Joseph D. Berry

Measurement of surface and interfacial tension using pendant drop tensiometry.

Pendant drop tensiometry offers a simple and elegant solution to determining surface and interfacial tension - a central parameter in many colloidal systems including emulsions, foams and wetting phenomena. The technique involves the acquisition of a silhouette of an axisymmetric fluid droplet, and iterative fitting of the Young-Laplace equation that balances gravitational deformation of the drop with the restorative interfacial tension. Since the advent of high-quality digital cameras and desktop computers, this process has been automated with high speed and precision. However, despite its beguiling simplicity, there are complications and limitations that accompany pendant drop tensiometry connected with both Bond number (the balance between interfacial tension and gravitational forces) and drop volume. Here, we discuss the process involved with going from a captured experimental image to a fitted interfacial tension value, highlighting pertinent features and limitations along the way. We introduce a new parameter, the Worthington number, Wo, to characterise the measurement precision. A fully functional, open-source acquisition and fitting software is provided to enable the reader to test and develop the technique further.

A multiphase electrokinetic flow model for electrolytes with liquid/liquid interfaces

A numerical model for electrokinetic flow of multiphase systems with deformable interfaces is presented, based on a combined level set-volume of fluid technique. A new feature is a multiphase formulation of the Nernst-Planck transport equation for advection, diffusion and conduction of individual charge carrier species that ensures their conservation in each fluid phase. The numerical model is validated against the analytical results of Zholkovskij et al. (2002) [1], and results for the problem of two drops coalescing in the presence of mobile charge carriers are presented. The time taken for two drops containing ions to coalesce decreases with increasing ion concentration.

Leidenfrost Vapor Layers Reduce Drag without the Crisis in High Viscosity Liquids.

The drag coefficient C_{D} of a solid smooth sphere moving in a fluid is known to be only a function of the Reynolds number Re and diminishes rapidly at the drag crisis around Re∼3×10^{5}. A Leidenfrost vapor layer on a hot sphere surface can trigger the onset of the drag crisis at a lower Re. By using a range of high viscosity perfluorocarbon liquids, we show that the drag reduction effect can occur over a wide range of Re, from as low as ∼600 to 10^{5}. The Navier slip model with a viscosity dependent slip length can fit the observed drag reduction and wake shape.

Decreasing the Wettability of Cellulose Nanocrystal Surfaces Using Wrinkle-Based Alignment

Cellulose nanocrystals (CNCs) are a particularly appealing format of the natural biopolymer due to their exceptional strength, nanoscale dimensions, and needle-like shape anisotropy. However, CNCs are hydrophilic and hence their wettability makes them impractical for many coating applications, with various approaches using chemical functionalization to overcome this. Here we show that CNC-coated surfaces can be rendered hydrophobic by alignment of the native CNCs using a wrinkled template-mediated printing process. We present a novel and simple method allowing full release of the CNCs from the template and their permanent adsorption into fine patterns onto the surface, thus preventing CNC repositioning during wetting. The aligned CNCs induce strong pinning effects that capture and retain water droplets with high contact angle and large roll-off angles, without becoming susceptible to oil contamination. The fabrication process for these coatings could be achieved by large-scale printing, making them a practical and cost-effective solution to hydrophobic coatings from raw cellulosic materials.

Predictions for optimal mitigation of paracrine inhibitory signalling in haemopoietic stem cell cultures

IntroductionRecent studies in the literature have highlighted the critical role played by cell signalling in determining haemopoietic stem cell (HSC) fate within ex vivo culture systems. Stimulatory signals can enhance proliferation and promote differentiation, whilst inhibitory signals can significantly limit culture output.MethodsNumerical models of various mitigation strategies are presented and applied to determine effectiveness of these strategies toward mitigation of paracrine inhibitory signalling inherent in these culture systems. The strategies assessed include mixing, media-exchange, fed-batch and perfusion.ResultsThe models predict that significant spatial concentration gradients exist in typical cell cultures, with important consequences for subsequent cell expansion. Media exchange is shown to be the most effective mitigation strategy, but remains labour intensive and difficult to scale-up for large culture systems. The fed-batch strategy is only effective at very small Peclet number, and its effect is diminished as the cell culture volume grows. Conversely, mixing is effective at high Peclet number, and ineffective at low Peclet number. The models predict that cell expansion in fed-batch cultures becomes independent of increasing dilution rate, consistent with experimental results previously reported in the literature. In contrast, the models predict that increasing the flow rate in perfused cultures will lead to increased cell expansion, indicating the suitability of perfusion for use as an automated, tunable strategy. The effect of initial cell seeding density is also investigated, with the model showing that perfusion outperforms dilution for all densities considered.ConclusionsThe models predict that the impact of inhibitory signalling in HSC cultures can be mitigated against using media manipulation strategies, with the optimal strategy dependent upon the protein diffusion time-scale relative to the media manipulation time-scale. The key messages from this study can be applied to any complex cell culture scenario where cell-cell interactions and paracrine signalling networks impact upon cell fate and cell expansion.

Effect of wall permittivity on electroviscous flow through a contraction.

The electroviscous flow at low Reynolds number through a two-dimensional slit contraction with electric double-layer overlap is investigated numerically for cases where the permittivity of the wall material is significant in comparison with the permittivity of the liquid. The liquid-solid interface is assumed to have uniform surface-charge density. It is demonstrated that a finite wall permittivity has a marked effect on the distribution of ions in and around the contraction, with a significant build-up of counter-ions observed at the back-step. The development length of the flow increases substantially as the wall permittivity becomes significant, meaning that the electric double-layers require a longer distance to develop within the contraction. Consequently, there is a corresponding decrease in the hydrodynamic and electro-potential resistance caused by the contraction. The effect of wall-region width on the flow characteristics is also quantified, demonstrating that the development length increases with increasing wall-region width for widths up to 5 channel widths.

Charge and Film Drainage of Colliding Oil Drops Coated with the Nonionic Surfactant C12E5

The interaction forces between colliding tetradecane drops were measured in the presence of the nonionic surfactant pentaethylene glycol monododecyl ether (C12E5). The force behavior was measured in the range of premicellar compositions of the nonionic surfactant in various salt solutions and was consistent with the presence of a surface charge even though the surfactant was nonionic in nature. The surface potential of oil drops was found to decrease with an increase in C12E5 concentration. The measured electrophoretic mobilities and ζ potentials of emulsified tetradecane drops also decreased with an increase in the C12E5 concentration. The surface potential decreased with an increase in the electrolyte at a constant C12E5 concentration, further confirming the presence of a charged oil-water interface. In addition to the charging behavior, the nonequilibrium film drainage between the tetradecane drops coated with C12E5 was also measured. In contrast to some existing experiments in the literature, it was found that oil drops coated with the nonionic surfactant were stable against coalescence, even when the drops were deformed on the order of their radii. These findings have significant implications on the stability of emulsions in food, personal care, and detergent industries.

Flow dynamics of a tethered elastic capsule

A two-dimensional model of a tethered capsule is used to elucidate the effects of capsule aspect ratio and capsule internal viscosity on capsule dynamics. Over the parameter space examined, the capsule initially elongates out into the flow and then slowly pivots toward the wall as the capsule relaxes to a steady-state shape. The region of the capsule membrane that would come into contact with the wall corresponds with a region of elevated traction-force magnitude. The effect of viscosity is found to be negligible at low shear rates, but at high shear rates, an increase in internal viscosity leads to an increase in the maximum capsule deformation and maximum force on the tether. At low shear rates, capsules with higher aspect ratios experience less force and deformation. Conversely, at high shear rates, capsules with higher aspect ratios experience greater force and deformation.

Mapping coalescence of micron-sized drops and bubbles

Emulsion formulation, solvent extraction and multiphase microfluidics are all examples of processes that require precise control of drop or bubble collision stability. We use a previously validated numerical model to map the exact conditions under which micron-sized drops or bubbles undergo coalescence in the presence of colloidal forces and hydrodynamic effects relevant to Brownian motion and low Reynolds number flows. We demonstrate that detailed understanding of how the equilibrium surface forces vary with film thickness can be applied to make accurate predictions of the outcome of a drop or bubble collision when hydrodynamic effects are negligible. In addition, we illuminate the parameter space (i.e. interaction velocity, drop deformation, interfacial tension, etc.) at which hydrodynamic effects can stabilise collisions that are unstable at equilibrium. Further, we determine conditions for which drop or bubble collisions become unstable upon separation, caused by negative hydrodynamic pressure in the film. Lastly, we show that scaling analyses are not applicable for constant force collisions where the approach timescale is comparable to the coalescence timescale, and demonstrate that initial conditions under these circumstances cannot be ignored.

Electrohydrodynamic Deformation And Interaction Of Microscale Drop Pairs

Binary drop electrocoalescence is the process of inducing two drops, suspended in an immiscible fluid, to coalesce in the presence of an external electric field. Electric forces have been known to accelerate the rupture of the interfacial film and enhance drop coalescence but the process has not been well characterized. The effects of the drop ion concentration and interfacial tension on the coalescence process are studied. It is shown that increasing interfacial tension, along with electric field makes it more likely that the drops stabilize after coalescence, as opposed to breaking up. This is due to the relative magnitudes of the drop deformation and charge separation timescales.