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Fundamental understanding of two-phase flow and the concomitant implications of wall wettability and viscosity are critical in areas like microfluidics and lab on chip applications. In this work, numerical investigation of the displacement of two immiscible fluids in a T-junction is presented. The study reveals the interplay of critical physicochemical determinants like capillarity, viscosity, and wettability on the dynamics of two-phase flow. Temporal evolution of the displacement of dispersed phase resulting into different regimes like squeezing, dripping, necking, droplet formation, and jetting for a combination of capillary numbers, viscosity ratios, and wettability scenarios is furnished in detail in order to elucidate the mechanism of droplet formation through the displacement behavior of two-phase flow. The findings establish the surface wettability to be the dominating factor in determining the time evolution of the dispersed liquid interface at a lower capillary number. With the increase in the hydrophobicity of the surface, the liquid interface transits from squeezing to dripping and then to droplet formation at a low capillary number. However, irrespective of wettability, the regime changes from jetting to parallel flows due to an increase in capillary number beyond a critical limit. Furthermore, the phenomenon of squeezing, necking, and breakage occurs relatively earlier with the increase in viscosity ratio and hydrophobicity of the surface. These results may bear significant implications toward designing of droplet dispensing systems with the substrate wettability as a critical controlling parameter.

Purpose The purpose of this study is to numerically analyze the thermal and entropy generation characteristics on two-dimensional, incompressible, laminar single-phase flow of Al2O3-water nanofluid in a micro-channel subjected to asymmetric sinusoidal wall heating with varying amplitude, length of fluctuation period and phase difference of applied heat flux for Reynolds number in the range of 25-1000. Design/methodology/approach The numerical computation is based on the Finite Element Method and the Lagrange finite element technique is used for approximating the flow variables within the computational domain. Findings The average Nusselt number increases with increasing Reynolds number (Re) for all the volume fractions of nanofluid. However, the total entropy generation decreases up to a critical value of Re and increases thereafter. Increase in volume fraction shifts the critical Re towards the lower Re regime. The average Nusselt number and total entropy generation increase with amplitude and length of fluctuation period of heat flux. The optimal choice of volume fraction for lesser entropy generation and higher heat transfer is found to be 3 per cent independent of the value of amplitude, length of fluctuation period and phase difference of the heat flux. Originality/value To the best of authors’ knowledge, the interplay of various parameters concerning non-uniform heating in achieving the maximum heat transfer with minimum irreversibility has not been investigated. Focusing on this agenda, the results of this study would benefit the industrial sector in achieving the maximum heat transfer at the cost of minimum irreversibilities with an optimal choice of inlet Reynolds number, volume fraction of nanofluid, amplitude, length of the period of fluctuation of heat flux and phase difference of applied heat flux.

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