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.bubble collision | colloidal forces | hydrodynamic interaction | soft matter | thin films B ubble dynamics has attracted scientific interest since the time of Leonardo da Vinci (1), yet the observation that some simple salts can prevent bubble coalescence at high concentrations whereas others cannot remains unexplained even after over a decade of systematic study (2). In themselves, bubble-bubble interactions are very important because they feature in diverse situations, from the basis of the bends in deep-sea divers, to the development of effective ultrasonic imaging contrast agents, through to enhancing the quality of champagne. However, the delicate and ephemeral nature of bubbles poses significant technical challenges to the precise quantification of the force-displacement characteristics of bubble collisions.As a vapor phase in a liquid, the interaction between bubbles should be amenable to a simple explanation in terms of basic physical and chemical principles. A detailed understanding of the interaction between moving bubbles can provide the foundation on which to study the fundamental coupling between forces and deformations that defines the dynamic interaction on the nanoscale between soft-matter materials, such as bubbles, drops, emulsions, biological cells, soft tissues, and gels. Here we report direct measurements of the dynamic force between two deformable microbubbles in water under a variety of accurately controlled collision protocols. The typical collision velocities are in the regime of Brownian particles of comparable dimensions. The experimental conditions are such that only attractive van der Waals-Lifshitz forces and hydrodynami...

Dynamic forces between a 50 microm radius bubble driven towards and from a mica plate using an atomic force microscope in electrolyte and in surfactant exhibit different hydrodynamic boundary conditions at the bubble surface. In added surfactant, the forces are consistent with the no-slip boundary condition at the mica and bubble surfaces. With no surfactant, a new boundary condition that accounts for the transport of trace surface impurities explains variations of dynamic forces at different speeds and provides a direct connection between dynamic forces and surface transport effects at the air-water interface.

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An atomic force microscope (AFM) operating in force modulation mode is used to study solvation forces at the interface between a graphite (HOPG) surface and the liquids octamethylcyclotetrasiloxane (OMCTS) and n-dodecanol. Simple analytical models are used that adequately describe the response of the cantilever as the modulation frequency and tip-sample interaction change. The analysis of AFM force curves yields the tip-sample interaction stiffness and damping. Hydrodynamic damping is significant for all the levers used and at present this limits the sensitivity of detecting weak tip-surface damping effects. The main results are: (i) Confinement of liquid between two surfaces can lead to oscillatory structural forces even when one of the surfaces has very high curvature. This could influence topographic images at the atomic level in liquids. In these experiments the typical radius for a sharp AFM tip is measured as Rtip ≈ 14 nm. (ii) The effective viscosity increases by ∼4 orders of magnitude for a sharp tip interacting with the OMCTS solvation layers nearest the surface. (iii) The AFM data are compared to published results obtained using the surface force apparatus (SFA). The AFM data for the interaction stiffness and damping are qualitatively similar to the SFA results, but the magnitude of the effects is smaller. This most likely arises from the limited interaction area over which the confined molecules can exhibit cooperative behavior (∼100 nm 2 ). (iv) Oscillatory solvation forces can also be observed with very blunt tips (Rtip ≈ 350 nm), and the data suggest that in this case tip microasperities dominate the tip-sample interaction.

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Atomic force microecopy is used to measure force profiea and friction forces for the block copolymer PEO/PS physisorbed on mica in xylene, 2-propanol, n-dodecane, and air. The force profiles show the distinctive repulsive forces associated with brushlike confiiations in good solvents and shorter range attractive forces in poor solvents. The brushlike profiles show that in addition to being compressed between the surfaces, the polymer chains can also bend out of the tip-surface contact region. The friction data show that the tip is beet regarded as a single asperity contact and on solid polymer surfacea there is a transition to plowing type friction as the applied force is increased which can be associated with the yielding of the polymeric material. No friction signal could be measured within the polymer brush in a good solvent. Topographic images of the adsorbed polymer in poor solvents are also shown. At submonolayer coverages the polymer agglomerates and during imaging the agglomerates were either broken up or moved if the tip scanning speed was too slow. The polymer could be more eaily imaged by adding 2-propanol which further collapsed the polymer chains and thus strengthed the agglomerate structures. In good solvents the tip tends to displace the molecules along the surface and it is concluded that further studies on these systems will be best undertaken with polymers chemisorbed onto the surface.

Microfabricated cantilever sensors were used to measure the surface stress induced by protein adsorption onto a gold surface. Two proteins, immoglobulin G (IgG) and albumin (BSA), were studied. The change of surface stress upon adsorption of IgG was found to be compressive, whereas that of BSA was tensile. This difference is elucidated in terms of protein deformation and packing. Most stress change occurs not on adsorption but over very long time scales, up to 12 h, as protein conformational changes occur. The ability to monitor slow protein changes (e.g., from protein denaturing) is a particular strength of the technique.

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