Protein adsorption in three dimensions.

The interaction between deformable drops or bubbles encompasses a number of distinguishing characteristics not present in the interaction between solid bodies. The drops can entrap a thin liquid film of the continuous phase that can lead to a stable film or coalescence. But before leading to either of these outcomes, the film must drain under the influence of an external driving force. This drainage process exhibits all the characteristic features of dynamic interactions between soft materials. For example, the spatial and temporal variations of forces and geometric deformations, arising from hydrodynamic flow, surface forces and variations in material properties, are all inextricably interconnected. Recent measurements of time-varying deformations and forces between interacting drops and bubbles confirmed that dynamic forces and geometric deformations are coupled and provide the key to understand novel phenomena such as the “wimple” in mechanically perturbed films. The counter-intuitive phenomenon of coalescence triggered by separating proximal drops or bubbles can also be elucidated using the same theoretical framework. One approach to modelling such systems is to use a fluid mechanics formulation of two-phase flow for which a number of parametric numerical studies have been made. Another popular approach focuses on describing the thin film between the interacting drops or bubbles with a flat film model upon which a phenomenological film drainage and rupture mechanism has been developed. While both models have a similar genesis, their predictions of the fate of the draining film are quite different. Furthermore, there have been few quantitative comparisons between results obtained from many different experimental approaches with either theory. One reason for this is perhaps due to difficulties in matching experimental parameters to model conditions. A direct attempt to model dynamic behaviour in many experimental studies is challenging as the model needs to be able to describe phenomena spanning six orders of magnitude in length scales. However, with the recent availability of accurate experimental studies concerning dynamic interaction between drops and bubbles that use very different, but complementary approaches, it is timely to conduct a critical review to compare such results with long-accepted paradigms of film stability and coalescence. This topic involves the coupling of behaviour on the millimetre–micrometre scale familiar to readers with an engineering and fluid mechanics background to phenomena on the micrometre–nanometre scale that is the domain of the interfacial science and nanotechnology community.

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Film drainage and coalescence between deformable drops and bubbles

Extractive fermentation using aqueous biphasic systems (ABS) is a promising separation process since it provides a nondenaturing environment for biomolecules and improves the stability of cells. Due to environmental concerns and toxicity issues related with common volatile organic solvents, ionic liquids (ILs), a new class of nonvolatile alternative solvents, are being currently investigated for extraction purposes. In this work, a wide range of imidazolium-based ILs was studied aiming at obtaining new insights regarding their ability toward the formation of ABS and their capacity to the extraction of biomolecules. On the basis of the IL cations 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium, the IL anion influence on ABS formation was assessed through their combination with chloride, bromide, acetate, hydrogensulfate, methanesulfonate, methylsulfate, ethylsulfate, trifluomethanesulfonate, trifluoroacetate, and dicyanamide. Ternary phase diagrams (and respective tie-lines) formed by these hyd...

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Evaluation of Anion Influence on the Formation and Extraction Capacity of Ionic-Liquid-Based Aqueous Biphasic Systems

The effect of ions on enzyme activity and stability usually follows the Hofmeister series (or the kosmotropicity order): kosmotropic anions and chaotropic cations stabilize enzymes while chaotropic anions and kosmotropic cations destabilize them. The effect of ionic liquids (ILs) on the enzyme activity/stability/enantioselectivity is complicated especially when there is no or little water presence in the IL media. However, when aqueous solutions of hydrophilic ILs are employed as reaction media, the enzyme seems to follow the Hofmeister series since ILs dissociate into individual ions in water.

Effect of ions and other compatible solutes on enzyme activity, and its implication for biocatalysis using ionic liquids

Effects of cyclodextrins on drug delivery through biological membranes.

Film thinning experiments have been conducted with aqueous films between two air phases in a thin film pressure balance. The films are free of added surfactant but simple NaCl electrolyte is added in some experiments. Initially the experiments begin with a comparatively large volume of water in a cylindrical capillary tube a few millimeters in diameter, and by withdrawing water from the center of the tube the two bounding menisci are drawn together at a prescribed rate. This models two air bubbles approaching at a controlled speed. In pure water, the results show three regimes of behavior depending on the approach speed; at slow speed (<1 microm/s) it is possible to form a flat film of pure water, approximately 100 nm thick, that is stabilized indefinitely by disjoining pressure due to repulsive double-layer interactions between naturally charged air/water interfaces. The data are consistent with a surface potential of -57 mV on the bubble surfaces. At intermediate approach speed (approximately 1-150 microm/s), the films are transiently stable due to hydrodynamic drainage effects, and bubble coalescence is delayed by approximately 10-100 s. At approach speeds greater than approximately 150 microm/s, the hydrodynamic resistance appears to become negligible, and the bubbles coalesce without any measurable delay. Explanations for these observations are presented that take into account Derjaguin-Landau-Verwey-Overbeek and Marangoni effects entering through disjoining pressure, surface mobility, and hydrodynamic flow regimes in thin film drainage. In particular, it is argued that the dramatic reduction in hydrodynamic resistance is a transition from viscosity-controlled drainage to inertia-controlled drainage associated with a change from immobile to mobile air/water interfaces on increasing the speed of approach of two bubbles. A simple model is developed that accounts for the boundaries between different film stability or coalescence regimes. Predictions of the model are consistent with the data, and the effects of adding electrolyte can be explained. In particular, addition of electrolyte at high concentration inhibits the near-instantaneous coalescence phenomenon, thereby contributing to increased foam film stability at high approach speeds, as reported in previous literature. This work highlights the significance of bubble approach speed as well as electrolyte concentration in affecting bubble coalescence.

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Stability of aqueous films between bubbles. Part 1. The effect of speed on bubble coalescence in purified water and simple electrolyte solutions.

The problem of hydrophobic hydration is at the boundary line between solution chemistry and surface thermodynamics, with focus on water structure and applications to biological self-assembly. In this retrospective, we trace the origins and evolution of the idea, analyze basic definitions, outline directions of research and consider current trends.

Hydrophobic hydration : Thermodynamics and theoretical aspects of colloid science

The stability of water films has been investigated with a Mysels-Scheludko type film balance. Minor trace impurities in water do not affect the lifetime of water films under vapor saturation, but significantly influence the stability in free evaporation. Trace amounts of positively adsorbed contaminants induce Marangoni-driven flow that destabilizes films under evaporation conditions whereas negatively adsorbed electrolytes actually prolong stability by reversing interfacial tension gradients and driving a steady circulation within the film. At high thinning rates, pure-water films develop exotic-appearing flow patterns and break due to a strong coupling between hydrodynamic and interfacial tension-gradient adsorption stresses. The most dominant factor of transient film stabilization in dynamic conditions under evaporation is a surface tension gradient created in the film. We discuss surface tension gradients in transient films created by temperature differences, impurity concentration, and expansion of the films.

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Vassili V. Yaminsky

Papers

Stability of aqueous films between bubbles. Part 1. The effect of speed on bubble coalescence in purified water and simple electrolyte solutions.

Hydrophobic hydration : Thermodynamics and theoretical aspects of colloid science

Stability of aqueous films between bubbles. Part 2. Effects of trace impurities and evaporation.