Optical Investigation of the Interfacial Phenomena during Coalescence of two Condensing Drops and Shape Evolution of the Coalesced Drop

Image analyzing interferometry is used to study the details of the shapes and coalescence of condensing drops of 2-propanol on a quartz surface. The measured thickness profiles give fundamental insights into the transport processes within the drop and its evolution from asymmetric to symmetric shape. The results show the presence of a thin flat film adjacent to the condensing drops and the adsorbed film thickness is found to be constant and independent of the point of measurement. We found that in addition to the apparent contact angle, the determination of the profiles of the local interfacial tangent, curvature and curvature gradient enhances the description of the growth and coalescence of the drops. We observed that the drops coalesce and there is a liquid flow from the smaller drop towards the bigger drop due to the difference in their shape dependent pressure fields. This was also shown by the calculations of the center of mass of the two coalescing drops and that of the coalesced drop. The curvature and its gradient describe the process more completely than the contact angle, unless we pick the contact angle at the inflection point. The asymmetric coalesced drop self-adjusts and evolves into a symmetric shape due to the capillary flow within the drop. The calculated pressure field in the coalesced, asymmetric drop is consistent with the required capillary assisted flow from the receding to the stationary front of the asymmetric drop, till it becomes symmetric. The calculations of the average shear stress, the decrease in the interfacial excess energy, interfacial pressure field, and the positions of the center of mass give a physical understanding of the spreading and coalescence of the drops. Observations and Results Naturally occurring interference fringes result due to the reflection of the monochromatic light (λ=543.5nm) at the liquid-vapor and liquid-solid interfaces (Zheng et al., 2002). Recorded images of the interference fringes for the condensing drops were analyzed to get the profiles of the thickness, slope (a measure of the apparent contact angle), and the curvature at each pixel location (0.177μm) along the drop using an image analyzing technique based on the reflectivity measurement (Gokhale et al., 2004a). Experimentally obtained optical micrographs of the condensing drops during the coalescence process are shown in Fig. 1. Figures 1(a) and 1(b) show that the two drops grow in size due to condensation and approach each other, still keeping their symmetric shape intact. Figure 1(c) depicts the merging of the two drops resulting in an elongated asymmetric drop. The drop shape evolves and eventually becomes symmetric [Fig. 1(d)]. The spreading velocities of the larger and the smaller drops before coalescence are 0.14ìm/s and 0.12ìm/s respectively. Assuming that the drops have a spherical cap shape before coalescence, Eq. (1) shows that the surface heat flux (q ′ ′ surface) is proportional to the rate of change of the radius of the drop.