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, i.e., 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. Strikingly, this transition is characterised by sudden expulsion of the inner water drop from the encapsulating oil droplet. We propose that this topological transition is triggered by the freezing of the encapsulating oil droplet from the outside in, putting tension on the inner water drop thus, destabilizing the W/O/W configuration. Using high-speed imaging we characterize the destabilization process. Interestingly, we find that below a critical size of the inner drop, Rin,crit ≈ 19 µm, the topological transition does not occur any more and the double-emulsion remains stable, in line with our interpretation.

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. We observe that these effects primarily emerge due to the hygroscopic nature of the pure droplet, which eventually leads to solutal-Marangoni effects. Additionally, heat transfer between the evaporating droplet and the underlying substrate also plays a crucial role in the overall dynamics. Thus, the overall spreading of a pure hygroscopic droplet is determined by a delicate interplay between solutal and thermal Marangoni effects.

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. This deformation strongly depends on the engulfment velocity V, even forming pointy-tip shapes for low values of V. We find such singular deformations are mediated by interfacial flows in nanometric thin liquid films separating the nonsolidifying dispersed droplets and the solidifying bulk. We model the fluid flow in these intervening thin films using a lubrication approximation and then relate it to the deformation sustained by the dispersed droplet.

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. We show that at sufficiently high undercooling, a peculiar freezing morphology exists that involves sequential advection of frozen fronts from the center of the droplet to its boundaries. This phenomenon is examined by combining elements of classical nucleation theory to the large-scale hydrodynamics on the droplet scale, bringing together two subfields which traditionally have been quite separated. Furthermore, we report a self-peeling phenomenon of a frozen splat that is driven by the existence of a transient crystalline state during solidification.

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. We establish that the presence of presolidified material at the advancing contact line fundamentally changes the resulting dynamics, namely, by modifying the local flow mobility that leads to an instability analogous to viscous fingering. Moreover, we delineate the interplay between the interfacial deformations of the impacting droplet and patterned growth of the solid phase as disconnected patterns emerge at faster impacts.

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. We combine experimental studies and theoretical analyses to reveal and rationalize the dynamics exhibited upon the coalescence between the contact line of a micron-sized oil droplet and the receding contact line of a millimeter-sized water drop that covers the oil droplet on the substrate. We find that after the coalescence a four-phase contact line is formed for a brief period. However this quadruple contact line is not stable, leading to a 'droplet splitting' effect and eventually expulsion of the oil droplet from the water drop. We then show that the interfacial tension between the different phases and the viscosity of the oil droplet dictate the splitting dynamics. More viscous oils display higher resistance to the extreme deformations of the droplet induced by the instability of the quadruple contact line and no droplet expulsion is observed in such cases.

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. On a substrate with uniform wettability, we find that the presence of bounding side walls enhances the local spreading and facilitates fluid coverage of the entire recessed region, but containment within the pixel is not guaranteed. In contrast, wettability patterning alone leads to robust containment of the fluid within the wetting region, but fluid coverage is reduced in the absence of side walls. Our theoretical calculations use a simplified numerical model of fluid redistribution via purely capillary effects, augmented by a Cox-Voinov spreading law. The neglect of fluid viscosity in this model means that, after an initial period of agreement, the predicted evolution is faster than in the experiments. Nonetheless, the simplified model achieves excellent predictions both for the liquid morphologies and for the conditions required for successful pixel filling on substrates with topographical and wettability variations.

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. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10-100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.

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. Here, we study electrowetting directly on graphitic surfaces with the use of aqueous electrolytes to show that reversible control of wetting can be achieved and quantitatively understood using models of the interfacial capacitance. We manifest that the use of highly concentrated aqueous electrolytes induces a fully symmetric and reversible wetting behavior without degradation of the substrate within the unprecedented potential window of 2.8 V. We demonstrate where the classical "Young-Lippmann" models apply, and break down, and discuss reasons for the latter, establishing relations among the applied bias, the electrolyte concentration, and the resultant contact angle. The approach is extended to electrowetting at the liquid|liquid interface, where a concentrated aqueous electrolyte drives reversibly the electrowetting response of an insulating organic phase with a significantly decreased potential threshold. In summary, this study highlights the beneficial effect of highly concentrated aqueous electrolytes on the electrowettability of carbon surfaces, being directly related to the performance of carbon-based aqueous energy-storage systems and electronic and microfluidic devices.