Effect of the fluid injection configuration on droplet size in a microfluidic T junction

Hydrodynamics of Liquid-Liquid Flows in Micro Channels and Its Influence on Transport Properties: A Review

Hydrodynamics plays a major role in transport of heat and mass transfer in microchannels. This includes flow patterns and flow regimes in which the micro-channels are operated. The flow patterns have a major impact the transport properties. Another important aspect is the pressure drop in micro-channels. In the present review, the experimental and Computational Fluid Dynamics (CFD) studies covering all the above aspects have been covered. The effect of geometrical parameters like shape of channel, channel size, material of construction of channels; operating parameters like flow velocity, flow ratio and fluid properties have been presented and analyzed. Experimental and analytical work of different pressure drop models has also been presented. All the literature related to influence of flow patterns on transport properties like volumetric mass transfer coefficients (VMTC) and heat transfer coefficients (HTC) have been presented and analyzed. It is found that most works in Liquid-Liquid Extraction (LLE) systems have been carried out in slug flow and T-junctions. Models for coupled systems of flow and mass transfer have been presented and works carried out for different coupled systems have been listed. CFD simulations match experimental results within 20% deviations in quantitative and qualitative predictions of flow phenomena for most research articles referred in this review. There is a disparity in prediction of a generalized regime map and a generalized regime map for prediction of flow patterns for various systems would need the help of Artificial Intelligence.

Controlled viscoelastic particle encapsulation in microfluidic devices

The encapsulation of particles in droplets using microfluidic devices finds application across several fields ranging from biomedical engineering to materials science. The encapsulation process, however, is often affected by poor single encapsulation efficiency, quantified by the Poisson statistics, with droplets containing more than one particle or with several empty droplets. We here demonstrate that viscoelastic aqueous solutions of xanthan gum enable controlled single particle encapsulation in microfluidic devices with a single encapsulation efficiency up to 2-fold larger than the one predicted by the Poisson statistics. We achieved such a result by identifying viscoelastic xanthan gum aqueous solutions that could drive particle ordering before approaching the encapsulation area and simultaneously form uniform droplets. This is the first experimental evidence of viscoelastic encapsulation in microfluidic devices, the existing literature on the subject being focused on Newtonian suspending liquids. We first studied the process of viscoelastic droplet formation, and found that the droplet length normalised by the channel diameter scaled as predicted for Newtonian solutions. At variance with Newtonian solutions, we observed that the droplet formation mechanism became unstable above critical values of the Weissenberg number, which quantifies the elasticity of the xanthan gum solutions carrying the particles. In terms of controlled encapsulation, we discovered that the single encapsulation efficiency was larger than the Poisson values in a specific range of xanthan gum mass concentrations. Finally, we introduced an empirical formula that can help the design of controlled viscoelastic encapsulation systems.

Analytical relations for long-droplet breakup in asymmetric T junctions

We develop accurate analytical relations for the droplet volume ratio, droplet length during breakup process, and pressure drop of asymmetric T junctions with a valve in each of the branches for producing unequal-sized droplets. An important advantage of this system is that after manufacturing the system, the size of the generated droplets can be changed simply by adjusting the valves. The results indicate that if the valve ratio is smaller than 0.65, the system enters a nonbreakup regime. Also the pressure drop does not depend on the time and decreases by increasing the valve ratio, namely, opening the degree of valve 1 to valve 2. In addition, the results reveal that by decreasing (increasing) the valve ratio, the droplet length of branch 1 decreases (increases) and the droplet length of branch 2 increases (decreases) linearly while the whole length of the droplet remains unchanged.