Raymond R. Dagastine

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.

Dynamic interactions between microbubbles in water

The interaction between moving bubbles, vapor voids in liquid, can arguably represent the simplest dynamical system in continuum mechanics as only a liquid and its vapor phase are involved. Surprisingly, and perhaps because of the ephemeral nature of bubbles, there has been no direct measurement of the time-dependent force between colliding bubbles which probes the effects of surface deformations and hydrodynamic flow on length scales down to nanometers. Using ultrasonically generated microbubbles (∼100 μm size) that have been accurately positioned in an atomic force microscope, we have made direct measurements of the force between two bubbles in water under controlled collision conditions that are similar to Brownian particles in solution. The experimental results together with detailed modeling reveal the nature of hydrodynamic boundary conditions at the air/water interface, the importance of the coupling of hydrodynamic flow, attractive van der Waals–Lifshitz forces, and bubble deformation in determining the conditions and mechanisms that lead to bubble coalescence. The observed behavior differs from intuitions gained from previous studies conducted using rigid particles. These direct force measurements reveal no specific ion effects at high ionic strengths or any special role of thermal fluctuations in film thickness in triggering the onset of bubble coalescence.

Measurement and analysis of forces in bubble and droplet systems using AFM

The use of atomic force microscopy to measure and understand the interactions between deformable colloids - particularly bubbles and drops - has grown to prominence over the last decade. Insight into surface and structural forces, hydrodynamic drainage and coalescence events has been obtained, aiding in the understanding of emulsions, foams and other soft matter systems. This article provides information on experimental techniques and considerations unique to performing such measurements. The theoretical modelling frameworks which have proven crucial to quantitative analysis are presented briefly, along with a summary of the most significant results from drop and bubble AFM measurements. The advantages and limitations of such measurements are noted in the context of other experimental force measurement techniques.

Characterization of nanoscale property variations in polymer composite systems: 1. Experimental results

A technique utilizing the indenting capabilities of the atomic force microscope is used to evaluate local changes in the response of polymer composite systems near the fiber-matrix interface. Room temperature and elevated temperature indentation response is measured for several model composite systems. Results of indentation studies are compared to finite element model predictions to understand the influence of interphase properties on the measured responses. For sized fiber systems, unexpected property variations are observed, leading to the discovery of a possible interphase formation mechanism in these systems. q 1998 Published by Elsevier Science Ltd. All rights reserved.

Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions.

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.

Anomalous Stability of Carbon Dioxide in pH-Controlled Bubble Coalescence†

Gas bubbles are formed as cavities in liquids, their pressure, shape, and deformability determined by the surface tension of the liquid. They are vital components in foams, microfluidics, sonochemical reactions, generation of atmospheric aerosol, and in the scent and taste delivery of soft drinks, beers, and champagne. In all of these cases, their stability or coalescence during inter-bubble collisions is a vital factor in determining bubble behavior and lifetime. It has been noted previously that, due to its high water-solubility and unusual aqueous chemistry, carbon dioxide may be expected to behave differently than inert gases, suggesting that a comparative study is needed. Here, we explore bubble coalescence as a function of pH and gas type, demonstrating that CO2 has a suprising and vital role, by comparing pure CO2 bubbles with air (which has CO2 as a minor component), argon, and nitrogen (pure, inert gases). Recently, advances in the technique of atomic force microscopy (AFM) have allowed direct measurements of the force and coalescence behavior between pairs of bubbles and drops with diameters around 100 mm to be made. Here, for the first time we use low velocities in order to understand the equilibrium forces acting between bubbles as they approach one another, and which ultimately determine their coalescence or stability. Bubble interaction events were measured by using an AFM cantilever to pick one bubble up in the size range 50– 200 mm from a glass substrate, and drive this bubble towards a substrate-immobilized bubble at a fixed, low speed (0.2 mms!1, chosen to eliminate the effects of hydrodynamic drainage forces between the bubbles), until either coalescence occurred, or until a fixed deflection of the cantilever was reached in the case of stable, repulsive interactions. The experiment is shown schematically in Figure 1B, and full details of experimental procedures used are included in the Supporting Information. During the axisymmetrical, close approach of two bubbles (Figure 1), they will deform and flatten in the presence of a repulsive interaction and a film of water will remain between the two air–water interfaces. The thickness of this film depends on the disjoining pressure which includes contributions from van der Waals and electrical double-layer interactions. By using a theoretical model (included in the Supporting Information) that couples a quantitative description of these forces to expressions that describe the interfacial profiles of the bubbles during an AFM measurement, information on both the surface forces and deformation can be calculated. The pH ranges investigated for each gas, and the regions in which coalescence were observed are shown in Figure 2. For the inert gases, argon and nitrogen, the window of coalescence is almost identical, between pH 3 and 7. For air, this region is smaller by half a pH unit at each extreme. In contrast, CO2 bubbles do not coalesce below pH 6, showing considerably enhanced stability. Surface potentials derived from model fits to the data as a function of pH for the gas bubbles are presented in Figure 3. It is found that the surface potentials for air bubbles are in general agreement with those obtained using microelectrophoresis, including the location of the isoelectric point close to pH 4. For argon and nitrogen bubbles, the surface Figure 1. A) Force vs. relative separation curves for the slow (0.2 mms!1) approach of two air bubbles in different pH conditions. At pH 7 and 10.3, stable interactions are observed; for pH 4 and 5.5, coalescence occurs. Colored points are the experimental data, and solid black lines are the model prediction. Dseparation is defined as the change in separation between the end of the cantilever and the solid surface. B) Schematic of the AFM experiment, showing a bubble attached to the cantilever approaching a surface-immobilized bubble.

The effect of milk processing on the microstructure of the milk fat globule and rennet induced gel observed using confocal laser scanning microscopy.

Confocal laser scanning microscopy (CLSM) was successfully used to observe the effect of milk processing on the size and the morphology of the milk fat globule in raw milk, raw ultrafiltered milk, and standardized and pasteurized milk prepared for cheese manufacture (cheese-milk) and commercial pasteurized and homogenized milk. Fat globule size distributions for the milk preparations were analyzed using both image analysis and light scattering and both measurements produced similar data trends. Changes to the native milk fat globule membrane (MFGM) were tracked using a MFGM specific fluorescent stain that allowed MFGM proteins and adsorbed proteins to be differentiated on the fat globule surface. Sodium dodecyl sulfate polyacrylamide gel electrophoresis confirmed the identity of native MFGM proteins isolated from the surface of fat globules within raw, UF retentate, and cheese-milk preparations, whereas only casein was detected on the surface of fat globules in homogenized milk. The microstructure, porosity, and gel strength of the rennet induced gel made from raw milk and cheese-milk was also found to be comparable and significantly different to that made from homogenized milk. Our results highlight the potential use of CLSM as a tool to observe the structural details of the fat globule and associated membrane close to its native environment.

Modular assembly of superstructures from polyphenol-functionalized building blocks

The organized assembly of particles into superstructures is typically governed by specific molecular interactions or external directing factors associated with the particle building blocks, both of which are particle-dependent. These superstructures are of interest to a variety of fields because of their distinct mechanical, electronic, magnetic and optical properties. Here, we establish a facile route to a diverse range of superstructures based on the polyphenol surface-functionalization of micro- and nanoparticles, nanowires, nanosheets, nanocubes and even cells. This strategy can be used to access a large number of modularly assembled superstructures, including core-satellite, hollow and hierarchically organized supraparticles. Colloidal-probe atomic force microscopy and molecular dynamics simulations provide detailed insights into the role of surface functionalization and how this facilitates superstructure construction. Our work provides a platform for the rapid generation of superstructured assemblies across a wide range of length scales, from nanometres to centimetres.

Measurement of the Hydrophobic Force in a Soft Matter System

The hydrophobic attraction describes the well-known tendency for nonpolar molecules and surfaces to agglomerate in water, controlled by the reorganization of intervening water molecules to minimize disruption to their hydrogen bonding network. Measurements of the attraction between chemically hydrophobised solid surfaces have reported ranges varying from tens to hundreds of nanometers, all attributed to hydrophobic forces. Here, by studying the interaction between two hydrophobic oil drops in water under well-controlled conditions where all known surface forces are suppressed, we observe only a strong, short-ranged attraction with an exponential decay length of 0.30 ± 0.03 nm-comparable to molecular correlations of water molecules. This attraction is implicated in a range of fundamental phenomena from self-assembled monolayer formation to the action of membrane proteins and nonstick surface coatings.

Hydrodynamic forces involving deformable interfaces at nanometer separations

A model is developed to describe the dynamic forces acting between two deformable drops, or between one drop and a solid surface, when they are in relative axisymmetric motion at separations of ≲100nm in a Newtonian liquid. Forces arise from hydrodynamic pressure in the draining liquid film that separates the interfaces and from disjoining pressure due to repulsive or attractive surface forces. Predictions of the model are successfully compared with recent experimental measurements of the force between two micrometer-scale surfactant stabilized decane drops in water in an atomic force microscope [S. L. Carnie, D. Y. C. Chan, C. Lewis, R. Manica, and R. R. Dagastine, Langmuir 21, 2912 (2005); R. R. Dagastine, R. Manica, S. L. Carnie, D. Y. C. Chan, G. W. Stevens, and F. Grieser, Science 313, 210 (2006)] and with subnanometer resolution measurements of time-dependent deformations of a millimeter-scale mercury drop approaching a flat mica surface in a modified surface force apparatus [J. N. Connor and R. G. Horn, Faraday Discuss. 123, 193 (2003); R. G. Horn, M. Asadullah, and J. N. Connor, Langmuir 22, 2610 (2006)]. Special limits of the model applicable to small and moderate deformation regimes are also studied to elucidate the key physical ingredients that contribute to the characteristic behavior of dynamic collisions involving fluid interfaces.A model is developed to describe the dynamic forces acting between two deformable drops, or between one drop and a solid surface, when they are in relative axisymmetric motion at separations of ≲100nm in a Newtonian liquid. Forces arise from hydrodynamic pressure in the draining liquid film that separates the interfaces and from disjoining pressure due to repulsive or attractive surface forces. Predictions of the model are successfully compared with recent experimental measurements of the force between two micrometer-scale surfactant stabilized decane drops in water in an atomic force microscope [S. L. Carnie, D. Y. C. Chan, C. Lewis, R. Manica, and R. R. Dagastine, Langmuir 21, 2912 (2005); R. R. Dagastine, R. Manica, S. L. Carnie, D. Y. C. Chan, G. W. Stevens, and F. Grieser, Science 313, 210 (2006)] and with subnanometer resolution measurements of time-dependent deformations of a millimeter-scale mercury drop approaching a flat mica surface in a modified surface force apparatus [J. N. Connor and R. G. ...