Microscopic Mechanism Of Nucleation During Vapor Condensation And The Interfacial Structure Effect

The heterogeneous nucleation of micro droplets on surfaces with different interfacial energies and micro structures are of great importance for the initial stage of vapor condensation process, which determine the nucleation rate, cluster and micro droplet size distribution, and the wetting mode of micro droplet. Meanwhile, the heterogeneous nucleation also exists widely in practical applications such as weather control, antifogging coating and condensation heat transfer process. For the nucleation of water droplet, the critical size of initial nucleus is usually in nanoscale. It is difficult to observe and investigate the physical process of nucleation stage directly, and the application of more sophisticated testing technique, simulation method and model analysis become necessary. In this paper, the reflection spectrum technique and molecular dynamics simulation (MD) method were adopted to investigate the formation mechanism of initial droplet. The interfacial structure parameters were introduced into heterogeneous nucleation rate model to investigate the effect of micro structure on the growth of clusters, spatial distribution of initial nuclei, and the wetting mode of micro droplet. And the effect of capillary force on the resting mode and dynamic behavior of droplets with the presence of micro structure were investigated as well.The reflection spectrum analysis was used to investigate the formation mechanism of initial droplets. The reflectivity of stainless steel surface with the presence of thin liquid film were calculated from theoretical correlations, and the experimental reflection spectrum data during dropwise condensation in literature was analyzed. For the initial condensation stage where the vapor molecules got in touch with the cooling surface and the surface renewal stage where the pre-existing droplets departed from the surface for re-nucleation, the reflectivity could be maintained between that of bare surface and surfaces covered with thin liquid film for a certain time period. The results revealed that the micro droplets were directly formed on the randomly distributed nucleation sites, instead of originating from the fracture of thin liquid film.The dynamic process of heterogeneous nucleation was investigated by molecular dynamics simulation (MD), and the effect of surface free energy distribution on the formation and growth of clusters was discussed. The results indicated that near-constant contact angles were already established for nanoscale nuclei on various surfaces, with the contact angle decreasing with solid-liquid interaction intensities linearly. The nucleation process was greatly affected by the surface free energy. As the solid-liquid interaction intensity was increasing, the latent heat that was released during cluster formation could be dissipated more rapidly through the surfaces, and the pre-existing clusters on surface also became more stable than that on low energy surfaces, yielding a relatively higher growth rate of clusters. The nucleation preferably initiated on high energy particles that embedded within low energy surface, and the clusters that formed on the heterogeneous particles were trapped around their original positions instead of migrating around as that observed on smooth surfaces. The results revealed that the spatial distribution of nucleation sites and nucleation sites number densities could be artificially controlled by altering the surface free energy distribution of solid surfaces, such as the application of hydrophobic-hydrophilic surfaces and hybrid surfaces with embedded high energy particles.The transient cluster size distribution (CSD) model was introduced into vapor condensation system to investigate the kinetics of initial condensation stage and the evolution of clusters and micro droplets. The CSD of heterogeneous process translated from monotonic decreasing to a unimodal distribution as time continued, and the CSD curve was found to be close to lognormal distribution. The peak value of the CSD curve shifted to larger cluster sizes and the cluster size distribution broadened with time, with the absolute value decreasing accordingly. The obtained CSD evolution within nanoscale was very similar to the reported experimental results of micron scale droplets, revealing that the subsequent experimental phenomenon at macroscopic scale was the direct result of further development of initial cluster size distribution.The geometrical parameters of conical micro structures were introduced into the classic heterogeneous nucleation theory, and a general formula of nucleation rate was proposed with the consideration of interfacial structure effect. Base on the modified model, the effect of micro structure on the nucleation rate, spatial distribution of initial nuclei, and the wetting mode of micro droplets were investigated. Meanwhile, experimental observations of initial condensation stage and MD simulations were also conducted on nano-array surfaces to support the model analysis. Due to the space-confining effect of micro cavities and grooves to the initial nuclei, the Gibbs free energies required for the nucleation of droplets were decreased, yielding a relatively higher nucleation rate for cavities and grooves. As the cross section angle β of cavity and groove decreased even further, the stable wetting state of droplets could change according to the combination of β and θ. For β<(2θ-π) condition, the droplet can suspend in the center of micro structures instead of immersing into the bottom of structures, which was beneficial to the formation of Cassie mode droplet. Experimental observation of initial condensation stage of water droplets on rough surfaces revealed that the initial droplets tended to accumulate around the micro grooves and cavities on rough surfaces. Vapor condensation experiments were conducted on a nano-array superhydrophobic surface to investigate the effect of micro structure on the possible wetting mode of initial droplet. The contact line of coalescing droplets could retract rapidly to form a larger spherical cap droplet. Meanwhile, migration and out-of-plane jumping phenomena could be observed for the coalescing droplets due to the excess interfacial energy that was released during coalescence. These results provided a basic design guideline to realize the spatial control of nucleation sites and manipulation of wetting mode of initial droplets by appropriate micro structures.Based on the force balance of local contact line and interfacial free energy minimization analysis, a criterion was proposed to determine the relationship between structure parameter and resting mode of droplet. Meanwhile, visualization investigation and MD simulation were conducted to investigate the droplet dynamic behavior that was actuated by capillary force with the presence of V-shaped grooves. The stable resting mode of droplet was determined by the apparent contact of inner wall and the cross section angle of the groove, and the criterion between immersing and suspending droplet was determined as β*=(2θ-π). For the grooves with higher apparent contact angle and low contact angle hysteresis, the droplets could move upward or downward depending on the variation of structure parameters and increase or decrease of droplet volumes. The movement of upper and lower meniscus for the suspending droplet was in stepwise manner. The driving force for the dynamic movement of upper contact line was provided by the deformation of upper meniscus, while the driving force for the lower one was provided by the fluctuation of local contact angle that was caused by the movement of upper meniscus. The results revealed that the wetting mode and dynamic behavior of droplets could be artificially controlled using the effect of geometrical structure and interfacial energies. The related phenomena can be regarded as a capillary actuator, which can be used in applications such as the artificial control of wetting mode and droplet transport in microfluidics.