Pikina Elena S, Ostrovskii Boris I, Pikin Sergey A
Landau Institute for Theoretical Physics, Russian Academy of Sciences, pr. akademika Semenova 1-A, 142432 Chernogolovka, Russia.
Soft Matter. 2020 May 21;16(19):4591-4606. doi: 10.1039/c9sm02292a. Epub 2020 May 4.
A theoretical study of the interaction and coalescence of isotropic droplets in overheated free-standing smectic films (FSSF) is presented. Experimentally it is clear that merging of such droplets is extremely rare. On the basis of the general thermodynamic approach to the stability of FSSF, we determined the energy gains and losses involved in the coalescence process. The main contributions to the critical work of drop coalescence are due to the gain related to the decrease of the surface energy of the merging drops, which is opposed by the entropic repulsions of elementary steps at the smectic interface between them. To quantify the evolution of the merging drops, we use a simple geometrical model in which the volume of the smectic material, rearranged in the process of coalescence, is described by an asymmetrical pyramid at the intersection of two drops. In this way, the critical work for drop coalescence and the corresponding energy barrier have been calculated. The probability of the thermal activation of the coalescence process was found to be negligibly small, indicating that droplet merging can be initiated by only an external stimulus. The dynamics of drop merging was calculated by equating the capillary force driving the coalescence, and the Stokes viscous force slowing it down. For the latter, an approximation of moving oblate spheroids permitting exact calculations was used. The time evolution of the height of the neck between the coalescing drops and that of their lateral size are in good agreement with experiments.
本文对过热的自支撑近晶薄膜（FSSF）中各向同性液滴的相互作用和聚并进行了理论研究。实验表明，这种液滴的合并极为罕见。基于FSSF稳定性的一般热力学方法，我们确定了聚并过程中涉及的能量得失。液滴聚并临界功的主要贡献来自于合并液滴表面能降低所带来的能量增加，这与它们之间近晶界面处基本步骤的熵斥力相互抗衡。为了量化合并液滴的演化，我们使用了一个简单的几何模型，其中在聚并过程中重新排列的近晶材料体积由两个液滴相交处的不对称棱锥来描述。通过这种方式，计算出了液滴聚并的临界功和相应的能垒。发现聚并过程的热激活概率小到可以忽略不计，这表明液滴合并只能由外部刺激引发。通过使驱动聚并的毛细力与减缓聚并的斯托克斯粘性力相等，计算了液滴合并的动力学。对于后者，使用了移动扁球体的近似方法以进行精确计算。聚并液滴之间颈部高度及其横向尺寸的时间演化与实验结果吻合良好。
