Papers by Author: Martin E.R. Shanahan

Abstract: The excellent spreading and wetting behaviour of superspreader solutions has been known and extensively studied over recent years. However, explanations for spreading dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the spreading exponents, but none of the models so far was able successfully to describe the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated the partial wetting of Silwet L-77® superspreader solutions of high concentrations (well above CMC) on polymer coated substrates of varying hydrophobicity. Results obtained can be explained in terms of the Marangoni effect as the major driving force for trisiloxane enhanced spreading. A simple theory, which involves surface tension gradients governing the spreading process, was developed in order to explain the specific evolution of the drop radius and consequent decrease in the contact angle. The proposed model was found to be in excellent agreement with the experimental results. Determined equation coefficients were shown to be dependent on both surfactant concentration and the hydrophobicity of the substrate.

Abstract: Preliminary results on a technique of Electrochemical-Impedance Spectroscopy (EIS) applied to the characterization of diffusion and absorption properties of water in an epoxy resin are reported. Thin films of polymer adhering to aluminum constitute simulated adhesive bonds. Immersion in water and assessment via electro-chemical impedance measurements allowed estimation of diffusion rates and water uptake. In parallel, classic gravimetric experiments were undertaken on thin, free films of the same polymer. Results of the two techniques are acceptably comparable, and thus EIS could be developed as a new tool for in situ assessment of adhesives.

Abstract: An experimental investigation into the evaporation of sessile nanofluid droplets is reported in this paper. The effect of nano-particle addition on the evaporative behaviour is studied using ethanol and Titanium Oxide nano-particles. The results show that a distinct ‘stick-slip’ pinning behaviour is observed when nano-particles are added to the base liquid. Increasing the nano-particle concentration was found to enhance the ‘stick-slip’ behaviour. This behaviour is attributed to the effects of evaporatively driven particle accumulation near the contact line. This in turn leads to an increase in localised viscosity, and an enhancement of contact line pinning.

Abstract: The wetting and evaporation behaviour of methanol-water droplets deposited on a smooth silicon substrate were investigated experimentally. Contact angle and droplet shape kinetics were studied using an optical technique. Drops were deposited onto a silicon substrate and enclosed in a cell with nitrogen as the ambient gas. Besides the case of pure water and pure methanol, three different volume fractions of methanol in water were investigated: 10%, 50% and 80%. Using a Kruss DSA100 contact angle analyser, the behaviour of the contact angle, droplet volume, and base width was determined as a function of time. Results show that evaporation of the droplet takes place in successive stages for mixtures. The more volatile component seems to evaporate principally in the first stage, during which the contact angle of the binary drop is closer to that of pure methanol. Because the wetting behaviour is partly dictated by the surface tension of the liquid-vapour interface, methanol is believed to be concentrated at the interface during this first stage. After complete evaporation of the methanol, the wetting behaviour of the droplet tends towards that of pure water. The mechanisms that dictate the evaporation and wetting behaviour of such binary droplets include many effects: diffusion of methanol in water in the liquid phase; accumulation of one of the component near the interface and preferential evaporation followed by diffusion of one component in another in the vapour phase. In order to model the phenomenon, the above effects must be taken into account. Solutal Marangoni stress as well as interfacial instabilities may also play an important role in the behaviour of theses systems.

Abstract: Water diffusion in polymers can often be approximated by a Fickian description, but a 2- phase model was proposed some years ago by Carter and Kibler (C&K), often referred to as “Langmuirtype” diffusion, by analogy with the Langmuir theory of adsorption. The two phases in question correspond to “mobile” and “bound” diffusant molecules. In this study, we have considered water uptake in an epoxy resin (an adhesive), employing gravimetry. A good, overall, empirical agreement with the C&K mathematical description of total mass increase with time has been obtained. In many applications of the C&K theory when used to quantify diffusion of water in polymers, only total water uptake is considered as a datum. However, a simple mathematical treatment of the theory enables the separate mobile and bound contributions to be isolated. These supplementary data have been used to try to get a better understanding of the meaning of the terms “mobile” and “bound” phases. Deuterium NMR analysis has been employed to study the mobility of the absorbed water. Decomposition of spectra has permitted us to assign two signals to the fractions of “mobile” and “bound” water. Analysis of peak evolution and a comparison with gravimetric data lead us to suggest that the “mobile” phase corresponds to diffusing molecules, whereas the “bound” phase corresponds to “clusters”.