Craig Priest

Controlled electrocoalescence in microfluidics: Targeting a single lamella

Electrocoalescence of aqueous droplets is investigated as a tool for microfluidic processing. Where droplets are separated by only thin lamellae, coalescence can be induced on demand within a fraction of a millisecond at low potentials (few volts). The authors show that in their approach electrocoalescence proceeds through an electric-field-induced dynamic instability of the oil/water interface. When the electrode geometry and applied potential are optimized, individual lamellae can be targeted for rupture within highly ordered droplet arrangements.

Generation of monodisperse gel emulsions in a microfluidic device

We demonstrate that high dispersed phase volume fraction emulsions (i.e., gel emulsions) can be prepared in situ for microfluidic applications. Previously, the production of gel-like emulsions in microfluidic devices, where the droplet size is less than the length-scale of the channel, required multistep splitting of larger droplets in a branched microchannel network. Instead, we employ an abrupt change in the aspect ratio of a single microchannel to rapidly destabilize a confined coflowing stream, forming highly monodisperse droplets (coefficient of variance <1.5%). Using this emulsification mechanism, gel emulsions can be prepared in a single production step.

Contact Line Pinning on Microstructured Surfaces for Liquids in the Wenzel State

The wettability of surfaces microstructured with square pillars was studied, where the static advancing contact angle on the planar surface was 72 degrees. We observed elevated advancing angles (up to 140 degrees) on these structures for droplets in the Wenzel state. No air was trapped in the structured surfaces beneath the liquid, ruling out the well-known Lotus leaf effect. Instead, we show that the apparent hydrophobicity is related to contact line pinning at the pillar edges, giving a strong dependence of wetting hysteresis on the fraction of the contact line pinned on pillars. Simulating the contact line pinning on these surfaces showed similar behavior to our measurements, revealing both strong pinning at the edges of the pillars as well as mechanistic details.

Static and Dynamic Electrowetting of an Ionic Liquid in a Solid/Liquid/Liquid System

A droplet of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, bmim.BF(4)) is immersed in an immiscible liquid (n-hexadecane) and electrowetted on a flat Teflon AF1600-coated ITO electrode. The static contact angle decreases significantly when voltage is applied between the droplet and the electrode: from 145 degrees down to 50 degrees (with DC voltage) and 15 degrees (with AC voltage). The electrowetting curves (contact angle versus voltage) are similar to the ones obtained in other solid/liquid/vapor and solid/liquid/liquid systems: symmetric with respect to zero voltage and correctly described by Young-Lippmann equation below saturation. The reversibility is excellent and contact angle hysteresis is minimal (approximately 2 degrees). The step size used in applying the DC voltage and the polarity of the voltage are unimportant. The saturation contact angle cannot be predicted with the simple zero-interfacial tension theory. Spreading (after applying a DC voltage) and retraction (after switching off the voltage) of the droplet is monitored. The base area of the droplet varies exponentially during wetting (exponential saturation) and dewetting (exponential decay). The characteristic time is 20 ms for spreading and 35 ms for retraction (such asymmetry is not observed with water-glycerol mixtures of a similar viscosity). The spreading kinetics (dynamic contact angle versus contact line speed) can be described by the hydrodynamic model (Voinovs equation) for small contact angles and by the molecular-kinetic model (Blakes equation) for large contact angles. The role of viscous and molecular dissipation follows the scheme outlined by Brochard-Wyart and de Gennes.

Asymmetric wetting hysteresis on hydrophobic microstructured surfaces.

The wetting behavior of hydrophobic, microstructured surfaces containing arrays of pillars or holes has been investigated. The size of the surface features was fixed (20 microm), while their separation was varied to adjust the area fraction (0-80%). The wettability of structured surfaces for liquids resting in the Cassie state is strongly dependent on the continuity of the solid component. Microstructured square pillars and holes showed distinct, asymmetric wetting hysteresis, consistent with our previous observations on flat, chemically heterogeneous surfaces. Furthermore, clear trends for the magnitude of contact angle hysteresis versus area fraction for the two types of microstructured surfaces are evident. The pinning energy associated with these surface features is estimated.

Surface patterning of bonded microfluidic channels.

Microfluidic channels in which multiple chemical and biological processes can be integrated into a single chip have provided a suitable platform for high throughput screening, chemical synthesis, detection, and alike. These microchips generally exhibit a homogeneous surface chemistry, which limits their functionality. Localized surface modification of microchannels can be challenging due to the nonplanar geometries involved. However, chip bonding remains the main hurdle, with many methods involving thermal or plasma treatment that, in most cases, neutralizes the desired chemical functionality. Postbonding modification of microchannels is subject to many limitations, some of which have been recently overcome. Novel techniques include solution-based modification using laminar or capillary flow, while conventional techniques such as photolithography remain popular. Nonetheless, new methods, including localized microplasma treatment, are emerging as effective postbonding alternatives. This Review focuses on postbonding methods for surface patterning of microchannels.

Influence of the work of adhesion on the dynamic wetting of chemically heterogeneous surfaces.

The velocity dependence of the dynamic contact angle for a glycerol-water mixture wetting two different chemically heterogeneous surfaces (mixed thiols on gold and partially methylated titania, 16 samples in all) was studied. The molecular kinetic theory (MKT) of wetting was used to interpret the dynamic contact angle data. The equilibrium displacement frequency ( K 0) was predominantly determined by the viscous contribution from the bulk liquid, with a minor contribution from the surface. The mean distance between surface sites (lambda) decreased with increasing work of adhesion. The contact line friction coefficient zeta 0 was found to vary exponentially with the work of adhesion, enabling the unit flow volume of the liquid to be obtained.

Inferring wettability of heterogeneous surfaces by ToF-SIMS.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been examined as a possible predictive tool for surface wettability. Heterogeneous surfaces were prepared with hydrophilic and hydrophobic regions of known surface coverage using self-assembled monolayers. The surface coverage of each component was then correlated with ToF-SIMS fragmentation of the hydrophobic and hydrophilic surface groups and static contact angle measurements. From these measurements, a clear relationship between the surface wettability and relative intensity of characteristic secondary ions was identified. Moreover, our results for planar surfaces can be extrapolated to predict the wettability of particulate samples for which direct contact angle measurements are not straightforward. The ability to infer particle wettability by ToF-SIMS is well suited to mineral characterization and in particular, the prediction of mineral flotation efficiencies.

Formation and stability of nanoparticle-stabilised oil-in-water emulsions in a microfluidic chip.

The formation and stability of drops in the presence of nanoparticles was studied in a microfluidic device to directly observe the early stages of Pickering emulsification (low interfacial coverage). We observed several key differences between oil droplet necking and rupture in aqueous phases of nanoparticles (methylated silica) and well-characterised surfactant systems. The presence of particles did not influence drop formation dynamics and thus the size of the drops generated. In addition, observations of in-channel drop stability shortly after formation (several milliseconds) indicated that particles in the aqueous phase slow film thinning processes, but do not prevent coalescence. In contrast, downstream collection and densification (at the microchannel outlet), showed that particle-stabilised drops do not coalesce for several weeks, above a critical particle concentration. The implications of our results for droplet microfluidics and our understanding of conventional emulsification systems are discussed.

Impedance nanopore biosensor: influence of pore dimensions on biosensing performance

Knowledge about electrochemical and electrical properties of nanopore structures and the influence of pore dimensions on these properties is important for the development of nanopore biosensing devices. The aim of this study was to explore the influence of nanopore dimensions (diameter and length) on biosensing performance using non-faradic electrochemical impedance spectroscopy (EIS). Nanoporous alumina membranes (NPAMs) prepared by self-ordered electrochemical anodization of aluminium were used as model nanopore sensing platforms. NPAMs with different pore diameters (25-65 nm) and lengths (4-18 μm) were prepared and the internal pore surface chemistry was modified by covalently attaching streptavidin and biotin. The performance of this antibody nanopore biosensing platform was evaluated using various concentrations of biotin as a model analyte. EIS measurements of pore resistivity and conductivity were carried out for pores with different diameters and lengths. The results showed that smaller pore dimensions of 25 nm and pore lengths up to 10 μm provide better biosensing performance.