wettable and conductive substrate, it is expected to spread into a thin, uniform film. Here, we demonstrate that this uniform spreading behaviour can be altered significantly by controlling the ambient relative humidity. We show that higher relative humidity not only promotes faster spreading of the droplet, it also destabilizes the moving contact line, resulting in a fingering instability.

Freezing-induced topological transition of double-emulsion.

Solidification of complex liquids is pertinent to numerous natural and industrial processes. Here, we examine the freezing of a W/O/W double-emulsion, , water-in-oil compound droplets dispersed in water. We show that the solidification of such hierarchical emulsions can trigger a topological transition; for example, in our case, we observe the transition from the stable W/O/W state to a (frozen) O/W single-emulsion configuration.

Thin-Film-Mediated Deformation of Droplet during Cryopreservation.

Freezing of dispersions is omnipresent in science and technology. While the passing of a freezing front over a solid particle is reasonably understood, this is not so for soft particles. Here, using an oil-in-water emulsion as a model system, we show that when engulfed into a growing ice front, a soft particle severely deforms.

Taming Electrowetting Using Highly Concentrated Aqueous Solutions.

Wetting of carbon surfaces is one of the most widespread, yet poorly understood, physical phenomena. Control over wetting properties underpins the operation of aqueous energy-storage devices and carbon-based filtration systems. Electrowetting, the variation in the contact angle with an applied potential, is the most straightforward way of introducing control over wetting.

Pattern Formation during the Impact of a Partially Frozen Binary Droplet on a Cold Surface.

The impact of a droplet on an undercooled surface is a complex phenomenon as it simultaneously instigates several physical processes that cover a broad spectrum of transport phenomena and phase transition. Here, we report and explain an unexpected but highly relevant phenomenon of fingered growth of the solid phase. It emerges during the impact of a binary droplet that freezes from the outside prior to the impact on the undercooled surface.

Fast-freezing kinetics inside a droplet impacting on a cold surface.

Freezing or solidification of impacting droplets is omnipresent in nature and technology, be it a rain droplet falling on a supercooled surface; in inkjet printing, where often molten wax is used; in additive manufacturing or metal-production processes; or in extreme ultraviolet lithography (EUV) for the chip production, where molten tin is used to generate the EUV radiation. For many of these industrial applications, a detailed understanding of the solidification process is essential. Here, by adopting an optical technique in the context of freezing-namely, total-internal reflection (TIR)-we elucidate the freezing kinetics during the solidification of a droplet while it impacts on an undercooled surface.

Splitting droplets through coalescence of two different three-phase contact lines.

Moving contact lines of more than two phases dictate a large number of interfacial phenomena. Despite their significance in fundamental and applied processes, the contact lines at a junction of four-phases (two immiscible liquids, a solid and gas) have been addressed only in a few investigations. Here, we report an intriguing phenomenon that follows after the four phases oil, water, solid and gas make contact through the coalescence of two different three-phase contact lines.

Sequential deposition of microdroplets on patterned surfaces.

We use a combination of experiments and numerical modelling to investigate the influence of physico-chemical-patterned substrates on the spreading of fluid deposited as a partially overlapping sequence of droplets via inkjet printing. Our investigation is motivated by the manufacture of polymeric organic light-emitting-diode displays, where the substrate is textured with a regular array of shallow recessed regions (pixels) that are highly wetting compared to the remainder of the substrate. We examine the roles of topography and wettability patterning separately and in combination.

Ultra-low voltage electrowetting using graphite surfaces.

The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices.