Investigation of Cassie-Wenzel Wetting transitions on microstructured surfaces

The Challenge of Superhydrophobicity: Environmentally Facilitated Cassie-Wenzel Transitions and Structural Design

Superhydrophobic materials can be used in various fields to optimize production and life due to their unique surface wetting properties. However, under certain pressure and perturbation conditions, the droplets deposited on superhydrophobic materials are prone to change from Cassie state to Wenzel state, which limits the practical applications of the materials. In recent years, a large number of works have investigated the transition behavior, transition mechanism, and influencing factors of the wetting transition that occurs when a superhydrophobic surface is under a series of external environments. Based on these works, in this paper, the phenomenon and kinetic behavior of the destruction of the Cassie state and the mechanism of the wetting transition are systematically summarized under external conditions that promote the wetting transition on the material surface, including pressure, impact, evaporation, vibration, and electric wetting. In addition, superhydrophobic surface morphology has been shown to directly affect the duration of the Cassie state. Based on the published work the effects of specific morphology on the Cassie state, including structural size, structural shape, and structural level, are summarized in this paper from theoretical analyses and experimental data.

Direct laser writing of hydrophobic and hydrophilic valves in the same material applied to centrifugal microfluidics

In this study, we utilize nanosecond and femtosecond direct laser writing for the generation of hydrophobic and hydrophilic microfluidic valves on a centrifugal microfluidic disk made of polycarbonate, without the need for wet-chemistry. Application of a femtosecond (fs) laser at 800 nm resulted in an increased contact angle, from ∼80° to ∼160°, thereby inducing the formation of a hydrophobic surface. In contrast, employing a nanosecond (ns) laser at 248 nm led to the formation of superhydrophilic surfaces. Morphological studies identified the enhancement in the surface roughness for the hydrophobic surfaces and the creation of smooth patterns for the hydrophilic surfaces. Chemical modifications in the laser-ablated samples were confirmed via Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. These spectroscopic examinations revealed an increase of hydrophilic chemical groups on both surfaces, with a more pronounced increase on the nanosecond laser-modified surface. Furthermore, these surfaces were used as a case study for centrifugal microfluidic valves. These modified surfaces demonstrated peculiar pressure responses. Specifically, the hydrophobic valves necessitated a 29% increase in pressure for droplet passage through a microchannel. On the other hand, the superhydrophilic valves exhibited enhanced wettability, decreasing the pressure requirement for fluid flow through the modified area by 39%. However, similarly to the hydrophobic valves, the fluid exiting the hydrophilic valve area required an increased pressure. Overall, our study shows the potential for tailoring valve functionality in microfluidic systems through precise surface modifications using laser technology.

Achieving High Efficacy and Low Safety Risk by Balancing Pesticide Deposition on Leaves and Fruits of Chinese Wolfberry (Lycium barbarum L.)

Pesticide residue has become the main technical barrier that restricts the export of Chinese wolfberry. Can we achieve high efficacy and low safety risk by balancing pesticide deposition on the leaves and fruits of Chinese wolfberry? In this research, the structural characteristics and wettability of leaves and fruits of Chinese wolfberry at different growth stages were studied. The adaxial and abaxial surfaces of leaves were hydrophobic, whereas the fruit surfaces were hydrophilic. Adding spray adjuvant could increase the retention of droplets on the leaf surfaces of Chinese wolfberry by 52.28-97.89% and reduce the retention on the fruit surfaces by 21.68-42.14%. A structural equation model analysis showed that the adhesion tension was the key factor affecting the retention of the solutions among various interface behaviors. When the concentrations of Silwet618, AEO-5, Gemini 31551, and 1227 were 2-5 times higher than their CMCs, the retention of pesticide solutions (pyraclostrobin and tylophorine) on Chinese wolfberry leaves significantly increased, and the control efficacies on aphids and powdery mildew also dramatically improved (65.90-105.15 and 41.18-133.06%, respectively). Meanwhile, the retention of pesticides on the fruit of Chinese wolfberry was reduced. This study provides new insights into increasing the utilization of pesticides in controlling pests and improving food safety.

Suppression of wetting transition on evaporative fakir droplets by using slippery superhydrophobic surfaces with low depinning force

This study experimentally investigated the evaporation and wetting transition behavior of fakir droplets on five different microstructured surfaces. Diamond-like carbon was introduced as the substrate, and the influence of varying the width, height, and pitch of the micropillars was assessed. The experimental results showed that the interfacial properties of the surfaces change the evaporation behavior and the starting point of the wetting transition. An important result of this study is the demonstration of a slippery superhydrophobic surface with low depinning force that suppresses the transition from the Cassie-Baxter state to the Wenzel state for microdroplets less than 0.37 mm in diameter, without employing large pillar height or multiscale roughness. By selecting an appropriate pillar pitch and employing tapered micropillars with small pillar widths, the solid-liquid contact at the three-phase contact line was reduced and low depinning forces were obtained. The underlying mechanism by which slippery superhydrophobic surfaces suppress wetting transitions is also discussed. The accuracy of the theoretical models for predicting the critical transition parameters was assessed, and a numerical model was developed in the surface evolver to compute the penetration of the droplet bottom meniscus within the micropillars.

Light-Triggered Manipulations of Droplets All in One: Reversible Wetting, Transport, Splitting, and Merging

Here, we establish different ways of light-triggered droplet manipulation such as reversible wetting, splitting, merging, and transport. The unique features of our approach are that the changes in the wetting properties of microscopic droplets of isotropic (oil) or anisotropic (liquid crystalline) liquids adsorbed on photoswitchable films can be triggered just by application of soft optical stimuli, which lead to dynamical, reversible changes in the local morphology of the structured surfaces. The adaptive films consist of an azobenzene-containing surfactant ionically attached to oppositely charged polymer chains. Under exposure to irradiation with light, the azobenzene photoisomerizes between two states, nonpolar trans-isomer and polar cis-isomer, resulting in the corresponding changes in the surface energy and orientation of the surfactant tails at the interface. Additionally, the local increase in the surface temperature due to absorption of light by the azobenzene groups enables diverse processes of manipulation of the adsorbed small droplets, such as the reversible increase of the droplet basal area up to 5 times, anisotropic wetting during irradiation with modulated light, and precise partition of the droplet into many small pieces, which can then be merged on demand to the desired number of larger droplets. Moreover, using a moving focused light spot, we experimentally demonstrate and theoretically explain the locomotion of the droplet over macroscopic distances with a velocity of up to 150 μm·s^-1. Our findings could lead to the ultimate application of a programmable workbench for manipulating and operating an ensemble of droplets, just using simple and gentle optical stimuli.

How to Achieve a Monostable Cassie State on a Micropillar-Arrayed Superhydrophobic Surface

Superhydrophobic surfaces with a monostable Cassie state possess numerous interesting applications in many fields, such as microfluidics, oil-water separation, drag reduction, self-cleaning, heat dissipation, and so on. How to guarantee a monostable Cassie state of a superhydrophobic surface is still an interesting problem. In this paper, considering the influence of external interferences that may induce the possible wettability transition, the whole wetting process of a droplet on a trapezoidal micropillar-arrayed superhydrophobic surface is divided into six possible stages. Both the Gibbs-free energy in each stage and the energy barrier between adjacent stages are obtained and analyzed theoretically. It is interesting to find that the finally stable wettability of a trapezoidal micropillar-arrayed superhydrophobic surface significantly depends on the apparent contact angle of the lateral surface of a single micropillar, which can be divided into three regions from 0 to 180°, corresponding to the Wenzel state, metastable Cassie state, and monostable Cassie state. Furthermore, the size of each region is explicitly related to and can be well-tuned by the geometry of microstructures. Such a wettability classification is well verified by a number of existing experimental results and our numerical simulations. As a relatively general case, such a trapezoidal micropillar-arrayed superhydrophobic surface can also be conveniently degenerated to the triangular or rectangular micropillar-arrayed surface. All the results should be useful for the precise design of functional surfaces of different wettabilities.

Hydrophobicity Evolution on Rough Surfaces

Hydrophobicity is abundant in nature and obtainable in industrial applications by roughening hydrophobic surfaces and engineering micropatterns. Classical wetting theory explains how surface roughness can enhance water repellency, assuming a droplet to have a flat bottom on top of micropatterned surfaces. However, in reality, a droplet can partially penetrate into micropatterns to form a round-bottom shape. Here, we systematically investigate the evolution of evaporating droplets on micropatterned surfaces with X-ray microscopy combined with three-dimensional finite element analyses and propose a theory that explains the wetting transition with gradually increasing penetration depth. We show that the penetrated state with a round bottom is inevitable for a droplet smaller than the micropattern-dependent critical size. Our finding reveals a more complete picture of hydrophobicity involving the partially penetrated state and its role in the wetting state transition and can be applied to understand the stability of water repellency of rough hydrophobic surfaces.

Explaining Evaporation-Triggered Wetting Transition Using Local Force Balance Model and Contact Line-Fraction

Understanding wettability and mechanisms of wetting transition are important for design and engineering of superhydrophobic surfaces. There have been numerous studies on the design and fabrication of superhydrophobic and omniphobic surfaces and on the wetting transition mechanisms triggered by liquid evaporation. However, there is a lack of a universal method to examine wetting transition on rough surfaces. Here, we introduce force zones across the droplet base and use a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value. For the first time, we provide a universal scale using the concept of the critical FPL value which enables comparison of various superhydrophobic surfaces in terms of preventing wetting transition during liquid evaporation. In addition, we establish the concept of contact line-fraction theoretically and experimentally by relating it to area-fraction, which clarifies various arguments about the validity of the Cassie-Baxter equation. We use the contact line-fraction model to explain the droplet contact angles, liquid evaporation modes, and depinning mechanism during liquid evaporation. Finally, we develop a model relating a droplet curvature to conventional beam deflection, providing a framework for engineering pressure stable superhydrophobic surfaces.

Light-Driven Wettability Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts

The directional light-induced mass migration phenomenon arising in the photoresponsive azobenzene-containing materials has become an increasingly used approach for the fabrication of controlled tridimensional superficial textures. In the present work we demonstrate the tailoring of the superficial wettability of an azopolymer by means of the light-driven reconfiguration of an array of imprinted micropillars. Few simple illumination parameters are controlled to induce nontrivial wetting effects. Wetting anisotropy with controlled directionality, unidirectional spreading, and even polarization-intensity driven two-dimensional paths for wetting anisotropy are obtained starting from a single pristine pillar geometry. The obtained results prove that the versatility of the light-reconfiguration process, together with the possibility of reversible reshaping at reduced costs, represents a valid approach for both applications and fundamental studies in the field of geometry-based wettability of solid surfaces.

Wetting transition energy curves for a droplet on a square-post patterned surface

Due to the property of water repellence, biomimetic superhydrophobic surfaces have been widely applied to green technologies, in turn inducing wider and deeper investigations on superhydrophobic surfaces. Theoretical, experimental and numerical studies on wetting transitions have been carried out by researchers, but the mechanism of wetting transitions between Cassie-Baxter state and Wenzel state, which is crucial to develop a stable superhydrophobic surface, is still not fully understood. In this paper, the free energy curves based on the transition processes are presented and discussed in detail. The existence of energy barriers with or without consideration of the gravity effect, and the irreversibility of wetting transition are discussed based on the presented energy curves. The energy curves show that different routes of the Cassie-to-Wenzel transition and the reverse transition are the main reason for the irreversibility. Numerical simulations are implemented via a phase field lattice Boltzmann method of large density ratio, and the simulation results show good consistency with the theoretical analysis.