The excellent wetting and spreading behavior of superspreader solutions has been known and extensively studied over recent years. However, explanations for wetting dynamics and accompanying mathematical models have not yet proved completely successful. Many attempts have been made to quantify the time exponents, but none of the models so far has been able to describe successfully the whole wetting process of trisiloxane solutions, especially on hydrophobic surfaces. We have investigated 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 diffusion at the “subsurface” as the key factor for trisiloxane enhanced wetting. A theory, involving concentration and surface tension gradients governing the dynamic wetting process, has been developed in order to explain the evolution of drop radius and consequent decrease in contact angle. The proposed model, which may explain exceptional wetting of trisiloxanes, was found to be in good agreement with experimental results. Coefficients obtained from application of the model to experimental results lend strong evidence supporting the importance of substrate hydrophobicity as one of the crucial aspects for wetting quality.
Solar energy has been one of the accessible and affordable renewable energy technologies for the last few decades. Photovoltaics and solar thermal collectors are mature technologies to harness solar energy. However, the efficiency of photovoltaics decays at increased operating temperatures, and solar thermal collectors suffer from low exergy. Furthermore, along with several financial, structural, technical and socio-cultural barriers, the limited shadow-free space on building rooftops has significantly affected the adoption of solar energy. Thus, Photovoltaic Thermal (PVT) collectors that combine the advantages of photovoltaic cells and solar thermal collector into a single system have been developed. This study gives an extensive review of different PVT systems for residential applications, their performance indicators, progress, limitations and research opportunities. The literature review indicated that PVT systems used air, water, bi-fluids, nanofluids, refrigerants and phase-change material as the cooling medium and are sometimes integrated with heat pumps and seasonal energy storage. The overall efficiency of a PVT system reached up to 81% depending upon the system design and environmental conditions, and there is generally a trade-off between thermal and electrical efficiency. The review also highlights future research prospects in areas such as materials for PVT collector design, long-term reliability experiments, multi-objective design optimisation, techno-exergo-economics and photovoltaic recycling.
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