Magnetowetting of Ferrofluidic Thin Liquid Films

Srinivas Tenneti, Sri Ganesh Subramanian, Monojit Chakraborty, Gaurav Soni, Sunando DasGupta
<span title="2017-03-17">2017</span> <i title="Springer Nature"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/tnqhc2x2aneavcd3gx5h7mswhm" style="color: black;">Scientific Reports</a> </i> &nbsp;
An extended meniscus of a ferrofluid solution on a silicon surface is subjected to axisymmetric, non-uniform magnetic field resulting in significant forward movement of the thin liquid film. Image analyzing interferometry is used for accurate measurement of the film thickness profile, which in turn, is used to determine the instantaneous slope and the curvature of the moving film. The recorded video, depicting the motion of the film in the Lagrangian frame of reference, is analyzed frame by
more &raquo; ... e, eliciting accurate information about the velocity and acceleration of the film at any instant of time. The application of the magnetic field has resulted in unique changes of the film profile in terms of significant non-uniform increase in the local film curvature. This was further analyzed by developing a model, taking into account the effect of changes in the magnetic and shape-dependent interfacial force fields. Process intensification in the micro-scale domain has been garnering attention due to their inherent abilities and advantages to control the transport phenomena with a minimal expense of resources 1,2 . Micro-Electro-Mechanical-Systems (MEMS) and nanotechnology have been paving way for newer and more advanced discoveries 3 such as Polymerase Chain Reaction (PCR) 4 , microsystems for DNA amplification and identification 5 , enzyme linked immunosorbent assay (ELISA) 6 , capillary electrophoresis 7 , electroporation 8 , biochips for the detection of hazardous chemical and biological agents 9 and microsystems for high throughput drug screening and selection 10,11 . Another well defined and established application of microfluidics and microfabrication is in the domain of device fabrications, utilizing photolithography 12,13 . Integrated circuits (IC's), the basic building blocks of any electronic device are becoming more complex, efficient and affordable due to the advancements in photolithography. However, the performance of IC's is limited by their significant heat generation, creating a need for the rapid development in alternate cooling strategies, such as micro-heat pipes 14 , thin film micro-refrigerators, etc 15,16 . Droplet based micro fluidics 17 that operate on electrowetting is also becoming a popular alternative for micro-chip cooling. The theory of electrowetting 18,19 along with its applications have been reviewed and reported by various researchers. The phenomenon of droplet electrowetting has been exploited on various surfaces and processes, such as paper 20 , carbon nanotubes 21 , liquid infused films 22 and more recently for energy harvesting and storage 23 and hot spot cooling enhancement 24,25 . Droplet motions with specific applications in lab on a chip devices have been extensively analyzed and reported by various researchers [26] [27] [28] [29] . Dynamics of droplets subjected to various modes of perturbation such as acoustic 30 , thermal 31 , optical 32 and magnetic 33 have been studied extensively, whereas droplet manipulation through magnetic actuation, with advancements in point of care medical diagnostics has been reported by Shin et al. 34 . The related concepts have been further extended to thin liquid films as well. Subjecting a thin liquid film to external perturbations resulting in the actuation of liquid motion is used to probe and manipulate the transport phenomena near the contact line region. The analysis of the dynamics of partially wetting thin liquid films, experiencing an external perturbation, is a complex process [35] [36] [37] . Evaporating thin liquid films may provide very high heat transfer coefficients across the liquid-vapour interface 38,39 . The physics associated with the dynamics of thin liquid films undergoing cycles of evaporation and condensation was first studied and quantified by Potash and Wayner 39 . The hydrodynamic flows associated with thin liquid films when subjected to continuous evaporative-condensing cycles in micro heat pipes has been studied and reported by Swanson et al. 40 . Since then, the response of thin liquid films subjected to various modes of perturbation such as thermal 38,41 and electrical 35 have been investigated and reported. Extensive reviews on intermolecular forces and interactions present in a thin liquid-solid-vapour system have been presented by Derjaguin et al. 42 . The contact angle hysteresis and
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