2.3.3. Three-Phase Contact Line Behavior

During a nanofluid boiling situation, it is usually expected that the dispersed nanoparticles deposit uniformly onto the heating surface by the gravitational effect. Given that the nanofluids are a very dilute type of fluid, the quantity of nanoparticles deposited by gravity can be assumed to be negligible. As such, when the first vapor bubbles nucleate, the heat transfer surface is hydrophobic and the evaporation rate at the solid, liquid, and vapor phases contact line is the main mechanism associated with the growth stage of the initial vapor bubbles. The published transient imaging of the contact line evaporation in the literature has already demonstrated that the contact line of the bubbles presents a radial type of motion across the heating surface [60], provoking the expansion or, alternatively, the contraction of the dry patch. This dry patch can be characterized by a higher temperature area at the center of the active nucleation point. Given that the evaporative heat flux has its maximum value at the solid–liquid–vapor contact line, the suspended nanoparticles will deposit at the contact line as it evaporates and expands. It has been reported that a ring-like deposition pattern was observed at the contact line of a sessile droplet deposit onto a hydrophobic surface due to the delay in the depinning of the contact line. Moreover, it is expected that the radial motion of the contact line will be more constrained with the addition of nanoparticles. As a consequence, the initial deposition happens over only a narrow region under the form of a ring. Furthermore, the deposition at the contact triple line becomes very intense with the boiling time and, thus, modifies the surface wettability that, in turn, aids in the continuous shrinkage of the contact line radius corresponding to the dry patch dimensions. This particular mechanism conducts to the smearing out of the thinned ring to a wider ring structure, which, consequently, improves the wettability of an extended inner region of the nucleation sites. When the rewetting of this region is complete after the detachment of the bubbles, the growth stage of the following bubbles slowly changes from the dynamics of the contact line to the entrapment of the microlayer dynamics. The microlayer thickness augments in the radially outward direction and, consequently, evaporates faster near the center of the nucleation site, leaving behind a small dry patch. Because of the higher evaporation rate at the center of the front of the microlayer, a radially inward capillary flow is generated inside it, which conducts the nanoparticles toward the evaporative front of the microlayer. It should be stated that as the fraction of nanoparticles augments around the evaporative front because of such inward flow, the clustering rate of the nanoparticles raises up to produce larger particles. When the droplet evaporation happens on hydrophilic surfaces, it has been highlighted that the nanoparticles adhere and settle at the contact line region, which is caused by the conjugated action of the shape of the liquid and vapor phases interface and the balance between friction forces and capillarity acting on the nanoparticles. Taking into account these phenomena, it is believed that a similar mechanism is responsible for the settlement of the nanoparticles at the evaporative front of the microlayer considering that the bigger clustered particles come in contact with the evaporative front because of the presence of induced inward capillary flows. The clustering of the nanoparticles at the inner area of the deposition pattern can be easily observed using AFM imaging wherein the sub micro-scaled particles are deposited, and this fact can justify the high density and thickness of the deposit at the central regions of the nucleation point. Moving away from the central regions, although the microlayer possess a relatively large fraction of the working fluid to maintain the suspension of nanoparticles, the thickness of the deposit gradually decreases radially outward since the complete evaporation of the microlayer at these points does not occur because of its increasing thickness. The augmented microlayer thickness renders extra ITR to the heat transfer flow and the microlayer has lower rates of evaporation, which in turn, averts

the complete dryout of the microlayer at the radially outward sites. The location of the dry patch can be estimated with the help of heat flux contour plots. If the maximum value of the dry patch radius and the deposited pattern radius is compared, it can be verified that the maximum dry patch radius is significantly smaller than the radius of the deposition pattern. Additionally, it has been found that the maximum dry patch radius is even smaller than the centrally deposited film radius. This fact that in the nanofluid boiling process, the microlayer does not evaporate totally in the radially outward regions and the dispersed nanoparticles settle primarily in such regions because of the gravity and attraction forces between the nanoparticles and surface like Van der Waals and electrostatic forces. Hence, the thickness of the deposit is radially decreased from the center of the nucleation points to their peripheral areas.
