Articles written in Sadhana

    • An image processing method to measure droplet impact and evaporation on a solid surface


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      We employ high-speed visualization to quantify the impact and evaporation of a droplet on a solid surface. The time-varying droplet shapes are recorded using a high-speed camera and an image processing method is developed for measurement for droplet dimensions, dynamic contact angle, and volume. The method is based on the liquid-gas and liquid-solid interface detection using Canny’s method and Otsu’s method for threshold digitalization. To obtain the dynamic contact angle, a higher-order polynomial is fitted along the pixelsnear the contact line. The droplet volume is measured assuming an axisymmetric geometry of the droplet and numerically integrating the identified pixels in the droplet. We demonstrate the detection of the complex topology of the deforming liquid-gas interface of an impacting and evaporating droplet on a solid surface. We plot the time-variation of droplet dimensions, namely, wetted diameter, maximum diameter, height; dynamic contact angle, and droplet volume. The contact angle and volume measurements are verified with known analytical values. Three case studies are considered to demonstrate the capability of the proposed method, namely, sessile evaporating droplet, bouncing droplet, and spreading viscous droplet on a solid surface

    • A review on coronavirus survival on impermeable and porous surfaces


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      We review recent studies on fomite transmission of COVID-19, caused by the novel coronavirus. In particular, we focus on survival time of coronavirus on solid and porous surfaces. Since the aqueous phase of a respiratory droplet serves as a medium for virus survival, evaporation of the droplet on a surface plays a crucial role in determining the virus survival time. While the bulk of the droplet takes a few seconds to evaporate, previous virus titer measurements revealed that the virus can survive for several hours or days on a surface. This long survival of virus has been attributed to a residual thin-liquid film which remains after drying of the bulk droplet. The evaporation of the thin-film is governed by the disjoining pressure within it and therefore, is a much slower process which causes the virus to survive longer. However, the aforesaid disjoining pressure is significantly modulated for the case of porous surfaces due to their typical geometries. This accelerates the thin-film evaporation on porous surfaces and thereby making them lesser susceptible to virus survival. Therefore, porousmaterials are deemed to be relatively safer for mitigating the spread of COVID-19 via fomite transmission. Using results of the reported research, we briefly discuss the possible recommendations to mitigate the spread of the disease.

    • A short review on optical interferometry techniques for characterization of a thin liquid film on a solid surface


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      Thin films of liquids formed on solid surfaces are of both fundamental and industrial importance. Therefore, the detection and analysis of thin-film profiles have been at the attention of the scientific community for decades. However, due to the small-scale nature, there exists a significant challenge to characterize thin films experimentally. The goal of the present review is to shed light on the recent developments in optical interferometry techniques for the characterization of thin-films. The review includes the efforts devoted to looking into thin-films of several applications, such as falling thin-films, and liquid crystal thin-films; by virtue of optical interferometry. Thereafter, how the technique has been extended to tribology has been reviewed. At last, the efforts devoted to combining reflectometry and interferometry for the characterization of thin liquid films with enhanced accuracy have been outlined.

    • Finite-element method based solver for an evaporating sessile droplet on a heated surface


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      We report the development of a finite-element-based solver to compute transport of mass, momentum and energy during evaporation of a sessile droplet on a heated surface. The evaporation is assumed to be quasi-steady and diffusion-limited. The heat transfer between the droplet and substrate and mass transfer of liquid-vapor are solved using a two-way coupling. In particular, here, we develop and implement the formulationof fluid flow inside the droplet in the model. The continuity and Navier–Stokes equations are solved in axisymmetric, cylindrical coordinates. Jump velocity boundary condition is applied on the liquid–gas interface using the evaporation mass flux. The governing equations are discretized in the framework of the Galerkin weight residual approach. A mesh of finite triangular elements with six nodes is utilized, and quadratic shapefunctions are used to obtain the second-order accurate numerical solution. Two formulations, namely, penalty function and velocity pressure, are employed to obtain discretized equations. The numerical results are the same using both methods, and the latter is around 30–50% faster than the former for the cases of refined grid. Computed flow fields are in excellent agreement with published results. The solver’s capability is demonstrated by solving the internal flow field for a case of a heated substrate.

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