Hydrophobicity Of Solid Surface Research Articles

The direct observation and distribution regularity of contact angle for spherical particle attached to air bubble in static three-phase system

Contact angle is a typical parameter to indicate the hydrophobicity of solid surface, and it is usually used to evaluate the wettability and floatability of mineral particles. Directly measurement on contact angle of particles is critical for laboratory research and industrial practice in flotation. By combining the image processing analysis and force balance calculation, a composite method to directly measure contact angle of particle was proposed. The good agreement between observed contact angle and calculated contact angle during lots of experiments proved the accuracy and convenience of this measurement method. The fundamental effects of particle size, hydrophobized time, and bubble size on contact angle distribution of particle were investigated using hydrophobized glass beads with different particle size. For glass beads with same surface hydrophobicity, the contact angle showed a nearly constant trend with varying particle size and bubble size. The essence of distribution regularity of contact angle was uncovered through force calculation analysis. Besides, the central angle is always larger than half of contact angle if a smooth spherical particle successfully attached to an air bubble and achieved equilibrium state.

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.

A new plate-hanging method for biofilm quantification and its application to evaluate the role of surface hydrophobicity

A novel procedure for the quantitative analysis of biofilm formation by bacteria and yeasts, the Plate-hanging method, was developed. In this system, various polymer disks were hung from the lid of a 6-well plate, immersed in a cell suspension, and moderately shaken (70 rpm). In order to verify the validity of the procedure, the effects of the solid surface hydrophobicity of the test disks and the cell surface hydrophobicities of microorganisms on biofilm formation were investigated. Biofilm formation of bacteria and yeasts on the solid surface strongly depended on hydrophobic interactions between the solid surface and the cell surface. A positive correlation between the hydrophobic properties of substratum and cell surfaces was observed. On the other hand, hydrophilic yeasts preferentially adsorbed onto relatively hydrophilic surfaces. Moreover, the plate-hanging method coupled with the periodic exchange of the liquid medium enabled the quantification of long-term biofilm growth.

New insights into the intensification of collector adsorption on fine particles induced by flow field

Solid surface hydrophobicity plays a crucial role in flotation recovering valuable components from fine particles, which is generally rendered by reagent adsorption. In this work, to compare the liquid − solid mixing and particle surface modification performance of two different energy input forms induced by structure design, namely the mechanical agitation and hydrodynamic flow, which corresponded to a stirred tank (ST) and a novel conditioning device with opposite rotational flow (CDORF) without any agitation impellers, flow field simulations and conditioning − flotation tests were conducted, together with the particle characterization. Results showed that compared with the radial flow dominated ST, integrated forms of convection and shear mixing effectively promoted pulp homogenization in the CDORF. Basically, electrostatic attraction could be easily completed between the changed functional groups of Si − O− and RNH3+, but hydrogen − bond affinity occurred between the species of RNH3+ and Si − OH or water molecules, thereby causing competitive adsorption to some extent, which required continuously transporting particles and reagent molecules through the area with high strain rate to clean particle surfaces and launch their interactions. Based on the simulation of particle distribution and strain rate, the CDORF exhibited compatible zones of large shear and dispersed particles. Therefore, it was found to obtain better response of the induction time and the flotation recovery of fine quartz particles per added power to the collector adsorption features. The research findings reveal that the collector − particle interaction between their functional groups could be flow field dependent, which is expected to provide references for designing and optimizing equipment related to flow process fields, especially in the solution containing surfactants.

Deciphering the molecular mechanism of water boiling at heterogeneous interfaces

Water boiling control evolution of natural geothermal systems is widely exploited in industrial processes due to the unique non-linear thermophysical behavior. Even though the properties of water both in the liquid and gas state have been extensively studied experimentally and by numerical simulations, there is still a fundamental knowledge gap in understanding the mechanism of the heterogeneous nucleate boiling controlling evaporation and condensation. In this study, the molecular mechanism of bubble nucleation at the hydrophilic and hydrophobic solid–water interface was determined by performing unbiased molecular dynamics simulations using the transition path sampling scheme. Analyzing the liquid to vapor transition path, the initiation of small void cavities (vapor bubbles nuclei) and their subsequent merging mechanism, leading to successively growing vacuum domains (vapor phase), has been elucidated. The molecular mechanism and the boiling nucleation sites’ location are strongly dependent on the solid surface hydrophobicity and hydrophilicity. Then simulations reveal the impact of the surface functionality on the adsorbed thin water molecules film structuring and the location of high probability nucleation sites. Our findings provide molecular-scale insights into the computational aided design of new novel materials for more efficient heat removal and rationalizing the damage mechanisms.

Effect of dynamic adsorption layer over colliding bubble on rate of solid surface dewetting in cationic surfactant solutions

Influence of a motion induced dynamic adsorption layer (DAL) at a liquid/air interface on a kinetic of three-phase contact line (TPCL) expansion in solutions of n-cetyltrimethylammonium bromide (CTAB - cationic surfactant) was studied. It was found that, independently of the solid surface hydrophobicity (assessed using determination of the receding contact angle values), the DAL presence influences profoundly the velocity of the TPCL expansion. For the quartz surface, where the contact angle was controlled by a CTAB concentration, the TPCL expansion rate correlateds with the degree of solid surface hydrophobicity. However, for similar values of the contact angle, much higher rate of the TPCL rate expansion was observed for the fully formed DAL at the colliding bubble surface. This effect was the most pronounced for receding contact angle equal to ca. 60–70° (i.e. for higher CTAB bulk concentrations). In the case of the highly hydrophobic poly(tetrafluoroethylene) (PTFE) surface, similar observations were reported, despite the fact that the receding contact angle values were independent of the CTAB bulk concentration. For the PTFE surface, additionally, similar TPCL expansion velocities were reported for pure water and CTAB solutions where the DAL structure was fully formed at the colliding bubble interface, suggesting similar liquid/gas interface’s hydrodynamic boundary conditions.

Impact of solid surface hydrophobicity and micrococcal nuclease production on Staphylococcus aureus Newman biofilms

Staphylococcus aureus is commonly associated with biofilm-related infections and contributes to the large financial loss that accompany nosocomial infections. The micrococcal nuclease Nuc1 enzyme limits biofilm formation via cleavage of eDNA, a structural component of the biofilm matrix. Solid surface hydrophobicity influences bacterial adhesion forces and may as well influence eDNA production. Therefore, it is hypothesized that the impact of Nuc1 activity is dependent on surface characteristics of solid surfaces. For this reason, this study investigated the influence of solid surface hydrophobicity on S. aureus Newman biofilms where Nuc1 is constitutively produced. To this end, biofilms of both a wild-type and a nuc1 knockout mutant strain, grown on glass, salinized glass and Pluronic F-127-coated silanized glass were analysed. Results indicated that biofilms can grow in the presence of Nuc1 activity. Also, Nuc1 and solid surface hydrophobicity significantly affected the biofilm 3D-architecture. In particular, biofilm densities of the wild-type strain on hydrophilic surfaces appeared higher than of the mutant nuc1 knockout strain. Since virulence is related to bacterial cell densities, this suggests that the virulence of S. aureus Newman biofilms is increased by its nuclease production in particular on a hydrophilic surface.

Mechanical robustness of monolayer nanoparticle-covered liquid marbles.

Powder-derived liquid marbles (LMs) are versatile nonwetting systems but are confronted with many limitations in application, as their surface particles are usually large and agglomerated. Recently, sol-gel film-derived LMs have come on the scene that are termed monolayer nanoparticle-covered (mNPc) LMs based on their unique characteristics, revealing great application potential but also generating many questions. Here, mechanical robustness, a very important yet to be addressed property, of mNPc LMs was systematically studied. Rolling, pendant contact, and compression experiments were designed using bare and coated glasses with water contact angles (WCAs) ranging from 23° to 157°. With rupture as a quality criteria, the mechanical robustness of mNPc LMs enhanced with the hydrophobicity of solid surfaces that exerted pressure on them, but maintained much weaker than typical powder LMs until the solid surface was superhydrophobic. In particular, when contacting hydrophilic surfaces of WCAs ≤53°, an mNPc LM did not have the capacity for nonwetting and ruptured immediately, even if the pressure approached zero. This was distinct from powder LMs and indicated that a particle shell as thin as ∼20 nm could not prevent intermolecular attractions between the internal liquid and external solid surface. An interface scenario consisting of solid surface microroughness was proposed to address this issue. On the other hand, mNPc LMs remained unruptured on superhydrophobic surfaces but presented degraded elasticity under extreme compression. Uncovering these properties could be of much help for developments of mNPc LMs and their counterparts, the mNPc liquid plasticines.

Interaction Forces between Water Droplets and Solid Surfaces across Air Films.

Wetting of solid surfaces occurs when the intervening air film between a water droplet and a solid surface ruptures. Although this rupturing phenomenon is well known, the underlying mechanism has not yet been well understood. In this work, the rupture of intervening air films is systematically studied by measuring the spatiotemporal thickness profiles of the air films between droplets of deionized water and flat solid surfaces using a synchronized triwavelength reflection interferometry microscope. It has been shown that the critical rupture thickness of the air film (hc) depends on the surface hydrophobicity of solid surfaces. The hc value was increased from 50 nm on a hydrophobic surface having an equilibrium water contact angle (θw) of 96° to 1.42 μm on a hydrophilic surface having a θw of 25°. In addition, an increase in the critical rupture thickness with decreasing surface hydrophobicity was found to be applicable not only to chemically treated quartz surfaces but also to a variety of natural mineral surfaces. By determining the pressure within the air films, we have shown that a strong attractive force is present between water droplets and hydrophilic surfaces, thereby accelerating the draining of air films. The measured forces might be of electrostatic origin, and the forces become less attractive with increasing hydrophobicity of solid surfaces. The present result provides a fundamental insight into the rupture of air films from the perspective of surface forces.

Bacterial response to spatially organized microtopographic surface patterns with nanometer scale roughness.

In this study, the influence of nanometer scale roughness on bacterial adhesion and subsequent biofilm formation has been evaluated using spatially organized microtopographic surface patterns for four major opportunistic pathogens of the genus Staphylococcus (S. epidermidis and S. aureus) responsible for associated-biofilm infections on biomedical devices. The results presented demonstrated that regardless of the strain employed the initial adhesion events to these surfaces are directed by cell-surface contact points maximisation and thus, bacterial cells actively choose their position to settle based on that principle. Accordingly, bacterial cells were found to preferably adhere to the square corners and convex walls of recessed surface features rather than the flat or concave walls of equal protruding features. This finding reveals, for the first time, that the particular shape of the surfaces features employed potentially determined the initial location of the adhering cells on textured surfaces. It was further shown that all surfaces patterns investigated produce a significant reduction in bacterial adhesion (40-95%) and biofilm formation (22-58%). This important observation could not be related to physical constrains or increased solid surface hydrophobicity, as previously suggested by other authors using engineered topographies with microscale surface roughness. It is evident that other causes, such as nanoscale surface roughness-induced interaction energies, might be controlling the process of bacterial adhesion and biofilm formation on surfaces with well-defined nanoscale topography.

Kinetics of the bubble attachment and quartz flotation in mixed solutions of cationic and non-ionic surface-active substances

The effect of cationic (n-cetyltrimethylammonium bromide − CTAB) and non-ionic (n-octanol) surface-active substances (SAS) on the kinetics of rising bubble attachment to a quartz surface and flotation recovery of quartz particles was investigated. The measured time of the three-phase contact (TPC) formation (tTPC) and the advancing contact angle (sessile drop) values, in pure and mixed CTAB and n-octanol solutions, allowed to evaluate the importance of electrostatic interactions and solid surface hydrophobicity in stability and rupture of the wetting films formed between a quartz surface and a colliding bubble. It was shown that electrostatic interactions inside the wetting film as well as the quartz hydrophobicity was changing with the composition of the mixed solutions. The experiments revealed that, within the concentration range of the SAS used in this study, CTAB modified properties of liquid/gas and liquid/solid interfaces, while non-ionic n-octanol adsorbed primarily at liquid/gas interface hindering its fluidity and, as a result, decreased bubble bouncing time. The correlation between the times of bubble attachment and the flotation recovery of quartz particles was established.

固体表面の動的撥水性に関する最近の話題

固体表面の動的撥水性に関する最近の話題

Effects of pH on wetting behavior of ‘star-like’ block copolymer surfactant solutions

Wetting behavior of aqueous solutions of four polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants (named A–D) are studied and pH effect is assessed. Surfactants A, B and C have a ‘star-like’ structure differing only by the number of polymeric branches: 4, 6 and 9 in the mentioned order while the forth one, D, is of a dendritic type with 4–6 primary and 2–3 secondary branches. Experimental data about liquid/air surface tension, contact angle θ and equilibrium wetting film thickness of the surfactant solutions in pH range of 1–12 are obtained. The liquid/solid and the air/solid surface tension are calculated using the empirical equation of state and Berthelot’s combining rule. Results from the studies indicate that pH affects polymer solution properties at liquid/solid and liquid/air interfaces, and influences parameters such as surface tension, solid surface hydrophobicity, equilibrium film thickness, etc.

Detection of cell surface hydrophobicity among uropathogenic isolates of Escherichia coli

A number of studies have shown that the hydrophobicity of solid surfaces influences adhesion of bacteria, eukaryotic cell and proteins. On the other hand bacteria and the other microbes including viral particles are evolving many different ways to use the hydrophobicity in order to adhere on to the substrates. A total of 20 clinical isolates of UPEC were subjected for antibiotic sensitivity pattern and cell surface hydrophobicity by salt aggregation method.Most of the strains showed resistant to all the antibiotics tested. 70% of isolates were showed cell surface hydrophobicity positivity. Judicial usage of antibiotics and proper detection of resistant mechanisms operating in the clinical isolates would yield better management of patient’s outcome. Cell surface hydrophobicity might associate with the adhesion of UPEC in both biotic and abiotic surfaces.

Evaluation of Dynamic Hydrophobicity of Nanoporous Silicon Surfaces Prepared By Metal-Assisted Chemical Etching

Wettability is one of the important properties of solid surfaces from both fundamental and practical aspects. Among various factors, surface energy and roughness are the dominant factors for the wettability. Static wettability, that is, water contact angle (WCA) has been commonly used as a criterion for the evaluation of hydrophobicity of solid surface. In recent years, WCA can be designed easily by controlling surface energy and roughness based on Young’s, Wenzel’s, and Cassie’s equations. On the other hand, dynamic wettabilities such as sliding property on the hydrophobic surface are poorly undersutood. Although water sliding angle (WSA: the critical angle where a water droplet with a certain weight begins to slide down the inclined plate) and contact angle hysteresis (the cosine of receding contact angle minus that of advancing contact angle) are often measured to evaluate the water-repellent property of the hydrophobic surface, comprehensive understanding of the water-repellent properties is still limited because the relationships between the WSA and the WCA are not clear [1-2]. In this study, we used nanoporous silicon surfaces with different surface morphologies, which were formed by metal-assisted chemical etching using Ag nanoparticles, as a solid surface. Static and dynamic wettabilities of their surfaces were investigated with focusing on the influences of pore size and surface roughness on wettability of porous silicon surfaces. p-type silicon (100) substrates were immersed in a mixed solution of 2×10-2 mol dm-3 AgNO3 and 5 mol dm-3 HF to deposit Ag particles on the entire surface of silicon substrates. After deposition of Ag particles, silicon substrates were immersed in a mixed solution of 10 mol dm-3 HF and 1 mol dm-3 H2O2at room temperature at different time. SEM images of obtained porous silicon after etching with different surface roughness are shown in Fig. 1. WCAs (approximately 135°) of etched silicon were higher than that of flat silicon (75°), presumably due to decrease of contact area between porous structure and droplet based on Cassie’s model. Although surface roughness increased with increasing etching time, however, WCAs of etched silicon were about the same (135°) despite the difference in surface morphologies (Fig. 1a-c). On the other hand, WSAs increased with increasing surface roughness. The changes in WSAs were thought to be caused by the pinning effect generated by increase of pore diameter and surface roughness during chemical etching. These results indicate that WSA is affected more significantly by physical inhomogeneity such as surface roughness of the etched surface than WCA. [1] M. Miwa, A. Nakajima, A. Fujishima, K. Hashimoto, T. Watanabe, Langmuir, 16, 5754-5760 (2000). [2] S. Suzuki, A. Nakajima, Y. Kameshima, K. Okada, Surface Science Letters, 557, L163-L168 (2004). Figure 1

Effects of biosurfactants, mannosylerythritol lipids, on the hydrophobicity of solid surfaces and infection behaviours of plant pathogenic fungi.

To investigate the effects of mannosylerythritol lipids (MELs) on the hydrophobicity of solid surfaces, their suppressive activity against the early infection behaviours of several phytopathogenic fungal conidia, and their suppressive activity against disease occurrences on fungal host plant leaves. The changes in the hydrophobicity of plastic film surfaces resulting from treatments with MEL solutions (MEL-A, MEL-B, MEL-C and isoMEL-B) and synthetic surfactant solutions were evaluated based on the changes in contact angles of water droplets placed on the surfaces. The droplet angles on surfaces treated with MELs were verified to decrease within 100 s after placement, with contact angles similar to those observed on Tween 20-treated surfaces, indicating decreases in surface hydrophobicity after MEL treatments. Next, conidial germination, germ tube elongation and the formation of appressorium of Blumeria graminis f. sp. tritici, Colletotrichum dematium, Glomerella cingulata and Magnaporthe grisea were evaluated on plastic surfaces that were pretreated with surfactant solutions. On the surfaces of MEL-treated plastic film, inhibition of conidial germination, germ tube elongation, and suppression of appressoria formation tended to be observed, although the level of effect was dependent on the combination of fungal species and type of MEL. Inoculation tests revealed that the powdery mildew symptom caused by B. graminis f. sp. tritici was significantly suppressed on wheat leaf segments treated with MELs. MELs exhibited superior abilities in reducing the hydrophobicity of solid surfaces, and have the potential to suppress powdery mildew in wheat plants, presumably due to the inhibition of conidial germination. This study provides significant evidence of the potential for MELs to be used as novel agricultural chemical pesticides.

Study on Wettability of Aluminum Alloy Superhydrophobic Surface

Wettability is interfacial phenomena by a solid interface changes from solid-gas interface to solid-liquid interface, and the contact angle is often used to characterize it. The super hydrophobic surfaces with contact angle over 150° have various excellent characteristics. In the paper, the wettability of aluminum alloy surfaces was studied. Based on the Wenzel and Cassie models, the relationship of wettability between roughnesses were investigated on body surface and coating surface. The results indicated that the hydrophobicity of surfaces could greatly enhance with high roughness on surface, and then coating process was analyzed, wettability of aluminum alloys were transformed from hydrophilicity to superhydrophilicity, when they were coated with organic materials with the low surface free energy, such as polypropylene (PP) or maleic anhydride grafted polypropylene (PP-g-MAH), so modified PP can enhanced the hydrophobicity of solid surface mostly.

Polyoxyalkylated diethylenetriamine polymeric surfactants: molecular structure effect on thin wetting films

Thin wetting films from aqueous solution of four polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants (named A, B, C, and D) are studied. Surfactants A, B, and C have a star-like structure differing only by the number of polymeric branches: four, six, and nine in the mentioned order while the forth one, D, is of a dendritic type with four to six primary and two to three secondary branches. The receding contact angles θr of the solution on hydrophilic SiO2 glass surface and the contact angle θaq of a drop of doubly distilled water on SiO2 glass surface pretreated with DETA polymeric surfactant solution are measured. The θr values on hydrophobicity of SiO2 glass surface, respectively, increase in the following order: surfactant A, surfactant C, surfactant B, and surfactant D. The equilibrium thickness heq of wetting films from DETA aqueous solution on hydrophilic SiO2 glass surface is measured using the micro-interferometric method. Results show an unexpected course of the heq vs. Cs curves with a maximum. Results from the studies indicate that differences in polymeric surfactant molecular structure affect the properties exhibited at air/liquid and solid/liquid interfaces, such as the value of surface tension, degree of hydrophobicity of solid surface, equilibrium film thickness, etc.

Dissipation of Film Drainage Resistance by Hydrophobic Surfaces in Aqueous Solutions

Understanding and minimizing the film drainage resistance (forces) from a moving fluid are of great importance both scientifically and technologically. The direct and accurate measurement of film drainage resistance was made possible by integrating a speaker diaphragm of large displacement range and rapid responses with a sensitive bimorph force sensor and high resolution digital camera. Our study demonstrates that the liquid film drainage resistance can be greatly diminished or accurately controlled by increasing or controlling the hydrophobicity of solid surfaces. The results show that for a given solid surface hydrophobicity, the film drainage resistance at the point where film ruptures increases linearly with increasing bubble approach velocity. The dependence of the film drainage resistance on bubble approach velocity decreases linearly with increasing hydrophobicity of the solid surface. This finding has important implications for biological processes, microfluidic devices, and design of new materials.

ChemInform Abstract: Control of Static and Dynamic Hydrophobicity of Solid Surface and Its Application

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