Advances in Colloid and Interface Science (v.127, #2)

The thermodynamic model of contact angles on rough, heterogeneous surfaces developed by Long et al. [J. Long, M.N. Hyder, R.Y.M. Huang and P. Chen, Adv. Colloid Interface Sci. 118 (2005) 173] was employed to study the role of energy barriers in determining contact angle hysteresis. Major energy barriers corresponding to metastable states and minor energy barriers corresponding to secondary metastable states were defined. Distributions of major and/or minor energy barriers as a function of apparent contact angle for various surfaces were obtained. The reproducibility of contact angle measurement, the effect of vibrational energy on contact angle hysteresis and the “stick-slip” phenomenon were discussed. Quantitative relations between contact angles and vibrational energy were obtained. It was found that receding contact angles are normally poorly reproducible for hydrophilic surfaces, but for extremely hydrophobic surfaces, advancing contact angles may have a poor reproducibility. When the vibrational energy available to a system increases, the measured advancing contact angle will decrease while the receding angle will increase until both reach a common value: the system equilibrium angle. This finding not only agrees well with the experimental observations in system equilibrium contact angle measurements, but also lays a theoretical foundation for such measurements. A small vibrational energy may result in a “stick-slip” phenomenon.
Keywords: Contact angle hysteresis; Energy barrier; Surface modeling; Rough-heterogeneous surface;

Quantification of particle–bubble interactions using atomic force microscopy: A review by Daniel J. Johnson; Nicholas J. Miles; Nidal Hilal (67-81).
The attachment of particles to bubbles in solution is of fundamental importance to several industrial processes, most notably in the process of froth flotation. During this process hydrophobic particles attach to air bubbles in solution, which allows them to be separated as froth at the surface. The addition of chemicals can help to modulate these interactions to increase the yield of the minerals of interest. Over the past decade the atomic force microscope (AFM) has been adapted for use in studying the forces involved in the attachment of single particles to bubbles in the laboratory. This allows the measurement of actual DLVO (Derjaguin, Landau, Vervey and Overbeek) forces and adhesive contacts to be measured under different conditions. In addition contact angles may be calculated from features of force versus distance curves. It is the purpose of this article to illustrate how the colloid probe technique can be used to make single particle–bubble interactions and to summarise the current literature describing such experiments.
Keywords: Atomic force microscopy; AFM; Particle–bubble interactions; Colloid probe; Force measurement;

Over decades, information about the rheological properties of the condensed monolayer phases has been obtained by introduction of a two-dimensional compressibility which is defined on the basis of the surface pressure–molecular area (ΠA) features of the monolayer. Since the last decade, fundamental progress was attained in the experimental determination of the main characteristics of Langmuir monolayers in microscopic and molecular scale. Already smallest changes in the molecular structure of the amphiphile can result in changes in the molecular arrangement in the monolayer and thus, in changes of the main characteristics of the monolayer such as, the surface pressure–area per molecule (ΠA) isotherms, the shape and texture of the condensed phase domains and the two-dimensional lattice structure. As the classical equations of state allowed only characterisation of the fluid (gaseous, liquid-expanded) state, thermodynamically based equations of state, which consider also the aggregation of the monolayer material to the condensed phase, have been developed.The present review focuses particularly to amphiphilic monolayers, the ΠA isotherms of which indicate the existence of two condensed phases. For this case, the experimental results of the differences in the structure features and phase properties are discussed. The generalisation of the equation of state for Langmuir monolayers developed for the case that one, two or more phase transitions in the monolayer take place, is in agreement with the experimental results that the two-dimensional compressibility of the condensed phases undergoes a jump at the phase transition, whereas the compressibility is proportional to the surface pressure within one of the condensed phases. An example is presented which explains the procedure of the theoretical analysis of ΠA isotherms indicating the existence of two condensed phases. An element of the procedure is the application of the general principle that the behaviour of any thermodynamic system is determined by the stability condition.An interesting anisotropy of the compressibility is revealed by GIXD studies of the S-phase of octadecanol monolayers. However, similar studies performed close to the LS–S-phase transition would result in a thermodynamically impossible negative compressibility. Close to this phase transition, the compressibility cannot be determined from the positions of the maxima because the monolayer is in a disordered state attributed to elastic distortions by fluctuations with the structure of the new phase in the surrounding matrix without destroying the quasi-long-range positional order.
Keywords: Langmuir monolayers; Compressibility; Thermodynamics; Phase transitions; Surface pressure-area isotherms; Brewster angle microscopy; Grazing incidence X-ray diffraction;