Elasticity can affect droplet coalescence

Abstract

Coalescence of two droplets on a solid substrate is an interfacial phenomenon that imposes the challenges of capturing the complex contact line motion and energy interaction between the solid-liquid interface. Recent investigations on the coalescence of polymeric droplets on a solid substrate have reported strong disagreements; the heart of the issue is whether coalescence of polymeric drops is similar to that of Newtonian fluid and is independent of molecular relaxation, or whether the role of entanglement of polymeric chains leads to a transition kinetics different from that of Newtonian fluid. Via this article, we resolve the disagreements through a discussion on the effects of merging method on the dominant forces governing the coalescence process, i.e., inertia, dissipation, and relaxation. In this regard, two methods of merging have been identified, namely droplet spreading method (DSM) and volume filling method (VFM). Our study unveils that the coalescence dynamics of polymeric drops is not universal and in fact, is contingent of the method by which the coalescence is triggered. Additionally, we demonstrate the spatial features of the bridge at different time instants by a similarity analysis. We also theoretically obtain a universal bridge profile by employing the similarity parameter in a modified thin film lubrication equation for polymeric fluids.

Sub-Newtonian coalescence in polymeric fluids

“…As the upper convective derivative is the predominant term, the present theory unifies these continuum based constitutive equations to eqn (5). However, for the coalescence of two sessile polymeric drops, 13 a theoretical model which is independent of characteristic stresses has been proposed for viscoelastic and elasticity dominated regimes using thin film lubrication approximation.…”

“…One such case that violates the condition is when P = 0 as the term M 2 is always negative. On substituting P = 0, we obtain eqn (13) that can be further simplified to obtain a cut-off radius in terms of material properties at which the coalescence is arrested. 36 As observed from Table 1, the value of the term j À M 1 M 2 j is bounded by O(10)/ Ec À1 .…”

“…Each subclass has a different micro-structure composition leading to distinct behaviors. However, there are few recent studies on a special class of non-Newtonian fluids i.e., macromolecular fluids [10][11][12][13][14] that have highlighted the deviation from proposed Newtonian behaviour. But a generalized theoretical framework unifying various constitutive laws to probe the phenomenon in viscoelastic droplets remains an open question.…”

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

“…Such a consideration neglects the change in the elasticity of the drops during the photopolymerization process. Given the significant impact of the elasticity effects during the coalescence of viscoelastic drops, ,− as revealed in a few recent studies, this is a notable limitation of this paper. In this paper, the same set of equations as considered in our previous paper have been used; however, instead of a single drop, in this paper, we consider the spreading and subsequent coalescence of two drops in the presence of in situ curing.…”

“…Interestingly, such an initial regime (where viscous forces balance capillary forces) in bridge growth has not been explicitly mentioned in previous studies, although several experimental studies (particularly for high-viscosity liquids) have pointed out the presence of a regime (prior to the regime where inertial forces balance the capillary forces) where the bridge growth is significantly retarded. 28,29 We also study the corresponding time evolution of the temperature and curing profiles inside the drop. The exothermic nature of the curing reactions ensures that for case 2 (characterized by rapid polymerization), there is a substantial reduction of temperature at the beginning of the coalescence, while for case 1, the temperature reduction is insignificant.…”