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Classical Mechanics

Coalescence dynamics of unequal sized drops

Profile image of Gautam BiswasGautam Biswas

2019, Physics of Fluids

https://doi.org/10.1063/1.5064516
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Abstract

In this study, coalescence dynamics of two unequal sized drops of the same liquid have been investigated using the coupled level set and volume of fluid method. A broad range of fluid properties is considered with two orders of magnitude variation of Ohnesorge numbers and Atwood number ranging between 0.01 and 0.9976. The pinch-off process and controlling parameters that lead to satellite generation have been investigated. The capillary waves are generated as a result of the sharp curvature produced near the contact region. Here we demonstrate that the capillary waves propagating along the interface of the lower drop can affect the eventual pinch-off of the satellite. The local curvature of the neck plays a crucial role in the pinch-off process. A sharper axial curvature of the neck increases the local capillary pressure which restricts the pinch-off. The critical diameter ratio above which a satellite pinches off during the coalescence of two free-falling drops increases with incre...

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    Kirti Sahu

    Physics of Fluids

    The coalescence dynamics of an ethanol droplet freely falling on a sessile ethanol droplet is investigated experimentally using a high-speed imaging system. The regime maps showing the partial coalescence and spreading behaviors in the plane of the Weber number (We) and the volume of the sessile droplet (V p) normalized with the volume of the impacting droplet (V i) have been presented. The partial coalescence phenomenon is observed when the ratio of the volume of the sessile droplet to that of the impacting droplet (V p /V i) is greater than two. For V p /V i = 2, the size of the daughter droplet is found to be about 0.1 times as that the impacting droplet, which increases with the increase in the Weber number (We) and normalized volume of the sessile droplet. In the present study, the negative curvature of the droplet coupled with the presence of the substrate leads to a different coalescence dynamics.

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    Effect of surface tension gradients on coalescence dynamics of two unequal-sized drops
    Arun Saha

    Physical Review Fluids

    The coalescence dynamics of two unequal-sized drops made of miscible but distinct liquids has been numerically simulated using the coupled level set and volume of fluid method. Effect of the surface tension ratio of two liquid drops, the Ohnesorge number and the diameter ratio of two parent drops on three different pinch-off regimes, namely, first-stage, second-stage, and no pinch-off, are explored. The result shows that the coalescence process of two miscible liquid drops exhibits a nonmonotonic behavior of partial coalescence from appearance to disappearance and then reappearance with decreasing surface tension ratio. The strong lifting force of the intense Marangoni flow causes the reappearance of partial coalescence at higher surface tension difference between two drops. When the Ohnesorge number increases, high viscous forces restrict the propagation of Marangoni flow and do not favor the pinch-off, even in the presence of a significant surface tension difference. The generation of secondary drops at a considerable surface tension difference is also prevented for small parent drop size ratio. A regime map for the distinct coalescence outcomes is presented for different surface tension ratios. The critical surface tension ratio necessary for partial coalescence to occur grows monotonically as the Ohnesorge number increases. In our simulation, the minimum value of critical surface tension ratio needed for partial coalescence to take place is 0.94 at small values of the Ohnesorge number and the Bond number.

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    A study of two unequal-sized droplets undergoing oblique collision
    Gautam Biswas

    Physics of Fluids, 2021

    The oblique collision of two unequal-sized liquid droplets in a gaseous environment is investigated numerically. It is found that the asymmetry in the flow field arising due to unequal-sized droplets and oblique collision greatly alters the collision outcomes observed in the case of the head-on collision of identical droplets. Our results reveal that permanent coalescence occurs at intermediate collision angles, but head-on and large-angle collisions result in reflexive separation and stretching separation, respectively. Moreover, we found that the end-pinching mechanism is operational in the case of head-on collision, and the capillary wave instability is responsible for the ligament breakup for large collision angles. It is also observed that the droplets coalesce permanently for low velocity ratios and high radius ratios, but for high velocity ratios and low radius ratios, the droplets coalesce temporarily and then split again. By conducting a large number of numerical simulations, the collision outcomes and the boundary separating them are plotted on R r À We and h À We planes, where We, R r , and h represent the Weber number, radius ratio of droplets, and collision angle, respectively.

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    Microstructural evolution of silicate immiscible liquids in ferrobasalts
    Prof. Marian B Holness

    Contributions to Mineralogy and Petrology, 2019

    An experimental study of the microstructural evolution of an immiscible basaltic emulsion shows that the Fe-rich liquid forms homogeneously nucleated droplets dispersed in a continuous Si-rich liquid, together with droplets heterogeneously nucleated on plagioclase, magnetite, and pyroxene. Heterogeneous nucleation is likely promoted by localised compositional heterogeneities around growing crystals. The wetting angle of Fe-rich droplets on both plagioclase and magnetite increases with decreasing temperature. Droplet coarsening occurs by a combination of diffusion-controlled growth and Ostwald ripening, with an insignificant contribution from coalescence. Characteristic microstructures resulting from the interaction of immiscible Fe-rich liquid with crystal grains during crystal growth can potentially be used as an indicator of liquid unmixing in fully crystallised natural samples. In magma bodies < ~ 10 m in size, gravitationally driven segregation of immiscible Fe-rich droplets is unlikely to be significant.

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