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In the surface tension-dominated microchannel T-junction, droplets can be formed as a result of the mixing of two dissimilar, immiscible fluids. This article presents results for very low Capillary numbers and different flow rates of the continuous and dispersed phases. Through three-dimensional lattice Boltzmann-based simulations, the mechanism of the formation of ''plugs'' in the squeezing regime has been examined and the size of the droplets quantified. Results for Re c ( 1 show the dependence of flow rates of the two fluids on the length of the droplets formed, which is compared with existing experimental data. It is shown that the size of plugs formed decreases as the Capillary number increases in the squeezing regime. This article clearly shows that the geometry effect, i.e., the widths of the two channels and the depth of the assembly, plays an important role in the determination of the length of the plugs, a fact that was ignored in earlier experimental correlations.

This paper reviews the past and recent studies on thermocapillarity in relation to microfluidics. The role of thermocapillarity as the change of surface tension due to temperature gradient in developing Marangoni flow in liquid films and conclusively bubble and drop actuation is discussed. The thermocapillary-driven mass transfer (the so-called Benard-Marangoni effect) can be observed in liquid films, reservoirs, bubbles and droplets that are subject to the temperature gradient. Since the contribution of a surface tension-driven flow becomes more prominent when the scale becomes smaller as compared to a pressure-driven flow, microfluidic applications based on thermocapillary effect are gaining attentions recently. The effect of thermocapillarity on the flow pattern inside liquid films is the initial focus of this review. Analysis of the relation between evaporation and thermocapillary instability approves the effect of Marangoni flow on flow field inside the drop and its evaporation rate. The effect of thermocapillary on producing Marangoni flow inside drops and liquid films, leads to actuation of drops and bubbles due to the drag at the interface, mass conservation, and also gravity and buoyancy in vertical motion. This motion can happen inside microchannels with a closed multiphase medium, on the solid substrate as in solid/liquid interaction, or on top of a carrier liquid film in open microfluidic systems. Various thermocapillary-based microfluidic devices have been proposed and developed for different purposes such as actuation, sensing, trapping, sorting, mixing, chemical reaction, and biological assays throughout the years. A list of the thermocapillary based microfluidic devices along with their characteristics, configurations, limitations, and improvements are presented in this review.

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Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for high Capillary numbers and different flow rates of the continuous and dispersed phases. Through lattice Boltzmann based simulations, a regime map is created that distinguishes parallel flows from droplet flows. Simulations show the dependence of flow rates and viscosity ratio on the volume of droplets formed, which is compared with existing experimental data. At high Capillary numbers, the transition zone which separates parallel and droplet flows shrinks, and is influenced by the viscosity ratio as well.

Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for transitional to high capillary numbers and different ratios of the continuous and dispersed phase flow rates and viscosities. Results are presented for the formation of micron-sized droplets using simulations performed based on a three-dimensional lattice Boltzmann method. The influence of viscosity and geometry of the device on the frequency and volume of droplets formed has been examined and the nondimensional drop size correlated with the capillary number and flow rate ratio. This work reveals two important and new physical features: (a) the transition zone separating droplet and jet flows narrows for high capillary numbers and (b) the critical flow rate ratio separating droplet and parallel flows increases as the dispersed to continuous channel width ratio increases, aspects which have been correlated using a simple relation for both transitions from the droplet-at-T-junction to droplet-in-channel and droplet-in-channel to parallel flow. In the droplet-at-T-junction regime, the droplet formation frequency was recorded as a function of the capillary number, flow rate ratio, and the channel width ratio as well. Results show that the transition to stable jets can be delayed and droplets can be formed even at very high flow rate ratios by significantly increasing the viscosity of the continuous phase.

Heat transfer in silica nanofluids at different acidity and base is measured for various ionic concentrations in a pool boiling experiment. Nanosilica suspension increases the critical heat flux 3 times compared to conventional fluids. The 10-nm particles possess a thicker double diffuse layer compared to 20-nm particles. The catalytic properties of nanofluids decrease in the presence of salts, allowing the particles to cluster and minimize the potential increase in heat transfer. Nanofluids in a strong electrolyte, i.e., in high ionic concentration, allow a higher critical heat flux than in buffer solutions because of the difference in surface area. The formation and surface structure of the deposition affect the thermal properties of the liquid.

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