Free Energy and Dynamics of Organic-Coated Water Droplet Coalescence

Organic matter is prevalent in the atmosphere. How such organic matter affects droplet coalescence is a critical issue in atmospheric sciences. In this paper, we investigated the effects of three different organic coatings formed by benzoic acid, heptanoic acid, and pimelic acid on coalescence. We studied the thermodynamics of coalescence by calculating the free energy change and decomposed it into energetic Delta U and entropic T Delta S contributions to understand the molecular details. Delta U was primarily determined by the flexibility of the organic compound, its solubility in water, and hydrogen bond networks. We found a strong positive correlation between T Delta S and the mobility of water and organic molecules. To understand the dynamics of coalescence, we studied collisions between two organic-coated water droplets of the same size at various initial speeds, impact parameters, and collision angles and reported the critical speeds at which coalescence transitions to separation/shattering. We showed that when a collision of the droplets leads to shattering, an organic coating can influence the size distribution of particulate matter that has implications for air pollution. We also discovered that collisions of organic-coated droplets can often induce rotations.