Surface force measurements previously conducted with thiolated gold surfaces showed a decrease in excess film entropy (£GSf), suggesting that hydrophobic force originates from changes in the structure of the medium (water) confined between hydrophobic surfaces. As a follow-up to the previous study, surface force measurements have been conducted using an atomic force microscope (AFM) with hydrophobic silica surfaces at temperatures in the range of 10 to 40¢XC. The silica sphere and silica plate were treated by both chemisorption of octadecyltrichlorosilane (OTS) and physical adsorption of octadecyltrimethylammonium chloride (C18TACl). A thermodynamic analysis of the results show similar results for both of the samples, that both ""Sf and excess film enthalpy ("Hf) become more negative with decreasing thickness of the water layer between the hydrophobic surfaces and decreasing temperature. |"Hf | > |T"Sf| represents a necessary condition for the excess free energy change ("Gf ) to be negative and the hydrophobic interaction to be attractive. Thus, the results obtained with both the silylated and C18TACl-adosrbed silica surfaces in the present work and the thiolated gold suefaces reported before show hydrophobic forces originate from structural changes in the medium. Thermodynamic analysis of SFA force measurements obtained at various temperatures revealed that "Sf were much more negative in the shorter hydrophobic force ranges than in the longer ranges, indicating a more significant degree of structuring in the water film when the two hydrophobic surfaces are closer together. It is believed that the water molecules in the thin liquid films (TLFs) of water form clusters as a means to reduce their free energy when they cannot form H-bonds to neighboring hydrophobic surfaces. Dissolved gas molecules should enhance the stability of structured cluster due to the van der Waals force between the entrapped gas molecules and the surrounding water molecules1, which may enhance the strength of the hydrophobic force. Weaker long-range attractive forces detected in degassed water than in air-equilibrated water was found in the present work by means of AFM force measurements, supporting the effect of dissolved gas on the structuring of water. At last, temperature effects on hydrophobic interactions measured in ethanol and the thermodynamic analysis revealed similar results as those found in water, indicating that the hydrophobic force originates from H-bond propagated structuring in the mediums. / Ph. D.

Modelling the ice accretion in glaze regime for the supercooled large droplets is one of the most challenging problems in the aircraft icing field. The difficulties are related to the presence of the liquid water film on the surface in the glaze regime and also the phenomena associated with SLD conditions, specifically the splashing and re-impingement. The steady improvement of simulation methods and the increasing demand for highly optimised aircraft performance, make it worthwhile to try to get beyond the current level of modelling accuracy. A semi-empirical method has been presented to characterize the thin water film in the icing problem based on both analytical and experimental approaches. The experiments have been performed at the Cranfield icing facilities. Imaging techniques have been used to observe and measure the features of the thin water film in the different conditions. A series of numerical simulations based on an inviscid VOF model have been performed to characterize the splashing process for different water film to droplet size ratios and impact angles. Based on these numerical simulations and the proposed methods to estimate the thin water film thickness, a framework has been presented to model the effects of the splashing in the icing simulation. These effects are the lost mass from the water film due to the splashing and the re-impingement of the ejected droplets. Finally, a new framework to study the solidification process of the thin water film has been explored. This framework is based on the lattice Boltzmann method and the preliminary results showed the capabilities of the method to model the dynamics, thermodynamics and the solidification of the thin water film.

Modelling the ice accretion in glaze regime for the supercooled large droplets is one of the most challenging problems in the aircraft icing field. The difficulties are related to the presence of the liquid water film on the surface in the glaze regime and also the phenomena associated with SLD conditions, specifically the splashing and re-impingement. The steady improvement of simulation methods and the increasing demand for highly optimised aircraft performance, make it worthwhile to try to get beyond the current level of modelling accuracy. A semi-empirical method has been presented to characterize the thin water film in the icing problem based on both analytical and experimental approaches. The experiments have been performed at the Cranfield icing facilities. Imaging techniques have been used to observe and measure the features of the thin water film in the different conditions. A series of numerical simulations based on an inviscid VOF model have been performed to characterize the splashing process for different water film to droplet size ratios and impact angles. Based on these numerical simulations and the proposed methods to estimate the thin water film thickness, a framework has been presented to model the effects of the splashing in the icing simulation. These effects are the lost mass from the water film due to the splashing and the re-impingement of the ejected droplets. Finally, a new framework to study the solidification process of the thin water film has been explored. This framework is based on the lattice Boltzmann method and the preliminary results showed the capabilities of the method to model the dynamics, thermodynamics and the solidification of the thin water film.

In case of metalworking operations, the purpose of lubrication is served by a complex mixture of two or more phases, these mixtures are known as metalworking fluids (MWFs). For many decades oil-in-water emulsions have been used as metalworking fluids. The particular advantage of using oil-in-water emulsion in metalworking operations is that it combines the cooling property of water and the lubrication property of the oil. To explain the lubrication mechanism for oil-in-water emulsions as metalworking fluids a variety of models and theories has been proposed. To understand the lubrication mechanism, the role of each ingredient in the tribological process needs to be studied. In the present study a model for lubrication which determines force and proximity regimes of droplets based on the droplet size distribution is proposed. Dynamic light scattering (DLS) is used to characterize the emulsions. The small droplets are found to be the ones which enhance lubricity. DLVO (Derjaguin-Landau-Verwey-Overbeek) theory is used to validate the results. The concentration and type of surfactant is found to be the performance controlling parameter. A further analysis of the three interfacial energetics; oil/water, oil/substrate, water/substrate, is studied in the presence and absence of surfactants with the help of a Goniometer, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM). Such energetics reflects the rate at which the excess surfactant molecules accumulate at the water/oil interface and desorb into the phases. The tribological response is recorded using AFM and the nanotribometer (NTR). Frictional response of the chemisorbed self-assembled monolayer of surfactant (sodium oleate) on the steel substrate reflects that a tribofilm helps in lubricating the contact under boundary lubrication by creating a low shear strength material. Water being the continuous phase in oil/water emulsion a thin water layer adjacent to steel substrate is always present. This thin layer on the solid substrate acts as a barrier to the lubricating oil droplets to reach the metal surface. The focus of the present work is also to investigate conditions which permit the disjoining of the water film to allow the oil to lubricate the metal substrate. AFM is used to study the interaction force between an oil droplet and the steel substrate through water. An oil encapsulated SiO2 colloidal probe used to simulate the oil droplet. The charge regulatory status of the substrates and interfaces are found to be critical in mapping the force characteristics when DLVO interaction is considered. The condition for activation of non-DLVO (hydration, hydrophobic, capillary) forces are also identified and found to be dependent on the physical states of surfaces. Disjoining of the thin film can be controlled by selecting surfactants based on interfacial energetics and attractive force characteristic can be achieved to facilitate lubrication.