Theoretical Study On The Relationship Between Wetting Behaviors Of Microdroplets And Substrate Microstructures

The living creature possesses lots of different characteristic after the evolution of so many years. Bionics means that we are able to improve our quality of life through mimicking the special function of living creature. A particular example is superhydrophobic phenomenon, like water droplet sliding away on the surface of louts leaf and water strider walking around on the surface of river. Experimental studies demonstrate that the hierarchical structure of the substrates is essential for their superhydrophobicity, and this findings provide new insight for the production of man-made superhydrophobic material. The robust superhydrophobic materials potentially used in the process of anti-icing, anti-fog, anti-corrosion et al requires a deep understanding on the relationship between droplet wetting behaviors and substrate textures.In this thesis, lattice Boltzmann method (LBM) was employed in this paper to study the wetting and dewetting behaviors of microdroplets, from aspects of both dynamics and thermodynamics, and their dependence on the microstructure of solid surfaces. The main results are given as follows:1. The application of Cassie equation to microscopic droplets is recently subjected to intensive critics because the microdroplet dimension is often of the same order of magnitude as the characteristic size of substrate heterogeneities, and the mechanism to describe the contact angle of microdroplets is not clear. By representing real surfaces statistically as an ensemble of patterned surfaces with randomly or regularly distributed heterogeneities (patches), lattice Boltzmann simulations in this part show that the contact angle of microdroplets has a wide distribution, either continuous or discrete, depending on the patch size. The origin of multiple contact angles observed is ascribed to the contact line pinning effect induced by substrate heterogeneities. We demonstrate that the local feature of substrate structure near the contact line determines the range of contact angles that can be stabilized, while the certain contact angle observed is closely related to the contact line width.2. In this part, three dimension Lattice Boltzmann method (LBM) is applied to investigate the dynamic characteristics of contact line motion for evaporating droplets on a patterned hydrophobic substrate. Firstly, we check the stick and slip mechanism for contact line motion, and we found that at the stick stage the contact line is in fact not motionless, but it moves at a relatively low rate. Secondly, intermittent motion of the microdroplet that is caused by the contact line depinning is found, with trajectories depending on the rate of evaporation. More interestingly, our simulation results indicate that contact line depinning events show a collective behavior, and occurrence of depinning seems to follow a power law in all our simulations, independent on the evaporation rate, and pillar spacing and distribution.3. It is commonly accepted for superhydrophobicity that Cassie wetting regime corresponds to a higher energetic state, and the irreversible Cassie to Wenzel transition occurs spontaneously or via an external stimulus. However, we demonstrate in this part through theoretical analysis and lattice Boltzmann simulation of droplet condensation that the Cassie state may be in fact energetically stable while the Wenzel state is metastable or unstable. On the hydrophobic substrate patterned with cone-shaped texture, for example, our results show that the growing droplets may undergo a spontaneous Wenzel to Cassie dewetting transition when the droplet grows to a size incommensurate with the pore encapsulated by the neighboring cones. While for pillared substrate, the dewetting transition more likely fails to occur spontaneously even though the Wenzel state is in fact energetic unfavorable, and we find that it is the work of adhesion between the droplet and the bottom substrate surface that impedes the transition.4. Superomniphobic surfaces display both superoleophobic and superhydrophobic properties, having a contact angle greater than for both water and oil droplets. In this part lattice Boltzmann simulations on droplets impacting on surface textures of various topologies are performed to understand the mechanism of how the superomniphobic properties can be achieved by optimizing the geometry of re-entrant surfaces and inherent hydrophobicity of substrates. Detailed kinetics for droplet impinging is analyzed for both liquid impalement and emptying, showing distinct dependences on geometrical details of re-entrant surfaces. The origins of enhanced stability of Cassie states are ascribed to (i) the barrier of the Cassie to Wenzel transition for the impalement process, (ii) the driving force for liquid receding in the emptying process, and (iii) the contact line pinning from the entrance effect and the edge effect. Finally, we check the strategies proposed here by designing a new re-entrance structure that possesses excellent property in maintaining droplet Cassie state. At the same time, we explain the reason why the droplet can bounce toward both directions on the non-uniformly distribute substrate.5. The problem of the dimension of wetting transition remains open. In this part, three dimension Lattice boltzmann method (LBM) was employed to observe when and how the one or two dimension wetting take place. Firstly, Wenzel and Cassie penetrating state was obtains in our simulations which performs on pillars uniform or random distribute on the substrate. Cassie penetrating state include two different final states which separated by pinning effect exist or not and the energy barrier caused by the pinning effect also calculated. Secondly, the process of wetting was observed and one or two dimension takes place in different cases. We also obtained a mix wetting process which located between one and two dimension wetting. The area of droplet wet as a function of time for the mix wetting is different from the one and two dimension wetting but close to two dimension. Finally, we analyze the mechanism of different wetting dimension through compare the force in horizon and vertical act on droplet.