Benjamin Cross

Using surface force apparatus, diffusion and velocimetry to measure slip lengths

Determining the slip lengths for liquids flowing close to smooth walls is challenging. The reason lies in the fact that the scales that must be addressed range between a few and hundreds of nanometres. Several techniques have been used over the last few years. Here, we consider three of them based on surface force apparatus, diffusion and velocimetry, respectively. The descriptions offered here incorporate recent instrumental progress made in the field.

Evidence for a disorder-driven phase transition in the condensation of 4He in aerogels

We report on thermodynamic and optical measurements of the condensation process of

Finite conductivity effects and apparent contact angle satutation in AC electrowetting

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A micro-nano-rheometer for the mechanics of soft matter at interfaces

He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.

Capacitive detection of buried interfaces by a dynamic surface force apparatus

We measured the electrowetting behavior of aqueous salt solutions. By varying the conductivity and the frequency of the applied AC voltage we determined the range of the validity perfect conductor assumption of the standard electrowetting theory for the case of AC voltage. We show that the contact angle reduction is dramatically reduced at high frequency and low salt concentration due to Ohmic losses with the liquid. A simple RC-equivalent circuit model explains the observations. It is demonstrated that finite conductivity effects are more pronounced for sessile droplets than for droplets confined between to parallel plates.

A Direct Sensor to Measure Minute Liquid Flow Rates

We present a nano-rheometer based on the dynamic drainage flow between a sphere and a plane from bulk regime to highly confined regime. The instrument gives absolute measurements of the viscosity of simple liquids in both regimes. For complex fluids, the measurements involve the viscosity and the elastic modulus. The device operates on distances ranging over four orders of magnitude from 1 nm to 10 μm, bridging rheological properties from the macroscopic to the molecular scale. This allows to measure an hydrodynamic or visco-elastic boundary condition and to explore the causes of the boundary condition at the microscopic level.

Ionic liquids at interfaces: general discussion

We present here a new type of distance sensor mounted on a Surface Force Apparatus (SFA), able to detect the position of a buried interface and therefore the thickness of a thin solid or soft matter film coating the SFA surface(s). This sensor relies on the capacitance created by the two metallized surfaces of the SFA. An harmonic oscillation of these polarized surfaces creates a pico- to femto-amps current indicating their relative position. One of the specificities of this sensor is the relatively weak polarization used for the measurements, minimizing the electrical forces and their impact on other interactions, hydrodynamical and mechanical forces measured by the SFA. This original and simple design is of high interest for studying the viscoelastic properties of thin films, to detect adsorbed liquid layers or slippage at liquid-solid interfaces, or even to study complex fluids such as ionic liquids under polarization. We demonstrate the use of this sensor to study the flow boundary condition of silicon oil on a metal surface, and the elastic modulus of a thin elastomer layer.

A Temperature-Controlled Device for Volumetric Measurements of Helium Adsorption in Porous Media

Nanofluidics finds its root in the study of fluids and flows at the nanoscale. Flow rate is a quantity that is both central when dealing with flows and notoriously difficult to measure experimentally at the scale of an individual nanopore or nanochannel. We show in this letter that minute flow rate can be directly measured accumulating liquid over time within the compliant membrane of a commercial piezoresistive pressure sensor. Our flow rate sensor is versatile and can be operated independently of the nature of the liquid, flow profile, and type of nanochannel. We demonstrate this method by measuring the pressure-driven flow of silicon oil in a single nanochannel of average radius 200 nm. This approach gives reliable measurement of the flow rate up to 1 pL/min. Unlike other nanoscale flow measurements methods based, for instance, on particle tracking, our sensor delivers a direct voltage output suitable for nanoflow control applications.

Phase behaviour and thermodynamics: general discussion

Andrew Abbott, Matthew Addicoat, Leigh Aldous, Radha Gobinda Bhuin, Natalia Borisenko, José Nuno Canongia Lopes, Ryan Clark, Samuel Coles, Margarida Costa Gomes, Benjamin Cross, Jeffrey Everts, Millicent Firestone, Ramesh Gardas, Matthieu Gras, Simon Halstead, Christopher Hardacre, John Holbrey, Toshiyuki Itoh, Vladislav Ivaništšev, Johan Jacquemin, Philip Jessop, Robert Jones, Barbara Kirchner, Sichao Li, Ruth Lynden-Bell, Doug MacFarlane, Florian Maier, Markus Mezger, Aǵılio Pádua, Octavian D. Pavel, Susan Perkin, Simon Purcell, Mark Rutland, John M. Slattery, Sefik Suzer, Kazuhisa Tamura, Morgan L. Thomas, Shraeddha Tiwari, Seiji Tsuzuki, Betul Uralcan, William Wallace, Masayoshi Watanabe and James Wishart