Microscale theory of surface tension

2003

This paper describes the equilibrium shape of a liquid drop under applied fields such as gravity and electrical fields, taking into account material properties such as dielectric constants, resistivities, and surface tension coefficients. The analysis is based on an energy minimization framework, scaling arguments, and on solutions of Maxwell's electrostatic equations. A rigorous and exact link is provided between the energy function corresponding to any given physical phenomena, and the resulting shape and size dependent force term in the (modified) Young's equation. It is shown that a dielectric solid and a perfectly conducting liquid is all that is needed to exactly recover the Young-Lippmann equation. A dielectric liquid on a conducting solid gives rise to line tension terms. Finally, a slightly resistive liquid on top of a dielectric, highly resistive solid gives rise to contact angle saturation and accurately predicts the experimental data that we observe in our electrowetting devices.

Lab on a Chip

The EDGE tensiometer allows access to nearly the full range of dynamic interfacial tension at relevant time scales for both droplet and bubble interfaces.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2011

We developed a microfluidic chip that is capable of measuring interfacial tensions between two liquids directly from a force balance. This functionality is obtained by using a tapered microchannel connected to a straight channel containing the second immiscible liquid. The equilibrium position of the liquid-liquid interface along the tapered channel depends on the interfacial tension, the contact angles with the channel walls, and externally controllable pressures. Using a device of PDMS with a channel height of 50 m and a width tapering from 300 to 20 m, equilibrium interfacial tensions (IFTs) could be measured for various systems with and without surfactant that display IFTs between 3 and 38 mN/m. Quantitative agreement with macroscopic measurements was obtained. In the absence of surfactant the resolution of our approach is limited by contact angle hysteresis and pinning. We demonstrate the potential of our device for measuring IFTs that change over time, by progressively exposing the W/O interface to surfactant. This microfluidic method offers interesting perspectives for measuring static or dynamic interfacial tensions on chip, combined with a flexible control over the composition of the continuous phase.

Advanced …, 2010

wileyonlinelibrary.com 431 acts analogously to interfacial tension in liquid drops. In conclusion, microfl uidic methods offer unique, useful, and simple techniques to measure the dynamics of a wide range of systems including two-phase systems, biological cells, and elastic capsules.

Journal of Physics: Condensed Matter, 2000

The propensity of liquid films to bead off poorly wettable substrates leads to a wide variety of liquid structures via mechanisms which are far from being fully understood. In particular, dewetting via unstable surface waves may be driven at least by dispersion forces, electrostatic forces, or by Marangoni-type transport. A hierarchy of dynamical instabilities finally transforms the initial homogeneous film into the final state, consisting of an ensemble of individual, isolated droplets. While these processes of self-organized structure formation are interesting in themselves, it may also be desirable to generate liquid structures in a more well-defined and predictable way. We have therefore investigated experimentally the behaviour of various liquids on substrates, the wettability of which has been laterally structured. The resulting artificial liquid objects display several remarkable properties, both statically and dynamically. Aside from potential applications as 'liquid microchips', it is shown how fundamental quantities can be extracted from the shapes of the liquid surfaces, as determined by scanning force microscopy. The three-phase contact line tensions obtained in this way are in fair agreement with theoretical predictions and might help to resolve long-standing debates on the role of wetting forces on the nanometre scale. 0953-8984/99/SA0057+18$19.50

Nature, 2005

The microfabrication technologies of the semiconductor industry have made it possible to integrate increasingly complex electronic and mechanical functions, providing us with ever smaller, cheaper and smarter sensors and devices. These technologies have also spawned microfluidics systems for containing and controlling fluid at the micrometre scale, where the increasing importance of viscosity and surface tension profoundly affects fluid behaviour. It is this confluence of available microscale engineering and scaledependence of fluid behaviour that has revolutionized our ability to precisely control fluid/fluid interfaces for use in fields ranging from materials processing and analytical chemistry to biology and medicine.

Langmuir, 2009

Emulsification in microdevices (microfluidic emulsification) involves micrometer-sized droplets and fast interface expansion rates. In addition, droplets are formed in less than milliseconds, and therefore traditional tensiometric techniques cannot be used to quantify the actual interfacial tension. In this paper, monodisperse droplets formed at flat microfluidic Y-junctions were used to quantify the apparent dynamic interfacial tension during (microfluidic) emulsification. Hexadecane droplets were formed in ethanol-water solutions with a range of static interfacial tensions to derive a calibration curve, which was subsequently used to access the dynamic interfacial tension of hexadecane droplets formed in surfactant solutions. For SDS and Synperonic PEF108, various continuous- and disperse-phase (hexadecane) flow rates were studied, and these conditions were linked to interfacial tension effects, which also allowed convective transport of surfactants to be investiagted. On the basis of these findings, various strategies for the formation of emulsion droplets can be followed and are discussed.

We present the influence of both the solid-liquid interfacial force (surface wettability) and liquid-liquid interfacial force (added surfactants) on water-oil twophase flow in microfluidic devices. Experimental results show that, in contrast to macroscale experiments, the surface wettability crucially determines the emulsion type created in the microchannels: O/W in hydrophilic channel and W/O in hydrophobic channel. Surfactants, however, determines the flow pattern, changing from droplet-based to stratified flow by decreasing  wo .

i love it.

https://www.irjet.net/archives/V6/i12/IRJET-V6I12283.pdf