Large Contact Angle Hysteresis Research Articles

Universal droplet propulsion by dynamic surface-charge wetting

Controllable droplet propulsion on solid surfaces plays a crucial role in various technologies. Many actuating methods have been developed; however, there are still some limitations in terms of the introduction of additives, the versatilities of solid surfaces, and the speed of transportation. Herein, we have demonstrated a universal droplet propulsion method based on dynamic surface-charge wetting by depositing oscillating and opposite surface charges on dielectric films with unmodified surfaces. Dynamic surface-charge wetting propels droplets by continuously inducing smaller front contact angles than rear contact angles. This innovative imbalance is built by alternately storing and spreading opposite charges on dielectric films, which results in remarkable electrostatic forces under large gradients and electric fields. The method exhibits excellent droplet manipulation performance characteristics, including high speed (~130 mm/s), high adaptability of droplet volume (1 μL–1 mL), strong handling ability on non-slippery surfaces with large contact angle hysteresis (CAH) (maximum angle of 35°), significant programmability and reconfigurability, and low mass loss. The great application potential of this method has been effectively demonstrated in programmable microreactions, defogging without gravity assistance, and surface cleaning of photovoltaic panels using condensed droplets.

Sulfobetaine Zwitterions with Embedded Fluorocarbons: Synthesis and Interfacial Properties.

We describe the preparation of a new set of fluorinated sulfobetaine (FSB) zwitterionic polymers in which fluorocarbon moieties are attached directly to the zwitterionic components. An efficient two-step modification to the conventional sulfobetaine methacrylate monomer synthesis gave access to a series of polymer zwitterions containing varying extents of fluorocarbon character. FSB methacrylates proved amenable to homo- and copolymerizations using reversible addition-fragmentation chain transfer (RAFT) conditions, affording polymers with molecular weights ranging from 5 to 20 kDa and with low molecular weight distributions. Thin films of FSB homopolymers on glass proved stable to aqueous environments and exhibited increasing hydrophobicity with fluorocarbon content, as well as remarkably large water contact angle hysteresis values that enable pinning of water droplets on hydrophobic surfaces, reminiscent of the "petal effect" found in nature. FSB-containing copolymers in aqueous media demonstrated markedly reduced oil-water interfacial tension values, even with moderate (20-50 mol %) FSB incorporation.

Relaxation phase during droplet impact on superhydrophobic surfaces with high contact angle hysteresis

The droplet impact process on a solid surface is divided into a spreading phase where the droplet reaches the maximum deformation followed by a retracting phase. However, in the case of surfaces with high contact angle hysteresis, these two phases are connected by a relaxation phase where the contact angle changes from the advancing to the receding contact angle almost without motion of the contact line. Although the relaxation time can represent a significant part of the total droplet contact time, this relaxation regime has been less explored, especially for superhydrophobic surfaces due to the challenge of designing such surfaces with controlled wetting properties. Here, we show that for superhydrophobic surfaces with large contact angle hysteresis, the relaxation time can be comparable to the spreading and retracting time. Our results indicate that both the contact angle hysteresis and the capillary forces play a major role in defining the relaxation time and that relaxation time scales with the inertial–capillary time when using the droplet relative deformation as the characteristic length scale for this relaxation regime.

Research Progress of Superhydrophobic Surfaces for Anti-icing

The problem of icing on metal surface seriously affects the development of many fields, such as architecture, aerospace, it can even causes significant security risks and economic losses. Thus, it is urgent to solve the problem of icing and ice accumulation on metal surfaces. The traditional anti-icing and deicing methods are not only expensive, but also inefficient. Therefore, it is urgent to develop a new generation of anti-icing and deicing technology. With the continuous development of bionics, inspired by the "lotus leaf effect", superhydrophobic surfaces, which exhibit large contact angle and low contact angle hysteresis have gradually attracted large numbers of attentions. The burgeoning superhydrophobic surfaces not only display excellent anti-icing and deicing performance, but also exhibit many advantages such as low energy consumption and high efficiency, showing good application prospect. In this paper, the fabrication methods of superhydrophobic surfaces in recent years was introduced, the influencing factors and internal mechanism of superhydrophobic surfaces for anti-icing was discussed, the anti-icing and deicing techniques of intelligent responsive superhydrophobic surfaces was summarized. In addition, the long-term durability and stability of superhydrophobic surfaces for anti-icing under low temperatures, large-scale fabrication and the future development of intelligent responsive superhydrophobic surfaces were prospected.

Dynamic wetting characteristics of submicron‐structured injection molded parts

AbstractThe generation of submicron structures on plastic part surfaces can be used to modify their wetting properties allowing the creation of superhydrophobic surfaces. The modified wetting behavior can allow functionalities such as fluid collection or transport, which are critical for microfluidics. Consistent manufacturing is achieved through surface replication and process characterization. In this work, submicron structures were generated on steel mold inserts using ultrafast femtosecond laser and then replicated by micro injection molding on polypropylene and polylactic acid. Samples with Laser‐Induced Periodic Surface Structures (LIPSS) were fabricated using a femtosecond laser and characterized to investigate the effects of mold temperature, texture orientation, and material selection on the dynamic contact angle. The dynamic wetting functionality of the surfaces was investigated by analysis of the advancing and receding contact angles. The hysteresis obtained from the dynamic measurements provides information about the fluid/texture interaction dynamics. The experimental results show that the effects of mold temperature and drop orientation are only significant for the advancing contact angles. The large contact angle hysteresis suggests a parahydrophobic wetting behavior for the manufactured plastic parts.

Toward Surfaces with Droplet Impact Robustness and Low Contact Angle Hysteresis

AbstractSuperhydrophobic surfaces show excellent water repellent performance. However, liquid impalement occurs when a droplet impacts on the surface at high velocity. To achieve higher resistance to liquid impalement at the same scale, a dense array of structures or structures with pinning at the structures top are typically used, but they also lead to increased contact angle hysteresis and higher energy dissipation. Changing the structure side wall shape can help to maintain low hysteresis and withstand high impact energy at the same scale. Droplet contact angle, contact angle hysteresis and droplet impact experiments are performed on both conical and cylindrical pillar surfaces. Comparing conical and cylindrical pillar structures with the same pitch and height, it is found that cylindrical pillars exhibit higher critical Weber number but result in larger contact angle hysteresis and larger liquid residue size when above a critical Weber number. A proper design of conical structures can maintain large contact angle, low hysteresis, strong resistance to impalement, higher number of bouncing and smaller liquid residue size. In addition, the critical Weber number for the conical structures in this work is higher than micro‐patterned pillar surfaces at the same pitch range, implying that we improve the anti‐wetting performance further.

Contact angle hysteresis: A new paradigm?

The wetting of a surface by a liquid is a crucial part of many natural and industrial processes. Despite numerous existing studies, some elements of wetting-drying such as contact angle variation during flow, are still poorly understood, yet it is a crucial piece of information needed for modeling fluid displacements in capillary-dominated flows. In the context of multi-phase flow in porous media, scarcity of direct contact angle measurements inside pores (in-situ) has added to the ambiguity. This work presents an unprecedented study of contact angle variations at different stages of flow, for a variety of fluid systems, using direct pore-scale measurements. Observations reveal that on average, contact angle patterns depend strongly on the type of nonwetting fluid: dodecane and decane, for instance, show large contact angle hysteresis between drainage (drying) and imbibition (wetting) cycles, while air or Soltrol® 220 oil exhibits little hysteresis. To further explore this behavior, we evaluate contact angle from the perspective of geometry, and show that the combined effects of fluid-fluid curvature and wetted-pore-radius variations can explain the observed trends. We hypothesize that, for certain nonwetting fluids, fluid-solid chemical interactions lead to pinning of fluid interfaces and the fairly enhanced hysteresis observed for some nonwetting fluids. Overall, this study suggests that contact angle is a result of the interplay between pore geometry and the forces deforming the fluid interface (including surface chemical interactions), and thus, - despite traditional practice -, contact angle alone is not always an accurate metric for wettability.

The wetting properties of frosted glass

Frosted glass is a common, low cost material. Its roughness can be used to control how it is wet by water. In this paper, the wetting properties of silicone oil and water are investigated. For the oil, wetting is total since the oleophilic character of the glass is enhanced by its roughness. Due to the remarkable optical properties of frosted glass, the spreading of oil droplets on its surface was recorded over three months. Frosted glass is a parahydrophilic surface because of its large contact angle hysteresis (up to 80° ). The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass. The trajectories (and the spreading) of the droplets were studied and phenomenological laws were deduced to describe the dependence of the droplet speed on the initial volume of the droplet and the angle of inclination. Such dependences of speed at long travel distances (100 times the capillary length) were deduced and rationalised with a simple model that takes into account the thickness of the wake. Moreover, we analysed the flow inside the wake of water droplets sliding on inclined frosted glass. Suggestions are given on how to exploit drainage of the water droplet wake and the high hysteresis of water within the framework of open microfluidics.

Direct observation of wetting behavior of water drops on single micro-scale roughness surfaces of rose petal effect

Hypothesis: It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.Experiments: An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.Findings: A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.

Field-Induced Wettability Gradients for No-Loss Transport of Oil Droplets on Slippery Surfaces.

Transporting oil droplets is crucial for a wide range of industrial and biomedical applications but remains highly challenging due to the large contact angle hysteresis on most solid surfaces. A liquid-infused slippery surface has a low hysteresis contact angle and is a highly promising platform if sufficient wettability gradient can be created. Current strategies used to create wettability gradient typically rely on the engineering of the chemical composition or geometrical structure. However, these strategies are inefficient on a slippery surface because the infused liquid tends to conceal the gradient in the chemical composition and small-scale geometrical structure. Magnifying the structure, on the other hand, will significantly distort the surface topography, which is unwanted in practice. In this study, we address this challenge by introducing a field-induced wettability gradient on a flat slippery surface. By printing radial electrodes array, we can pattern the electric field, which induces gradient contact angles. Theoretical analysis and experimental results reveal that the droplet transport behavior can be captured by a nondimensional electric Bond number. Our surface enables no-loss transport of various types of droplets, which we expect to find important applications such as heat transfer, anticontamination, microfluidics, and biochemical analysis.

Droplet Spreading and Wettability of Abrasive Processed Aluminum Alloy Surfaces

One of the main cause of a droplet metastable state is found to be surface roughness. This state is characterized by a large contact angle hysteresis and condition when the static contact angle is larger than the advancing dynamic contact angle. Besides the texture, other factors can influence the deviation from the equilibrium state, in particular, the fluid flow rate (the growth rate of a droplet) affecting the contact line speed. An experimental study was done to determine the effect of roughness and fluid flow rate on wetting of aluminum-magnesium alloy surfaces with random roughness processed by abrasive polishing. Three-dimensional roughness parameters were used to evaluate their texture. The correlations between these parameters, static, advancing and receding dynamic contact angles, hysteresis, and contact line speed were obtained. The molecular-kinetic theory of wetting was used to interpret the dynamic contact angle data.

Nanostructures and wetting properties controlled by reactive ion etching of fluorinated ethylene propylene

We study the surface morphology, mass loss and wetting of fluorinated ethylene propylene surfaces which have been treated using reactive ion etching. We find that etching with argon gas alone results in a network structure, while oxygen combined with argon results in pyramidal hair-like structures, and by adding CF4 globules are found. We demonstrate that small alterations in the preparation allows one to create samples with either homogeneous nanostructures exhibiting large contact angles and very low hysteresis or more irregular structures which exhibits pinning sites that allow very large contact angle hysteresis. The advancing and receding contact angles are estimated based on the tilting plate method, and the pinning force measured for the samples is investigated for large droplets. The results reported here indicate that surface structure and contact angle hysteresis of fluorinated polymers can be manipulated for applications where either large or small contact angle hysteresis is needed.

Investigation of time dependent water droplet dynamics on porous fuel cell material via synchrotron based X-ray imaging technique

Proper water management in proton exchange membrane fuel cells (PEMFCs) remains a challenge, hindering the commercialization of this technology. Liquid water emerges in the form of droplets from micrometer-scaled pathways of porous gas diffusion layers (GDLs) into millimetre-scaled gas flow channels. Afterward, droplets grow to certain critical sizes before being removed by the convective gas flow. The opaque nature of fuel cell components, including GDL materials, poses significant challenges to understand droplet dynamics and develop more efficient water management techniques. In this work, an advanced synchrotron X-ray radiographic imaging technique available at the Canadian Light Source (CLS) was employed to visualize time dependent droplet behaviour in a serpentine PEMFC flow channel for the first time. High spatial and temporal resolution coupled with high energy photons of an X-ray beam provided high resolution images of growing droplets. From the captured images, the whole growth cycle of liquid droplets was obtained under various gas flow rates. A cyclic pattern of droplet dynamics including emergence, growth, and detachment was visualized from high resolution images. From the X-ray images, the time-evolution of dynamic contact angles, height, and chord of a growing droplet was quantitatively analyzed. At higher gas velocities, the critical detachment diameter of droplets became smaller and contact angle hysteresis was found be lower, while larger contact angle hysteresis and bigger droplets were observed at lower gas velocities. The critical droplet size at which the detachment of droplets occurred was compared to the predicted results from an analytical approach and good agreement was found.

Droplet Regulation and Dropwise Condensation Heat Transfer Enhancement on Hydrophobic-Superhydrophobic Hybrid Surfaces

ABSTRACTEfficient Dropwise condensation on nanostructured superhydrophobic surfaces (SHS) has received extensive attention. However, good heat performance only occurs in two conditions: the presence of non-condensable gas, or low surface subcooling. For industrial pure steam condensation, large droplets tend to be pinned on the surfaces due to the large contact angle hysteresis (CAH) and impede heat transfer process. In this study, the SHSs and hydrophobic surface (HS) were integrated in spaced band pattern with different width and CAH. It was observed that the condensed droplets experience a fast horizontal shifting from hydrophobic zones to superhydrophobic zones driven by adhesive force. In this way, the departure path of droplets were altered and the refreshing frequencies are increased for both SHS and HS regions. The heat transfer coefficients for some 1 mm × 1 mm hybrid surfaces were found to be elevated by about 25% at subcooling of 6K compared to the weighted mean value of individual HSs and SHSs. For surfaces with short dividing distance (about 0.5 mm × 0.5 mm), the bridging effects between droplets in SHS regions were also observed, which is undesirable for heat transfer. These work indicates that strong adhesive force of SHSs can function as a promoter of droplet condensation for hybrid surfaces with proper choice of dividing width.

Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient.

In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

Numerical study of hydrophobic micron particle’s impaction on liquid surface

In this study, a simulation method is established for the impaction of micron particles on liquid surfaces, by which the processes of two impaction modes (submergence and oscillation) are studied. The submergence is found to go through three stages, each of which shows different characteristics of particle velocity and gas–liquid interface variance. The dominant forces of the early and late times of the submergence mode are hydrodynamic force and surface tension, respectively, the accumulated work of which is in the same order. The lost particle kinetic energy is converted to the surface energy of the interfaces, the internal energy and the kinetic energy of fluids. The primary part of the oscillation is the first cycle, and the characteristics of its sinking process are similar to that of the submergence. In the reverting stage, the particle rising velocity increases first and then decreases, and the cavity retracts until the gas–liquid interface flattens. The dominant forces of the early and late times of the reverting stage are surface tension and hydrodynamic force, respectively. The positive accumulated work of surface tension on the particle is considerably limited due to the large contact angle hysteresis at the early times of the reverting stage. The negative accumulated work of the hydrodynamic force on the particle at the late times causes a fast decrease in particle kinetic energy, which leads to particle floating on the gas–liquid interface. The results are helpful in understanding the mechanism of micron particle impaction and developing the prediction method of attachment efficiency.

Scale effects of graphene and graphene oxide coatings on pool boiling enhancement mechanisms

Surface modifications through conductive carbon based coatings, such as graphene (G) and graphene oxide (GO) are studied to enhance the pool boiling heat transfer performance. In this study, four different mechanisms for G/GO coatings in the nanoscale and microscale were evaluated. We report the results of specifically designed experiments and discuss the effects of (i) thermal conductivity of graphene films, (ii) wettability, (iii) contact angle hysteresis, and (iv) morphological effects. For the nanoscale coatings, an atmospheric pressure chemical vapor deposition (APCVD) process was used to exercise control over the number of layers. For these samples, it was seen that the thermal conductivity and wettability through increased wickability were not contributing factors, but the large contact angle hysteresis (∼50°) was seen as a possible mechanism. The microscale coatings were developed through a dip-coating technique. Morphological features were generated by varying the dip-coating duration between 2 and 20min. Pool boiling tests were conducted with distilled water at atmospheric pressure which resulted in a maximum CHF of 192W/cm2 corresponding to the sample with ridge microstructures. In addition to the contact angle hysteresis in these samples, the roughness was seen to be responsible for the CHF enhancement with longer dip-coating durations (in excess of 5min) which was further verified by using a roughness based CHF model. In the case of shorter duration coatings, the unique ridge microstructures enhanced the microlayer evaporation. Bubble growth rates in the initial inertia controlled region were obtained to provide further details on the heat transfer mechanism. In summary, contact angle hysteresis, roughness, and evaporation from ridge partitioned microlayer were identified as the three mechanisms associated with nanoscale and microscale G/GO coated surfaces.

In-situ observation of hydrophobic micron particle impaction on liquid surface

Particle impaction on a droplet surface directly affects the particle capture by the droplet, and this phenomenon is the basic process of wet scrubbing and wet deposition. The experimental system is established to in-situ observe the behavior of micron particles impacting on the liquid surface. 50–200μm hydrophobic PMMA and PS particles were used. The particles showed two types of motion behavior, namely, submergence and oscillation, after impaction onto the liquid surface. During particle sinking, the advancing contact angle remained constant. The shape of the liquid surface met the Young–Laplace equation under the quasi-static assumption. After the angle of the three-phase line reached the critical value, the liquid surface was closed, and the particle submerged. A small bubble formed, adhering to the particle trailing surface. If the particle velocity decreases to zero before the angle of the three-phase lines reaches the critical value, the particle will reverse the direction of its motion. When the particle moves to the horizontal liquid surface, the movement ceases. Reciprocating oscillation behavior was not observed. During particle reversion, the receding contact angle gradually decreased and then remained unchanged. The large contact angle hysteresis is the main cause of failure observation in rebound motion mode in the experiment. The changing rule of the critical submergence/oscillation velocity with the particle diameter and surface tension coefficient was studied. As the particle diameter increases, the critical submergence/oscillation velocity decreases. As the surface tension coefficient increases, the critical submergence/oscillation velocity increases. The observed critical submergence/oscillation velocity in this experiment demonstrated the accuracy of the proposed model.

Programmable Oil/Water Separation Meshes: Water or Oil Selectivity Using Contact Angle Hysteresis

We report a novel method for oil/water separation using stainless steel meshes functionalized with amphiphilic copolymer, poly(methacrylic acid‐co‐ethylhexylmethacrylate) (PMAA‐co‐EHMA), brushes. Because the PMAA‐co‐EHMA brush‐covered surfaces show a large contact angle hysteresis at the oil/water contact line, the meshes can be programmed to act as either water‐selective or oil‐selective filters simply by pre‐wetting the mesh with one of these liquids. These programmable meshes can separate oil/water mixtures to high filtrate purities (more than 99 % mol/mol) in both oil‐selective and water‐selective modes.image

A simple approach to fabricate the rose petal-like hierarchical surfaces for droplet transportation

Precise transportation of liquid microdroplets is a great challenge in the microfluidic field. A sticky superhydrophobic surface with a high static contact angle (CA) and a large contact angle hysteresis (CAH) is recognized as the favorable tool to deal with the challenging job. Some approaches have been proposed to fabricate such surface, such as mimicing the dual-scale hierarchical structure of a natural material, like rose petal. However, the available approaches normally require multiple processing steps or are carried out with great expense. In this study, we report a straightforward and inexpensive method for fabricating the sticky superhydrophobic surfaces. The fabrication relies on electroless galvanic deposition to coat the copper substrates with a textured layer of silver. The whole fabrication process is carried out under ambient conditions by using conventional laboratory materials and equipments, and generally take less than 15min. Despite the simplicity of this fabrication method, the rose petal-like hierarchical structures and the corresponding sticky superhydrophobic wetting properties were well achieved on the artificial surfaces. For instance, the surface with a deposition time of 10s exhibits the superhydrophobity with a CA of 151.5°, and the effective stickiness with a CAH of 56.5°. The prepared sticky superhydrophobic surfaces are finally shown in the application of droplet transportation, in which the surface acts as a mechanical hand to grasp and transport the water droplet.