(142cs) Dropwise Condensation On a Cold Surface: Drop Distributions and Surface Functionalization

      In order to design effective surfaces for large-scale dropwise condensation, an understanding of how the surface functionalization affects drop growth and coalescence is needed. The long term technological goal is to be able to propose a set of design conditions to NASA to help them achieve maximum heat transfer rates under microgravity conditions in their management of waste heat generated from electronics and the cabin environment. We are specifically interested in how the shape of a drop size distribution is tied to the surface functionalization.
      Transient dropwise condensation from a vapor phase onto a cold and chemically treated surface was studied. The experimental setup consisted of a glass slide sitting on top of an aluminum plate which rested on a glass Pyrex container filled with boiling water. The aluminum plate had a rectangular shaped hole cut into it which exposed the glass slide to the vapor. Water was condensed onto the underside of the glass slide which had been chemically treated to yield a uniform contact-angle. Vapor deposition with dodecyltrichlorosilane and heptadecafluorotrichlorosilane yielded contact-angles ranging from neutral wetting to hydrophobic. The temperature of the vapor and the condensing surface were not controlled over the course of the experiment which lasted for 50 minutes. Hence, the vapor cooled and the glass slide heated up over the course of the experiment. The drops were imaged from above and statistics related to their size and population density were collected.
      The process of condensation occurs by the three concurrent processes of nucleation, growth, and coalescence of drops. The growth of a drop can be divided into two categories: that by vapor diffusion onto the liquid-gas interface and that by surface “diffusion” where smaller drops merge with the perimeter of an existing drop. Coalescence is when two drops of comparable size grow large enough to touch and merge into a single drop.
      For a typical condensation experiment, the progression of a generation of drops advances through two stages: an increase in drop density as a result of nucleation and a decrease in drop density as a result of macroscopic coalescence events. How quickly the transition from one stage to the next occurs is dependent upon the characteristics of the surface. It was found that for increasing contact-angle, the time needed to reach the maximum drop density (i.e. transition point) increases. In addition, the rate of decrease in the drop density increases as the contact-angle increases.
      The transition point can also be determined by observing the time evolution of drop size distributions (number of drops versus drop radius). Initially, the distribution starts off as exponential as the first generation of drops comes into view. The distribution then transitions to a gamma distribution at the transition point. When the drop coalescence events are large enough to regenerate large areas of the surface for subsequent nucleation events, a bimodal distribution is observed. For long times, an exponential distribution (with a longer tail) is again observed. Because the experiment is transient in nature, the shape of the distribution can be used to predict the number of drop generations and their stage of development.