Molecular dynamics simulation of the coalescence of nanometer-sized water droplets in n-heptane

Abstract

Molecular dynamics simulations using a modified Drieding 2.21 force field were carried out to study the coalescence behavior of nanometer-sized water droplets in vacuum and in n-heptane. The coalescence mechanisms of the water droplets in the above-noted environments are fairly similar in a sense that the water droplets form a bridge linking the droplets before they merge. However, in the latter situation, due to the presence of n-heptane molecules in between the water droplets, the coalescence was observed to be slowed down considerably, especially in the first 10 ps of the process. However, once the bridge is formed, the water droplets, in both situations, spend about the same amount of time to form a single droplet. The maximum distance between the droplets above which coalescence does not occur was found to be 10 A. In terms of the dynamics, the diffusion coefficient of n-heptane in the emulsion system was very close to its value in the pure liquid form. This may be because n-heptane is the continuous phase. Nonetheless, the dynamic behavior of water in n-heptane is different from that of pure water during and after the coalescence. In particular, the self-diffusion coefficient of water molecules in n-heptane is about 20% higher than the experimental value of pure water. Due to the lack of strong attraction forces between water and n-heptane molecules, the n-heptane molecules were observed to orient themselves perpendicularly to the water/n-heptane interfaces so that the contacting area is minimized.

Drop coalescence in technical liquid/liquid applications: a review on experimental techniques and modeling approaches

“…The application of MD to liquid/liquid systems is computationally more expensive and only two publications are known to the authors so far. Zhao and Choi (2004) investigated clusters of 100 water molecules coalescing in n-heptane showing that the dynamics of the coalescence process until the formation of the coalescence bridge differs from the case in vacuum. However, the confluence of the droplets after the bridge formation was comparable in n-heptane and vacuum.…”

Coalescence Behavior of Water Nanoclusters: Temperature and Size Effects

“…Initially, the two nanoclusters in equilibrium were placed with their center-to-center distance at L 0 ) 2R + 3 Å (as stated previously and depicted in Figure 1), where R denotes the radius of the nanoclusters and the separation distance (3 Å) is selected the same as that utilized in related studies. 7,9,16,17 Thus at the beginning of the coalescence, L equals to L 0 and the value of shrinkage is zero. As the nanoclusters start to coalesce, L becomes less than L 0 and the value of shrinkage grows from zero.…”

“…In addition, the obtained morphology is found to be similar to that of Koplik and Banavar 8 as well as Zhao and Choi. 9 Since arrangements of water molecules are quite different at the three coalescence stages of two water nanoclusters, temperature variations as well as the standard deviation of the temperature at the three stages are noteworthy and are examined in …”

“…Any distance small enough to be within the interaction range of the intermolecular forces is suitable for the simulations. In the present study, the value of 3 Å employed by Zhao et al 9 for coalescence of water droplets in vacuum was also invoked in the simulations. It is noted that this value was also appropriate for coalescence (or sintering) analysis of metal nanoparticles using molecular dynamics simulations.…”

Inertial impedance of coalescence during collision of liquid drops

“…2012; Anthony 2017). For length scales below the continuum limit, coalescence of liquid drops has also been investigated using molecular dynamics simulations (Koplik, Pal & Banavar 2002; Zhao & Choi 2004). The limits of continuum mechanics and models for fluid flows, and transition to molecular scales, have been reviewed in detail by Hadjiconstantinou (2006).…”