Large Contact Angle Research Articles

Melting Process of Frozen Sessile Droplets on Superhydrophobic Surfaces.

Superhydrophobic surfaces can exhibit icephobicity in many ways due to their large contact angles and small rolling angles. The melting process of frozen droplets on superhydrophobic surfaces is still unclear, hindering the understanding of surface icephobicity. In this experimental study of the melting process of frozen sessile droplets on superhydrophobic surfaces, we find two types of melting morphologies with opposite vortex directions on a single-scale nanostructured (SN) superhydrophobic substrate and a hierarchical-scale micronanostructured (HMN) superhydrophobic substrate. Melting pattern visualizations and flow field measurements showed Marangoni convection and natural convection occurring in the melting sessile droplets. For the HMN superhydrophobic substrate, the internal flow was found to be dominated by Marangoni convection due to the temperature gradient along the surface of the droplet. For the SN superhydrophobic substrate, Marangoni convection was inhibited by the superhydrophobic particles at the surface of the droplet, which were shed from the fragile superhydrophobic substrate during the freezing-melting process, as confirmed by surface characterizations of the substrate and flow measurements of a water pool. These results will help researchers better understand the melting process of frozen droplets and in designing novel icephobic surfaces for numerous applications.

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.

Replaceable Dielectric Film for Low-Voltage and High-Performance Electrowetting-Based Digital Microfluidics.

Electrowetting-on-dielectric (EWOD) technology has been considered as a promising candidate for digital microfluidic (DMF) applications due to its outstanding flexibility and integrability. The dielectric layer with a hydrophobic surface is the key element of an EWOD device, determining its driving voltage, reliability, and lifetime. Hereby, inspired by the ionic-liquid-filled structuring polymer with high capacitance independent on thickness, namely ion gel (IG), we develop a polymer (P)-ion gel-amorphous fluoropolymer, namely, PIGAF, composite film as a replaceable hydrophobic dielectric layer for fabrication of a high-efficiency and stable EWOD-DMF device at relatively low voltage. The results show that the proposed EWOD devices using the PIGAF-based dielectric layer can achieve a large contact angle (θ) change of ∼50° and excellent reversibility with a contact angle hysteresis of ≤5° at a relatively low voltage of 30 Vrms. More importantly, the EWOD actuation voltage did not change obviously with the PIGAF film thickness in the range of several to tens of microns, enabling the thickness of the film to be adjusted according to the demand within a certain range while keeping the actuation voltage low. An EWOD-DMF device can be prepared by simply stacking a PIGAF film onto a PCB board, demonstrating stable droplet actuation (motion) at 30 Vrms and 1 kHz as well as a maximum moving velocity of 69 mm/s at 140 Vrms and 1 kHz. The PIGAF film was highly stable and reliable, maintaining excellent EWOD performance after multiple droplet manipulations (≥50 cycles) or long-term storage of 1 year. The proposed EWOD-DMF device has been demonstrated for digital chemical reactions and biomedical sensing applications.

Carboxymethylation enhances the low oil absorption of freeze-thawed tapioca starch in fried ham sausage batter

Excessive consumption of high oil content deep-fried foods increases the risk of obesity and cardiovascular disease. In this study, carboxymethylation was utilized to improve the oleophobic ability of freeze-thawed tapioca starch for deep-fried ham sausage batter. The smallest specific surface area, largest contact angle and oil distribution of the freeze-thawed carboxymethyl tapioca starch (CM-FTS) proved that it had the weakest oil absorption capacity. Carboxymethylation damaged long-range crystalline structures in FTS, but the oil absorption did not increase. Furthermore, carboxymethylation appropriately increased the viscosity of FTS batter which evenly improved its coagulation and adhesion and reduced oil absorption during frying. The decrease of tan δ also indicated that the improvement of gel property of batter inhibited the oil absorption in CM-FTS-batter for the lowest oil penetration. These results demonstrate that carboxymethylation enhances the oleophobic structure of FTS and maintains low oil absorption properties of the batter and the internal ham sausage.

CHARACTERIZATION OF SUPERHYDROPHOBIC COATINGS BASED ON PDMS AND MQ RESIN ON TEXTURED SURFACES

The paper presents the results of studying coatings based on poly(dimethylsiloxane) rubber crosslinked with MQ resin at different contents of the latter. The PDMS :MQ resin ratios have been taken to be 2 : 1, 1 : 1, and 1 : 2. It has been shown that the application of a hydrophobic fluorinated-group-free composite with composition PDMS :MQ = 1 : 1 onto a pre-textured surface by spin-coating or deep-coating results in the formation of superhydrophobic coatings. The coatings are characterized by extremely large contact angles (170°) and rolling angles no larger than 4°. The analysis of variations in the contact angle, surface tension, and contact diameter and volume of a water droplet that has been in contact with a coating for a long time has indicated a high hydrolytic resistance of the obtained coatings.

Flowrate effects on the lateral coalescence of two growing bubbles

AbstractThe coalescence of two growing bubbles presents unique characteristics compared to static bubble coalescence. The gas injection flowrate significantly affects the different stages of bubble evolution, which is poorly understood. In this study, we investigate the flowrate effects on the lateral coalescence of two growing bubbles experimentally. The synchronous bubbling from adjacent needles is achieved using water to push air. During the bubble growth process, we find that the initial nonlinear evolution of bubble volume is because the bubble emerges as a small spherical cap with a large curvature radius and apparent contact angle. As the neck expands after bubble coalescence, the injection flowrate accelerates the neck evolution compared to the case without air injection. We find the neck expansion time decreases linearly with increasing flowrate, while the expansion speed increases with flowrate, but only in the early stage. Moreover, we propose a new theoretical expression that predicts the neck radius well at all the flowrates. At the post‐coalescence oscillation stage, the average projection area of the coalesced bubble increases linearly with time, except for periodic oscillations. Besides, we find that the injected air primarily influences the coalesced bubble's height, which in turn affects the projection area.

Influence of contact angle on droplet parameters in ellipsoidal wettability model

Contact angle is one of the most important indexes to evaluate the hydrophobicity of solid surface. In order to explore the wetting characteristics of droplets on anisotropic solid surfaces, including contact angle, droplet radius, droplet height, contact radius, contact area and projected area, an ellipsoidal droplet contact angle model was established. Different from the ordinary plane ellipsoidal cap model, the major axis and minor axis of the elliptical cap shape in this model are not exactly the same in different planes. This model studied the relationship between contact angle and interface parameters in different planes. By collecting the droplet size parameters of fluororubber (FKM) prepared by template method, the correctness of the theoretical model was verified. Among them, the maximum error between the theoretical value of droplet radius and the actual measured value was 4.3%, and the maximum error of droplet projected area was 2.1%. It was found that the contact angle was inversely proportional to the droplet-solid contact radius and contact area, and directly proportional to the projected area of the droplet. In addition, for the same droplet on the surface of the same solid material, it was observed that the contact radius between the droplet and the solid was small in the direction of large contact angle. This discovery is helpful to explain the hydrophobic mechanism of the material surface with anisotropic contact angle.

The Effects of Nano-Based Bio-Carbonates in Superhydrophobic Concrete—A Review

Concrete must be a hydrophilic compound that is easily fabricated by nature. At the nanoscale, mechanical and chemical reactions alter the quality of cement-based substances. Continuous sprinkling of nano-silica solution synthesised with minimal surface solvents has been used to create a superhydrophobic (SH) concrete surface while similarly modifying the surface’s chemical composition and dynamical intrinsic structure. In this study, we examine the impacts of admixtures in SH concrete including nano-based bio-carbonate. The fundamental characteristics and dispersal techniques of nanoparticles often employed in cement-based compounds are reviewed initially in this paper. Investigations of the large contact angle, small slide angle, and carbonated thickness have been employed to analyze the impacts of admixtures. Additionally, the industry and uses of nanoparticles for concrete substances are addressed, and the expense is inventively represented by a survey questionnaire. Finally, this article identifies the obstacles that now occur in the field of research and offers appropriate future viewpoints.

Inclined colloidal drops: Evaporation kinetics and pattern formation

The drying of solute-laden drops is ubiquitous in everyday life, from paints and printers to the raindrops drying on our windows. Nonetheless, scientific interest has primarily focused on understanding the evaporation kinetics on flat surface, with the key parameter of substrate inclination only recently started being addressed. This work focuses on the influence of moderate substrate inclinations at 20° and 40° on the evaporation kinetics and associated deposit patterns of colloidal drops. Inclination altered the shape of the drops which formed a lower contact angle at the upper side of the drop (rear edge) and larger contact angle at the lower side (front edge). As evaporation rate is a function of contact angle, which in turn is a function of inclination, the evaporation lifetime was extended by 43% and 61% for 20° and 40°, respectively, compared to a flat drop. A theoretical approximation of the evaporative flux across the liquid-vapour interface of the drops showed the contribution of each edge to the evaporation kinetics. These differences in the evaporative fluxes altered the internal flows within the drop and in turn the coffee-ring formation mechanism. The particle deposit shape at the two edges for each drop was visualised which combined with the theoretical arguments allowed the proposition of the particle deposition mechanism in inclined drops: inclination added a gravitationally-driven velocity flow component within the drops, which is perpendicular and hence negligible in flat drops. This additional flow hindered or enhanced the number of particles arriving at the rear and front edges of the inclined drops, respectively, and hence influenced the dimensions of the coffee-ring patterns. Eventually, the particle deposits grew sufficiently tall to effectively stagnate the outward flow which resulted in enhanced particle accumulation at the interior of the drops as inclination increased.

Hydrophobic interface-assisted synthesis of K2CO3/Al2O3 adsorbent pellets for CO2 capture

Suitable molding and granulation methods are crucial, and an outstanding sphere structure can play a better role in the future scale-up industrial application of K2CO3-based CO2 adsorbent. Water will form a large contact angle on the surface of hydrophobic solution and become sphericity water. In this work, hydrophobic powders (polyethylene (PE), TiO2, and SiO2) were selected in the hydrophobic interface-assisted method. The CO2 adsorption capacity of TiO2 pellets is 0.50 mmol/g, while that of PE pellets is 0.24 mmol/g, and finally that of SiO2 pellets is 0.36 mmol/g. Then, experiment proved that high temperature (>500 ℃) makes the hydrophobic powders (i.e, TiO2 and SiO2) lose hydrophobicity which make water molecules diffuse faster, while PE is completely decomposed at high temperature. Compared with the pellets calcined at 300 ℃, the TiO2 pellets calcinated at 500 ℃ have an adsorption capacity of 0.70 mmol/g, which is over 1.72 times that of the original pellets, and the other two hydrophobic surfaces are also improved to some extent. Additionally, the appropriate ratio of low-temperature pore forming agent (i.e., urea) can help improve the adsorption performance, while excessive addition will lead to poor CO2 adsorption performance. In addition, the K2CO3/Al2O3 pellets also exhibited desirable mechanical properties, with an optimal compressive strength of 41 Mpa. Therefore, the loss of hydrophobicity leads to more active diffusion of water molecules, which may provide a new idea for the future development of CO2 adsorbents and other areas.

Sustainable acid hydrotropic fractionation for bamboo-based nanofilms with unparalleled hydrophobicity and UV resistance

Bamboo, as an important biomass raw material, has a very favorable prospect for Nature-based Solution. Here we provide a method to prepare bamboo-based lignocellulosic nanofibrils (LCNFs) and films with excellent impedance performances by p-toluenesulfonic acid (p-TsOH) under mild condition. The combined delignification factor (CDF) and combined hydrolysis factor (CHF) were introduced to regulate the hydrolysis severity. The resultant LCNFs with uniform diameter retained a plentiful reactivity without surface functional group changing. The ascendant thermal stability (Tmax > 360 °C) of the LCNFs were also obtained. The films made by the LCNFs can block about 99 % UV light with transparency greater than 50 % which exhibited remarkable UV resistance in both UVA and UVB regions. Furthermore, the films presented excellent hydrophobic stability which showed large contact angles (water, 89–99°) and maintained at 64° after 300 s. These properties make bamboo-based materials substitute bamboo for plastics and can be used in aviation, construction, automotive industry, packaging and many other fields, conforming to international initiatives.

초정밀 레이저를 이용한 연꽃잎 표면 자연 모사 연구

In a pilot natural super-hydrophobic surfaces study, a super-hydrophobic surface was made by coating, etching, laser ablation, chemical vapor deposition and lithography. In this study, cone-shaped periodic micro and nano-structures were constructed on a silica surface with femtosecond and picosecond laser, and the period of micro-structures between cone shape patterns was increased with 10 μm intervals. The contact angle and image of the super-hydrophobic surface were analysed and the cone (Aspect-ratio 1.27) shape model with micro-protrusion structure similar to the surface of the lotus leaf was made to measure the contact angle. To analyse the differences in the contact angles between the cone shapes and heights of the micro-protrusion, different samples with cone (Aspect-ratio 1.27), sphere (Aspect-ratio 1.00) shapes were made through laser micro-machining technology. To mimick the natural lotus leaves, the optimum condition was a cone shape. Samples of PDMS with different shapes and mixed micro/nano-structures were fabricated with a PDMS mold insert. The largest contact angle was measured at 170.42° which is similar to the contact angle of the lotus leaf. This mold insert could be used repeatedly. The molding process is advantageous for large areas and mass production.

Toughened polybenzoxazine coating matrix with improved comprehensive performance through hydrogen bond tuning with new polyurethanes

Polybenzoxazine has been explored in coating applications because of its excellent properties, including good processability, excellent thermal stability, and large water contact angles, but its toughness must be improved without compromising its other properties. Herein, a new serious of polyurethanes with varying numbers of ether groups were synthesized to enhance the toughness of polybenzoxazine via tuning the hydrogen bonds. The curing mechanism, chemical structures, and phase morphology of the blends are discussed. The results show that hydrogen bonding is the main reason for improved properties of the cured products. By varying the hydrogen bond content of the modified system, a champion toughness of 20.4 kJ/m2 without sacrificing processability, thermal stability, and coating performances can be realized, which the char yield is varied between 26.0 % and 28.9 % at 800 °C and corresponding coating hardness and adhesion can reach to 6H and 5B, respectively. Furthermore, the modified system-based coatings exhibit a reversible hydrophobic-hydrophilic state by different stimulus with a champion oil-water separation efficiency of 3379 L·m−2·h−1, illustrating their potential intelligent applications.

Influence of the Atmosphere on the Wettability of Polymer Brushes

Polymer brushes, i.e., end-tethered polymer chains on substrates, are sensitive to adaptation, e.g., swelling, adsorption, and reorientation of the surface molecules. This adaptation can originate from a contacting liquid or atmosphere for partially wetted substrates. The macroscopic contact angle of the aqueous drop can depend on both adaptation mechanisms. We analyze how the atmosphere around an aqueous droplet determines the resulting contact angle of the wetting droplet on polymer brush surfaces. Poly(N-isopropylacrylamide) (PNiPAAm)-based brushes are used due to their exceptional sensitivity to solvation and liquid mixture composition. We develop a method that reliably measures wetting properties when the drop and the surrounding atmosphere are not in equilibrium, e.g., when evaporation and condensation tend to contaminate the liquid of the drop and the atmosphere. For this purpose, we use a coaxial needle in the droplet, which continuously exchanges the wetting liquid, and in addition, we constantly exchange the almost saturated atmosphere. Depending on the wetting history, PNiPAAm can be prepared in two states, state A with a large water contact angle (∼65°) and state B with a small water contact angle (∼25°). With the coaxial needle, we can demonstrate that the water contact angle of a sample in state B significantly increases by ∼30° when a water-free atmosphere is almost saturated with ethanol, compared to an ethanol-free atmosphere at 50% relative humidity. For a sample in state A, the relative humidity has little influence on the water contact angle.

C-shaped porous polypropylene fibers for rapid oil absorption and effective on-line oil spillage monitoring

Development of efficient absorbent materials for detection and treatment of offshore oil spillages remained a challenge. In this work, C-shaped polypropylene oil-absorbent fibers with sub-micron internal pores were prepared by combining spun-bonding technique and thermally induced phase separation (TIPS). The effect of drawing speed on the phase separation and the porous morphology of the shaped fiber non-woven fabric (NWF) was investigated. C-shaped NWF with porous morphology had large water contact angle, higher porosity, larger specific surface area, and increased oil absorption speed and capacity. An online oil spillage detection system was developed using porous C-shaped NWF and an oxygen sensing probe, showing shorter response time and higher signal-to-noise (STN) ratio. The response time for detecting the spillage of soybean oil and diluted crude oil (0.5 mL/0.8 L) in water were only 24 s and 10 s, respectively. The reliable oil detection low detection limit (RLDL) of the oxygen sensing probe was reduced 173 times (from 36.5 g/L to 0.21 g/L) when combined with C-shaped porous fiber NWF.

Bioinspired flexible, breathable, waterproof and self-cleaning iontronic tactile sensors for special underwater sensing applications

Flexible tactile sensors are under high pursuit for wearable healthcare devices. However, challenges still exist in achieving both wearing comfortability through air permeability and hydrophobicity to resist water splash or even to be used in the special underwater circumstance. Inspired by the smart bio-structure of lotus leaf, we develop a flexible, breathable, waterproof and self-cleaning tactile sensor by sandwiching one ionogel electrolyte between two polydopamine (PDA)/MXene/stearic acid (STA) fabric electrodes. The micro-channels in the fabric and the through-holes in the gel endow the sensor with a high air breathability of 723.5 mm s−1 and the STA micro-sheets decorated on the outside surface of the sensor provide hydrophobicity with a large water droplet contact angle of 140.7° for the unique properties of self-cleaning and washability. By taking advantage of the supercapacitive sensing mechanism, the microstructures of MXene nanosheets on microfibers and the using of an interlayer with an internal aperture to initially separate electrodes and electrolyte, an extremely high sensitivity of up to 1677.79 kPa−1 is achieved. Practical sensing applications of the developed flexible sensors worn on different parts of human body for physiological signal monitoring, motion detection and silent information communication through Morse code in the special underwater circumstance are demonstrated.

New Functional Polymer Materials via Click Chemistry-Based Modification of Cellulose Acetate.

Cellulose acetate (CA) was partially acrylated, and the resulting cellulose acetate acrylate (acryl-substitution degree of 0.2) underwent quantitative thio-Michael click reactions with various thiols. A toolbox of functional CA polymers was obtained in this way, and their properties were studied. The modification with fatty alkyl thiols led to hydrophobic materials with large water drop contact angles. Octadecylthio-, butoxycarbonylpropylthio-, and furanylthio-modifications formed highly transparent materials. The new derivative CAASFur disintegrated completely under industrial composting conditions. Films of modified CA polymers were cast and investigated in terms of barrier properties. The nanocomposite of CAAS18 compounded with a synthetic layered silicate (hectorite) of a large aspect ratio showed permeabilities as low as 0.09 g mm m-2 day-1 for water vapor and 0.16 cm3 mm m-2 day-1 atm-1 for oxygen. This portfolio of functional CA polymers opens the door to new applications.

Facile and Economical Fabrication of Superhydrophobic Flexible Resistive Strain Sensors for Human Motion Detection

Superhydrophobic flexible strain sensors have great application value in the fields of personal health monitoring, human motion detection, and soft robotics due to their good flexibility and high sensitivity. However, complicated preparation processes and costly processing procedures have limited their development. To overcome these limitations, in this work we develop a facile and low-cost method for fabricating superhydrophobic flexible strain sensor via spraying carbon black (CB) nanoparticles dispersed in a thermoplastic elastomer (SEBS) solution on a polydimethylsiloxane (PDMS) flexible substrate. The prepared strain sensor had a large water contact angle of 153 ± 2.83° and a small rolling angle of 8.5 ± 1.04°, and exhibited excellent self-cleaning property. Due to the excellent superhydrophobicity, aqueous acid, salt, and alkali could quickly roll off the flexible strain sensor. In addition, the sensor showed excellent sensitivity (gauge factor (GF) of 5.4–7.35), wide sensing ranges (stretching: over 70%), good linearity (three linear regions), low hysteresis (hysteresis error of 4.8%), and a stable response over 100 stretching-releasing cycles. Moreover, the sensor was also capable of effectively detecting human motion signals like finger bending and wrist bending, showing promising application prospects in wearable electronic devices, personalized health monitoring, etc.

Wetting of porous thin films exhibiting large contact angles.

Porous solid films that promote large apparent contact angles are interesting systems since their wetting properties are dependent on both the surface structure and water penetration into the film. In this study, a parahydrophobic coating is made by sequential dip coating of titanium dioxide nanoparticles and stearic acid on polished copper substrates. The apparent contact angles are determined using the tilted plate method, and it is found that the liquid-vapor interaction decreases and water droplets are more likely to move off the film when the number of coated layers increases. Interestingly, it is found that under some conditions, the front contact angle can be smaller than the back contact angle. Scanning electron microscopy observations demonstrate that the coating process led to the formation of hydrophilic TiO2 nanoparticle domains and hydrophobic stearic acid flakes that allows heterogeneous wetting. By monitoring the electrical current through the water droplet to the copper substrate, it is found that the water drops penetrate the coating layer to make direct contact with the copper surface with a time delay and magnitude that depends on the coating thickness. This additional penetration of water into the porous film enhances the adhesion of the droplet to the film and provides a clue to understand the contact angle hysteresis.

A comparison of antibacterial activity in dark-UV light in perspective of surface and structural properties of spray pyrolysis grown Cu doped Cr2O3 thin films

Thin films of Cr2O3 and Cu doped Cr2O3 thin films were prepared using the spray pyrolysis technique with different Cu/Cr ratios. Precursor solution of 0.2 M was sprayed on a pre-heated (≈ 400 °C) soda lime glass substrate using air as the carrier gas. XRD analysis confirmed polycrystalline nature of the films with rhombohedral structure. Crystallite size and lattice strain have been estimated using Williamson-Hall method. These films were porous and presented a rough morphology and showed a strong signature on the presence of vibrational Cr-O bonding. The solid-liquid surface contact angle measurements revealed a large contact angle which is indicative of reduced surface energy. Vickers hardness measurement reveals that Cr2O3 thin film with 2% Cu/Cr ratio have high hardness value of 903 HV (0.2 kgf). The antibacterial activity of the films was investigated against two gram-positive (Bacillus Subtilus and Bacillus Meurellus) and two gram-negative bacteria (Escherichia Coli and Acetobacter Rhizopherensis). A comparison was established both under dark and UVA light in terms of structural properties of the films. Antibacterial activity was significantly enhanced with the increase in Cu/Cr ratio. Interestingly, the UVA light further facilitated to boost the antibacterial effect against both types of bacteria. A good correlation was found between the structural parameters and the zone of inhibition.