Coalescence of Sessile Polymer Droplets: A Molecular Dynamics Study

Abstract

Droplet coalescence is ubiquitous in nature and, at the same time key to various technologies, such as inkjet printing. Herein, this study reports on the coalescence of polymer droplets with different chain lengths coalescing on substrates of different wettability. By means of molecular dynamics simulations of a coarse‐grained model, it is found that the rate of bridge growth is higher in the case of droplets with smaller contact angles (more wettable substrates) and decreases with the increase of the chain length of the polymers. Different behavior is also identified in the dynamics of the approach of the two droplets during coalescence with the substrate wettability playing a more important role compared to the chain length of the polymers. While the dynamics of the droplet are greatly affected by the latter parameters, the density profile and flow patterns remain the same for the different cases. Thus, this study anticipates that it provides further insights into the coalescence of liquid polymer droplets on solid substrates with implications for relevant technologies.

Molecular dynamics simulation of the coalescence of surfactant-laden droplets

“…A complementary measure of the coalescence dynamics is obtained by monitoring the total length of the droplets in the X direction, whence the velocity of approach u = Ẋ can be obtained. 25,82 Data for droplets with C10E4 surfactant are presented in Fig. 11, and show that the fastest approach takes place in the middle of the process at a time about 150 τ , which is similar for different surfactants.…”

“…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

“…Recent MD simulations of a CG model have indicated the presence of the thermal regime and subsequent inertial scaling law (b ∝ t 1/2 ) for the bridge growth for both aqueous droplets with and without surfactant. 83,84 In the case of the coalescence of sessile droplets, 34,45,81,[85][86][87][88][89][90][91] for water droplets on non-wettable substrates (contact angle θ s ≥ 90 • , Figure 1) it has been suggested that the bridge grows with time as b ∝ t 1/2 , as has been observed in the case of freely suspended droplets. In contrast, for wettable substrates, namely θ s < 90 • , the bridge is predicted to grow with a new power law b ∝ t 2/3 .…”

“…[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.…”

“…66 The bulk fluid properties also affect the bridge growth. For example, in the case of polymer droplets 34,92,93 , their viscosity can result in a lower rate of coalescence in comparison with water droplets. One finds power-law exponents lower than 1/2 for nonwettable substrates (equilibrium contact angle greater than 90 • ) and power-law exponents lower than 2/3 for wettable substrates.…”