Experimental investigation of contact angles of low surface tension fluids

Condensation is a widespread process in many industrial fields, in particular in electrical energy production, process industry, refrigerating systems and heat pumps. The interest in achieving efficient condensation with compact devices, low charges of heat transfer fluids, reduced temperature differences is therefore a crucial aspect in the actual research. In this context, the possibility of achieving dropwise condensation (DWC) in a stable and effective way, appears to be a game-changer in many of the fields that currently requires condensation. Dropwise condensation, indeed, allows to reach much higher heat transfer coefficient than the case of filmwise condensation, which is currently the most common mechanism. Dropwise condensation is achieved when the condensation process takes place in the form of discrete droplets formed on the surface of the heat exchanger, instead of having a film of liquid that wets all the surface. Then, the droplet’s size increases due to coalescence of droplets and condensate formation, until the droplet slides away, leaving the substrate free for the formation of new droplets. To achieve and characterize DWC it is therefore very important to study the behavior of the droplets, that need to have good mobility, avoiding spreading as a film on the surface. The work of this thesis addresses to the study of wettability of low surface tension fluids through experimental measurements of dynamic and static contact angles. The necessity of focusing on low surface tension fluids is given by the great adoption of these fluids as refrigerants in heat pumps and chillers, which are currently subject of great interest considering their importance in terms of reducing fossil fuels consumption and emissions. On the other hand, low surface tension fluids are much more critical in terms of achieving DWC, given their tendency to form low contact angles and wet the whole surface. This explains the few research that is already available, compared to water vapor condensation. For contact angles measurement, a modified Wilhelmy plate method has been adopted, and great importance has been given to the validation of this method by comparing it to a standard sessile drop method. The validation has been carried out by measuring static and dynamic contact angles of three different fluids with decreasing surface tension (water, ethylene glycol and ethanol) on four different surfaces (pure aluminum, pure copper, PTFE and M7T3, a sol-gel which has already been tested for DWC of water vapor). In parallel, the uncertainty of the optical method has been evaluated through numerical simulations focusing on the grade of the interpolating polynomial and on the error of the coordinates of the points of the meniscus. Then, the measurements have been performed for three refrigerants, HFE-7000, R1233zd(E) and R1234ze(E), at a saturation pressure which is higher than the atmospheric, to assess the possibility of adopting this technique also for low surface tension fluids. Finally, a series of measurements have been performed with two surfaces that are currently under investigation to achieve DWC with saturated water vapor: a lubricant-infused surface (LIS) and hybrid octyl-modified silica coating (O2T8), which could represent possible substrates to obtain DWC also for refrigerants. This had the purpose of investigating liquid-substrate interactions to better understand and characterize wettability in the case of low surface tension fluid to achieve DWC in particular with refrigerants.