The dynamics of the pendant drop coalescing with a sessile drop to form a single daughter droplet is known to form a bridge. The bridge evolution begins with a point contact between the two drops leading to a liquid neck of size comparable to the diameter of the drops. To probe this phenomenon in polymeric fluids, we quantify the neck radius growth during coalescence using high-speed imaging. In this study, we unveil the existence of three regimes on the basis of concentration ratio c/c*, namely, inertioelastic c/c* < c e /c*, viscoelastic c e /c* < c/c* < 20, and elasticity dominated regimes c/c* > 20. Our results suggest that the neck radius growth with time (t) obeys a power-law behavior t b , such that the coefficient b has a steady value in inertioelastic and viscoelastic regimes, with a monotonic decrease in elasticity dominated regime. On the basis of this dependence of b on concentration ratios, we propose a new measurement technique, rheocoalescence, which possibly can predict the relaxation time of these fluids in the elasticity dominated regime. We also show a deviation from universality proposed in the literature for the elasticity dominated regime.

A quantitative COVID-19 model that incorporates hidden asymptomatic patients is developed, and an analytic solution in parametric form is given. The model incorporates the impact of lock-down and resulting spatial migration of population due to announcement of lock-down. A method is presented for estimating the model parameters from real-world data, and it is shown that the various phases in the observed epidemiological data are captured well. It is shown that increase of infections slows down and herd immunity is achieved when active symptomatic patients are 10-25% of the population for the four countries we studied. Finally, a method for estimating the number of asymptomatic patients, who have been the key hidden link in the spread of the infections, is presented.

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Complex fluids show deviations from their Newtonian counterparts in terms of droplet coalescence. The figure shows transition of such coalescence kinetics from Newtonian into the sub-Newtonian regime.

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