Hierarchical structured surfaces enhance the contact angle of the hydrophobic (meta-stable) state

The relation between wetting properties and geometric parameters of fractal surfaces are widely discussed on the literature and, however, there are still divergences on this topic. Here we propose a simple theoretical model to describe the wetting properties of a droplet of water placed on a hierarchical structured surface and test the predictions of the model and the dependence of the droplet wetting state on the initial conditions using simulation of the 3-spin Potts model. We show that increasing the auto-similarity level of the hierarchy - called n - does not affect considerably the stable wetting state of the droplet but increases its contact angle. Simulations also explicit the existence of metastable states on this type of surfaces and shows that, when n increases, the metastability becomes more pronounced. Finally we show that the fractal dimension of the surface is not a good predictor of the contact angle of the droplet.

A phase diagram of morphologies for anisotropic particles sculpted from emulsions

HYPOTHESIS

A micron-scale oil-in-water emulsion droplet frozen in the presence of surfactants can be induced to eject the crystallizing solid from its liquid precursor. This dynamic process produces highly elongated solids whose shape depends critically on the rate of crystallization and the interfacial properties of the tri-phase system.

EXPERIMENT

By systematically varying the surfactant concentration and cooling protocol, including quenching from different temperatures as well as directly controlling the cooling rate, we map out the space of possible particle morphologies as a function of experimental control parameters. These results are analyzed using a non-equilibrium Monte Carlo model where crystallization rate and interfacial energies can be specified explicitly.

FINDINGS

Our model successfully predicts the geometry of the resulting particles as well as emergent phenomena including how the particle shape depends on nucleation site and deformation of the precursor droplet during crystallization.

Modeling oil-water separation with controlled wetting properties

Several oil-water separation techniques have been proposed to improve the capacity of cleaning water. With the technological possibility of producing materials with antagonist wetting behavior, for example, a substrate that repels water and absorbs oil, the understanding of the properties that control this selective capacity has increased with the goal of being used as the mechanism to separate mixed liquids. Besides the experimental advance in this field, less is known from the theoretical side. In this work, we propose a theoretical model to predict the wetting properties of a given substrate and introduce simulations with a four-spin cellular Potts model to study its efficiency in separating water from oil. Our results show that the efficiency of the substrates depends both on the interaction between the liquids and on the wetting behavior of the substrates itself. The water behavior of the droplet composed of both liquids is roughly controlled by the hydrophobicity of the substrate. Predicting the oil behavior, however, is more complex because the substrate being oleophilic does not guarantee that the total amount of oil present on the droplet will be absorbed by the substrate. For both types of substrates considered in this work, pillared and porous with a reservoir, there is always an amount of reminiscent oil on the droplet, which is not absorbed by the substrate due to the interaction with the water and the gas. Both theoretical and numerical models can be easily modified to analyze other types of substrates and liquids.

A minimum energy optimization approach for simulations of the droplet wetting modes using the cellular Potts model

Wetting modes of a droplet on a periodical grooved surface were simulated by using the Cellular Potts Model (CPM). An optimization approach based on the Synthesis Minimum Energy (SME), which is defined as the lowest energy of the simulation system, was proposed for determining the droplet wetting modes. The influence of the fluctuation parameter (T) was discussed. The results showed that the SME optimization approach increased the accuracy of the wetting mode simulation. For the values of T used in the SME, an increase in the range of T and a decrease in the step size of T will not only cause an increase in the accuracy of the SME but also will cause an increase in the total consumption of calculation time and a decrease in the ability of accuracy improvement. A high value of the fluctuation parameter T generated the Cassie mode transition for the droplet. With an increase in the pillar height, the droplet wetting mode transited from Wenzel mode to Cassie mode, while it transited from Cassie mode to Wenzel mode with an increase in the interpillar distance.

Synchrotron x-ray imaging visualization study of capillary-induced flow and critical heat flux on surfaces with engineered micropillars

Over the last several decades, phenomena related to critical heat flux (CHF) on structured surfaces have received a large amount of attention from the research community. The purpose of such research has been to enhance the safety and efficiency of a variety of thermal systems. A number of theories have been put forward to explain the key CHF enhancement mechanisms on structured surfaces. However, these theories have not been confirmed experimentally because of limitations in the available visualization techniques and the complexity of the phenomena. To overcome these limitations and elucidate the CHF enhancement mechanism on the structured surfaces, we introduce synchrotron x-ray imaging with high spatial (~2 μm) and temporal (~20,000 Hz) resolutions. This technique has enabled us to confirm that capillary-induced flow is the key CHF enhancement mechanism on structured surfaces.

Modeling of Droplet Evaporation on Superhydrophobic Surfaces

When a drop of water is placed on a rough surface, there are two possible extreme regimes of wetting: the one called Cassie-Baxter (CB) with air pockets trapped underneath the droplet and the one called the Wenzel (W) state characterized by the homogeneous wetting of the surface. A way to investigate the transition between these two states is by means of evaporation experiments, in which the droplet starts in a CB state and, as its volume decreases, penetrates the surface's grooves, reaching a W state. Here we present a theoretical model based on the global interfacial energies for CB and W states that allows us to predict the thermodynamic wetting state of the droplet for a given volume and surface texture. We first analyze the influence of the surface geometric parameters on the droplet's final wetting state with constant volume and show that it depends strongly on the surface texture. We then vary the volume of the droplet, keeping the geometric surface parameters fixed to mimic evaporation and show that the drop experiences a transition from the CB to the W state when its volume reduces, as observed in experiments. To investigate the dependency of the wetting state on the initial state of the droplet, we implement a cellular Potts model in three dimensions. Simulations show very good agreement with theory when the initial state is W, but it disagrees when the droplet is initialized in a CB state, in accordance with previous observations which show that the CB state is metastable in many cases. Both simulations and the theoretical model can be modified to study other types of surfaces.