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Wetting phenomena of underwater systems do not always follow the classical Wenzel and Cassie–Baxter configurations.

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We have derived a modified one-dimensional lubrication equation to describe the early coalescence behavior of a symmetric sessile drop coalescence for under-liquid substrates, which takes into account the viscosities of both the drop and the surrounding medium. We found a time scale, which governs the process, and there exists a crossover time between the universal scaling of the bridge height growth h^{*}∼t^{*} (valid for both under-liquid and air) and a much slower bridge growth h^{*}∼t^{*}^{0.24} occurring at a later time. It is also found that the drop coalescence bridge profile has a self-similarity, which breaks up much earlier for under-liquid substrates as opposed to symmetric coalescence in air.

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Modeling and simulation are very powerful tools and have become an integral part in the design and development of engineering systems. In the microscale domain, modeling and simulation have been applied in the development of microfluidic devices. Microdevices developed for sample mixing, sample dispersion, drug discovery, biochemical analysis and micropower systems have become possible only due to the impetus and insight gained from the simulation studies of fundamental microfluidic problems. Microfluidic device modeling comprises of dealing with interplay of multiphysics phenomena such as fluid flow, structure, surface and interfaces, etc. As the surface area to volume ratio increases with the decrease of the system feature size of microdevices, some physical phenomena which are insignificant in the macro domain become prominent in the micro domain. Some of the tried and tested macroscale theory and experimental results no longer show similar trends in the microscale. Hence dealing with simultaneously widely differing physics becomes too complicated in the microscale which include microfluidics, microtransport, microthermal, micromechanics, microlectronics and optics with biochemical thermodynamics and reaction kinetics. In this article, the use of numerical modeling and simulation techniques for flow in microchannels applied to microfluidic devices are presented encompassing all the relevant physics at the microscale by the state of the art multiphysics simulation tools such as CFD-ACE+, COMSOL, Fluent to name a few. Numerical simulation of electroosmotic effect on pressure-driven flows in the serpentine channel of a microfuel cell with variable zeta potential on the side walls is investigated and reported. The Poisson-Boltzmann and Navier-Stokes equations are solved numerically to investigate the electroosmotic driven flow phenomena. It is observed that vortices are developed at the straight portion of the microchannel due to the electroosmosis. Flow control in the serpentine microchannel by regulating the zeta potential at the bend has also been demonstrated. Capillary driven flow in a microchannel with alternate hydrophilic-hydrophobic patterns on the bottom wall is investigated for bioreactor applications. The transient flow is modeled by coupling the incompressible Navier-Stokes equation with the interface evolution equation using volume of fluid (VOF) methodology. Higher order surface reconstruction method is adopted for interface tracking. Flow instability increases as the fluid traverses alternately between the hydrophilic and hydrophobic regions. Such flow phenomena in the microchannel indicate that flow control is possible by patterning the channel walls for applications related to 1 Corresponding Author; Email: skmitra@iitb.ac.in www.intechopen.comRecent Advances in Modelling and Simulation 284 microfluidic devices. The simulation of dynamic interaction between an elastic membrane structure and fluid in a two-dimensional microchannel is also reported here. The viscous and pressure forces imposed by t...

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