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 frame, 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.
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Here, we report the intriguing movements of an extended liquid meniscus on a silicon substrate under the influence of sinusoidal alternating current (AC) voltages at different operating frequencies. As opposed to droplet electrowetting, wherein the droplet spreads and experiences oscillations at the free surface, the application of AC voltage to a thin liquid film results in distinct and uniform dewetting, in conjunction with augmented wetting. Image analyzing interferometry is used for the precise measurement of the film thickness profile and other associated parameters. We postulate that the classic Young-Lippmann equation fails to explain the dynamics of an extended meniscus and evince that the dynamics of film displacement could be successfully explained by considering the product of the applied electric field and its gradient, as opposed to the existing consideration of a square dependence on the applied voltage. The physics of the hitherto unreported phenomena is elucidated by developing a mathematical model, taking into consideration all of the germane forces governing the dynamics of the thin liquid film. We affirm that the present study would serve as a fundamental background for a fascinating mode of liquid actuation, with inherent application potential in several existing and novel microfluidic systems.
Acoustic waves with screw dislocations at their wavefronts, or acoustic vortices, are characterized by an azimuthal phase dependence. The time average of an acoustic vortex forms a circular region of high pressure which can be used to trap particles, and by manipulating the vortex axis, it can controllably translate them. In addition, acoustic vortices carry orbital angular momentum which can be transferred to absorbing particles causing them to rotate. This paper describes various new observations of these phenomena. In the first example, microparticles are manipulated in a host liquid at high ultrasonic frequencies (2 MHz), and a controlled translation and rotation is demonstrated. In this viscously dominated system, small particles rotate slowly with the liquid and larger particles are drawn into the centre of the vortex. The particle dynamics are explained as a fine balance between the transfer of angular momentum and the action of acoustic radiation forces. In the second example, much larger particles are levitated in air in the low ultrasonic range (40 kHz) and controlled translation and rotation is again observed. Here, viscosity is shown to play a lesser role in the particle dynamics, and conditions are observed under which the particles are rapidly ejected from the vortex core.
Diabetes, a chronic condition, is one of the prevalent afflictions of the 21st century, and if left unchecked, this ailment could lead to severe life-threatening complications. A widely accepted methodology for monitoring diabetes is the estimation of the glucose and ketone contents in the body fluids, viz. blood, urine, etc. Additionally, certain conditions such as starvation, and following a protein-rich diet (e.g., keto-diet) could also lead to significant changes in the ketone content, thereby resulting in a false-positive diagnosis. Hence, a precise, portable, and on-demand procedure for the rapid and combined estimation of glucose and ketone in bodily fluids is of utmost importance. To that end, paper-based analytical devices (μPADs) are promising tools, owing to their multitudinous advantages, and compatibility with biofluids. Although, numerous researchers have contributed substantially in the fundamental investigation, design, and fabrication of μPADs for various applications, a combined platform capable of rapid, accurate, and on-demand glucose and ketone detection, that is easy to fabricate, is still relatively unexplored. Moreover, the flow dynamics of an analyte, in combination with enzyme-catalyzed (for glucose) and uncatalyzed reactions (for ketone), within a porous paper matrix is also vaguely understood. Herein, we present a facile laser-printing-based fabrication of colorimetric sensors on a filter paper, for rapid, and non-invasive estimation of glucose and ketone contents in urine. The urine sample, upon being deposited in a particular expanse, is wicked through the paper matrix, and reacts with specific reagents in the designated zone(s), giving rise to final color, concomitant with the glucose or ketone content in the sample. The device design enables the liquid to be wicked into the porous matrix in a way that would concentrate the colored product in a dedicated detection zone, thereby augmenting the feasibility for accurate colorimetric detection. Furthermore, we present for the first time, a detailed dynamic model of the flow-field in a variable cross-section paper device using the Richards equation, while also considering the species transport and reaction kinetics within the porous media. The results of the numerical simulation agree well with those observed experimentally, thereby validating the present model. Finally, we also developed a web and desktop-based application that would enable the user to upload the images of the colored zones to provide an accurate estimate of the glucose and ketone content in the sample. We believe that our model, in combination with the proposed fabrication methodology, and the in-house developed app., would enable rapid and reliable fabrication of μPADs for various fundamental investigations, and applications pertaining to affordable healthcare monitoring.
Drying of complex fluids leads to the formation of several intricate patterns where each component plays a unique role in determining the final dried morphologies. Salts are essential to body fluids (viz. blood, urine, saliva) and buffer solutions. Hence, understanding the sole effect of salts on the dried patterns of complex fluids has become imperative. In the present investigation, the exclusive effects of some commonly available salts on the final dried patterns of model solutions in the presence of polystyrene particles have been explored. Fascinating results have been observed as the sole presence of salts was found to alter the final dried patterns of the particle suspensions. To delve deeper into the physics of evaporation dynamics, a qualitative analysis has also been undertaken to estimate the predominant forces affecting the dried droplet morphologies. This investigation can serve as a baseline for understanding the underlying mechanisms involved during the drying of complex fluids to further aid in disease diagnosis.
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