http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artifact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g., of oil) on (hydrophobic) substrates immersed in water, as they show similar properties and can easily be confused with surface nanobubbles and as they are produced in a similar way, namely, by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation. The review starts with how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. Molecular dynamic simulations and theories to account for the long lifetime of the surface nanobubbles are then reported on. The crucial element contributing to the long lifetime of surface nanobubbles and nanodroplets is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace’s equation, and Henry’s law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas or oil oversaturation ζ that determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation. The review also covers the potential technological relevance of surface nanobubbles and nanodroplets, namely, in flotation, in (photo)catalysis and electrolysis, in nanomaterial engineering, for transport in and out of nanofluidic devices, and for plasmonic bubbles, vapor nanobubbles, and energy conversion. Also given is a discussion on surface nanobubbles and nanodroplets in a nutshell, including theoretical predictions resulting from it and future directions. Studying the nucleation, growth, and dissolution dynamics of surface nanobubbles and nanodroplets will shed new light on the problems of contact line pinning and contact angle hysteresis on the submicron scale.

/pdf/surface-nanobubbles-and-nanodroplets-17od1qfe93.pdf

Surface nanobubbles and nanodroplets

Foaming, foam films, antifoaming and defoaming

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.

/pdf/film-drainage-and-coalescence-between-deformable-drops-and-2akulbb9px.pdf

Film drainage and coalescence between deformable drops and bubbles

On the basis of the theory of capillarity, the process of formation of a wetting perimeter when a spherical particle touches a planar liquid surface is analysed taking into account the line energy of this perimeter. The minimum size of particle which can float is calculated and an accurate estimate is made for the minimum time of touching of particle and surface. The kinetic energy of collision between an air bubble and the particles is used to calculate the maximum size of particles which can remain attached for flotation. Flotation data suggest that a lower limit for the size of isolated particles which can be floated is of the order of 1 µm and a comparison of this radius with the theoretical limit gives a value of 10–4 to 10–5 dyn for the line energy of the wetting perimeter. Predictions of the flotation region in terms of particle size and contact angle using this energy do not conflict with practical findings.

Attachment of particles to a liquid surface (capillary theory of flotation)

Critical thickness of thin liquid films: Theory and experiment

Line tension in three-phase equilibrium systems

A new method for experimental determination of the line tension at the contact line between a Newton black (NB) film and the bulk solution, “the critical bubble method,” has been developed. The theoretical basis of the method is discussed. In a special device small bubbles of radii R = 10–100 μm are generated and observed. The smallest bubbles do not form NB films. Bubbles of radii greater than the “critical radius” Rc always form NB films. It is assumed that the formation of NB films is hindered by a force barrier due to a positive line tension. The buoyancy force of the bubbles of radii greater than Rc overcomes this barrier. On this basis values of the line tension ϰ are calculated from the experimentally obtained Rc. The dependence of ϰ on the electrolyte concentration is studied in the small CNacl range in which the ϰ values are positive for sodium lauryl sulfate aqueous solutions.

Line tension of Newton black films. I. Determination by the critical bubble method

The effect of the line tension κ on condensation of liquid from supersaturated vapors onto solid spherical nuclei is studied. A negative line tension, which is considered here, leads to a decrease in critical supersaturation and a new mechanism of barrierless phase formation. Critical supersaturation is interpreted as the supersaturation corresponding to the onset of such barrierless condensation. The result is presented graphically, in dimensionless coordinates, for three different wetting angles θ ∞ of an infinitely large drop. Numerical values of critical supersaturation as a function of nucleus radius R 0 are presented for κ = −1.5 × 10 −5 dyn and surface tension for water σ = 72 dyn/cm at 25°C. The most important conclusion is that in a very wide range of R 0 the critical supersaturation remains constant and equals the critical supersaturation at condensation on a flat substrate (i.e., at R 0 = ∞).

A. Scheludko

Papers

Attachment of particles to a liquid surface (capillary theory of flotation)

Critical thickness of thin liquid films: Theory and experiment

Line tension in three-phase equilibrium systems

Line tension of Newton black films. I. Determination by the critical bubble method

Condensation of vapors on spherical nuclei and the line tension effect