Energies associated with the spreading of a small sessile drop

In this work, the spreading of a small sessile drop on a smooth, flat surface has been analyzed. A comprehensive energy balance was developed that consists of wetting energy, air-liquid interfacial energy, gravitational energy, and heat due to flow dissipation. The net change in these energies is predicted to vary widely with the wettability of the solid surface. On the least wettable surfaces, energy released by contraction of the air-liquid interface drives spreading. On the most wettable surfaces, spreading is powered by wetting energy released from the advancing contact line. On surfaces with intermediate wettability, spreading is fueled by a combination of energies released from both the air-liquid interface and the contact line. Generally, gravitational energy is minuscule compared to the other energies. Overall, the model suggests that spreading is exothermic, as expected for a spontaneous process. Small liquid drops with a wide variety of surface tension and density are anticipated to give similar results.