Numerical Study of the Coalescence and Mixing of Drops of Different Polymeric Materials

Abstract

In this study, we employ direct numerical simulation (DNS) to investigate the solutal hydrodynamics dictating the three-dimensional coalescence of microscopic, identical-sized sessile drops of different but miscible shear-thinning polymeric liquids (namely, PVAc or polyvinyl acetate and PMMA or polymethylmethacrylate), with the drops being in partially wetted configuration. Despite the ubiquitousness of the interaction of different dissimilar droplets in a variety of engineering problems ranging from additive manufacturing to understanding the behavior of photonic crystals, coalescence of drops composed of different polymeric and non-Newtonian materials has not been significantly explored. Interaction of such dissimilar droplets often involves simultaneous drop spreading, coalescence, and mixing. The mixing dynamics of the dissimilar drops are governed by interphase diffusion, the residual kinetic energy of the drops stemming from the fact that coalescence starts before the spreading of the drops have been completed, and the solutal Marangoni convection. We provide the three-dimensional velocity fields and velocity vectors inside the completely miscible, dissimilar coalescing droplets. Our simulations explicate the relative influence of these different effects in determining the flow field at different locations and at different time instances and the consequent mixing behavior inside the interacting drops. We also show the non-monotonic (in terms of the direction of migration) propagation of the mixing front of the miscible coalescing drops over time. We also establish that the overall mixing (on either side of the mixing front) speeds up as the Marangoni effects dictate the mixing. We anticipate that our study will provide an important foundation for studying miscible multi-material liquid systems, which will be crucial for applications such as inkjet or aerosol jet printing, lab-on-a-chip, polymer processing, etc.

Coalescence of 3D Polymeric Drops in the Presence of In Situ Photopolymerization

“…A more interesting example will be the case where the heterogeneity stems from the two coalescing droplets being of different but miscible materials. For example, in our previous paper, 19 we studied the coalescence (without in situ curing) of PVAc and PMMA droplets; the large Marangoni effects stemming from the difference in the surface tension values of these materials profoundly influenced the coalescence dynamics. The same problem, if studied in the presence of in situ curing (both PVAc and PMMA are photocurable polymers; hence, both will undergo photocuring) will be extremely important.…”

“…In the first study, 18 the effect of finite impact speed on the postimpact coalescence of two polymeric drops of identical material (of the same or different sizes) was investigated. In the second study, 19 the coalescence of two polymeric drops of different materials was probed. Other notable related works include the papers by Finotello et al 20 and Verdier 21 (both these studies probe the polymeric drop coalescence in the absence of a solid substrate), Dekker et al, 22 Yue et al, 23 and Pawar et al 24 (these three papers consider the coalescence of viscoelastic drops), and so forth.…”

Molecular dynamics simulation of the coalescence of surfactant-laden droplets

“…water or polymer droplets. 6–30 From the point of view of numerical simulations, these have by and large provided descriptions of the macroscopic and dynamic properties of coalescence, 7,8,20–22,31–33 but they generally continue to suffer from inadequate resolution at the pinching point between droplets at the initial stage of coalescence, despite progress in this area. 12 Moreover, a detailed molecular-level description of the mass transport mechanism of surfactant is beyond the reach of any continuum model.…”

Coalescence of surfactant-laden droplets

“…[21][22][23][24] Apart from such natural processes, droplet coalescence is relevant for many industrial applications as well, such as inkjet printing, 25 microfluidics, [26][27][28][29] and water treatment during crude oil and natural gas separation. 30,31 Further control of the process may involve the use of various additives, [32][33][34][35][36][37][38][39] such as surfactant, [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58] which can reduce surface tension at fluid interfaces, crucial in multi-phase systems. For example, surfactant can stabilize droplets' surface or prevent their coalescence, thus improving the bio-compatibility in certain systems 59 or affecting the fusion, mixing, and manipulation of droplets in microfluidic devices.…”