Contact Angle Hysteresis Measurements Research Articles

Investigation of the anti-biofouling properties of graphene oxide aqueous solutions by electrowetting characterization

In this study, we investigate the anti-biofouling properties of graphene oxide (GO) nanosheets with bovine serum albumin (BSA). These properties are characterized by contact angle hysteresis (CAH) measurements and contact angle (CA) modification under electrowetting (EW), in order to determine the surface modification due to BSA adsorption. The experiments show two different behaviors. The droplets containing only BSA or BSA with low GO concentration exhibit high CAH and irreversible CA modification under EW due BSA adsorption on the surface. However, in the presence of a high GO concentration, the CAH is largely lowered and CA modification under EW is reversible, suggesting preferential adsorption of BSA on GO nanosheets.

Wetting Transitions on Hierarchical Surfaces

The current study reports the fabrication and characterization of superhydrophobic surfaces with increasing nanoroughness by decreasing silica nanoparticle size in a sol–gel matrix. Using small-angle X-ray scattering (SAXS) measurements allowed for the direct quantification of air entrapped at the interface, revealing for the first time that significant air remains on hierarchical surfaces despite observed droplet pinning through hysteresis measurements. Combining contact angle hysteresis and SAXS measurements of the surfaces immersed in sodium dodecylsulfate (SDS) solutions with Cassie and Tadmor’s model, a series of predicted contact angles were generated, comparing wetting transition mechanisms based on wetting line advance, droplet adhesion/pinning, and interfacial air entrapment. The study provided confirmation of key theoretical assumptions on wetting of hierarchical surfaces: (i) Cassie wetting of the nanofeatures is the preferred wetting progression on hierarchical surfaces; and (ii) the presence of an intermediate petal state is dependent on the level of nanoroughness as compared to the microroughness.

A Physical Approach To Specifically Improve the Mobility of Alkane Liquid Drops

Seamless control of resistance to liquid drop movement for polar (water) and nonpolar alkane (n-hexadecane, n-dodecane, and n-decane) probe liquids on substrate surfaces was successfully demonstrated using molten linear poly(dimethylsiloxane) (PDMS) brush films with a range of different molecular weights (MWs). The ease of movement of liquid drops critically depended on polymer chain mobility as it relates to both polymer MW and solvent swelling on these chemically- and topographically identical surfaces. Our brush films therefore displayed lower resistances to liquid drop movement with decreasing polymer MW and surface tension of probe liquid as measured by contact angle (CA) hysteresis and tilt angle measurements. Subsequently, while mobility of water drops was inferior and became worse at higher MWs, n-decane drops were found to experience little resistance to movement on these polymer brush films. Calculating CA hysteresis as Δθ(cos) = cos θ(R) - cos θ(A) (θ(A) and θ(R) are the advancing and receding CAs, respectively) rather than the standard Δθ = θ(A) - θ(R) was found to be advantageous for estimation of the actual dynamic dewetting behavior of various probe liquids on an inclined substrate.

Droplet Compression and Relaxation by a Superhydrophobic Surface: Contact Angle Hysteresis

In this article, the contact angle hysteresis (CAH) of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the contact angle (CA) of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different. In the contact line withdrawal regime, the contact line will withdraw with the CA remaining at the receding angle. In the contact line pinning regime, however, the contact line will be pinned at the final position and the CA will decline to a certain value higher than the receding angle. Furthermore, the advancing pinning behavior can also be realized through a successive compression-relaxation process. On the basis of the liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of the droplet during the compression-relaxation process; both contact line withdrawal and pinning regimes can also be identified. The results of the experiment and simulation agree with each other very well.

Facile preparation of superhydrophobic polymer surfaces

A simple and general method has been developed for the preparation of polymeric materials with superhydrophobic surfaces. The process is applicable to a large number of polymers, thermoplastic or thermoset. In this manuscript preparation and characteristics of superhydrophobic surfaces prepared from a segmented polydimethylsiloxane–urea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) are discussed. All samples were prepared onto glass surfaces by using a simple, multi-step spin-coating procedure. In the first step a thin film of the desired polymer was coated onto the glass slide. This was followed by spin-coating of two layers of hydrophobic fumed silica using a dispersion in tetrahydrofuran. Finally to obtain a durable surface, a very thin film of the parent polymer was spin-coated from a very dilute solution containing 2.5% by weight hydrophobic silica and 0.25% by weight matrix polymer in tetrahydrofuran. Surfaces were characterized by scanning electron microscopy (SEM), which clearly showed the formation of rough surfaces with homogeneously distributed silica particles in 1–10μm range. Static water contact angle and contact angle hysteresis measurements proved the formation of superhydrophobic surfaces. Samples displayed static water contact angles larger than 170° and very low contact angle hysteresis of less than 3°.

Measurement of Contact-Angle Hysteresis for Droplets on Nanopillared Surface and in the Cassie and Wenzel States: A Molecular Dynamics Simulation Study

We perform large-scale molecular dynamics simulations to measure the contact-angle hysteresis for a nanodroplet of water placed on a nanopillared surface. The water droplet can be in either the Cassie state (droplet being on top of the nanopillared surface) or the Wenzel state (droplet being in contact with the bottom of nanopillar grooves). To measure the contact-angle hysteresis in a quantitative fashion, the molecular dynamics simulation is designed such that the number of water molecules in the droplets can be systematically varied, but the number of base nanopillars that are in direct contact with the droplets is fixed. We find that the contact-angle hysteresis for the droplet in the Cassie state is weaker than that in the Wenzel state. This conclusion is consistent with the experimental observation. We also test a different definition of the contact-angle hysteresis, which can be extended to estimate hysteresis between the Cassie and Wenzel state. The idea is motivated from the appearance of the hysteresis loop typically seen in computer simulation of the first-order phase transition, which stems from the metastability of a system in different thermodynamic states. Since the initial shape of the droplet can be controlled arbitrarily in the computer simulation, the number of base nanopillars that are in contact with the droplet can be controlled as well. We show that the measured contact-angle hysteresis according to the second definition is indeed very sensitive to the initial shape of the droplet. Nevertheless, the contact-angle hystereses measured based on the conventional and new definition seem converging in the large droplet limit.

Solvent resistant superhydrophobic films from self-emulsifying carnauba wax–alcohol emulsions

By dispersing molten carnauba wax in warm isopropyl alcohol or ethanol with subsequent mild ultrasonic mixing, highly stable surfactant-free natural wax-in-alcohol emulsions were prepared for the first time. These emulsions can easily suspend hydrophobic colloidal organics or solids. We demonstrate fabrication of superhydrophobic films from sub-micron polytetrafluoroethylene (PTFE) particle-dispersed emulsions by a simple drop and/or spray casting procedure and subsequent thermal annealing above the melting point of carnauba wax. We investigate effect of wax and Teflon concentration and thermal annealing on the degree of superhydrophobicity. Moreover, the films display superior resistance to solvent etching against aggressive solvents such as chloroform, toluene, acetone and alcohols upon immersion into such solvent baths for one hour. Composite films remain superhydrophobic after solvents evaporate from their surfaces following their removal from the solvent baths. Moreover, detailed contact angle hysteresis measurements revealed that solvent bath immersion does not downgrade “self-cleaning” superhydrophobicity to “sticky” superhydrophobicity.

Chemical Nano-Heterogeneities Detection by Contact Angle Hysteresis: Theoretical Feasibility

The theoretical feasibility of detecting chemical nanoheterogeneities on solid surfaces by measurement of contact angle hysteresis (CAH) was studied, using simplified models of cylindrical (2D) and axisymmetric (3D) drops on corresponding models of chemically heterogeneous, smooth solid surfaces. This feasibility depends on the ratio between the external energy input to the drop and the energies needed to deform its liquid-gas interface and move the contact line across energy barriers. A ubiquitous source of external energy is building vibrations, since most contact-angle measurements are done in buildings. The energy barriers that oppose the motion of the contact line were numerically calculated for various parameters of the two systems. The variations of the liquid-gas interfacial energy are discussed in terms of orders of magnitude. By comparing these energies, it is concluded that under regular ("barely perceptible") building vibrations CAH measurements may detect chemical heterogeneities at the few nanometers scale.

Fabrication of superhydrophobic polyurethane/organoclay nano-structured composites from cyclomethicone-in-water emulsions

Nano-structured polyurethane/organoclay composite films were fabricated by dispersing moisture-curable polyurethanes and fatty amine/amino-silane surface modified montmorillonite clay (organoclay) in cyclomethicone-in-water emulsions. Cyclomethicone Pickering emulsions were made by emulsifying decamethylcyclopentasiloxane (D 5), dodecamethylcyclohexasiloxane (D 6) and aminofunctional siloxane polymers with water using montmorillonite particles as emulsion stabilizers. Polyurethane and organoclay dispersed emulsions were spray coated on aluminum surfaces. Upon thermosetting, water repellent self-cleaning coatings were obtained with measured static water contact angles exceeding 155° and low contact angle hysteresis (<8°). Electron microscopy images of the coating surfaces revealed formation of self-similar hierarchical micro- and nano-scale surface structures. The surface morphology and the coating adhesion strength to aluminum substrates were found to be sensitive to the relative amounts of dispersed polyurethane and organoclay in the emulsions. The degree of superhydrophobicity was analyzed using static water contact angles as well as contact angle hysteresis measurements. Due to biocompatibility of cyclomethicones and polyurethane, developed coatings can be considered for specific bio-medical applications.

Experimental Study of Thermodynamic Surface Characteristics and pH Sensitivity of Silicon Dioxide and Silicon Nitride

In this report, we have introduced a revision of the chemical treatment influence on the surface thermodynamic properties of silicon dioxide (SiO(2)) and silicon nitride (Si(3)N(4)) solid thin layers. Some characterization techniques might be used to quantify the thermodynamic properties of solid surface and predict its ability in the adhesion phenomenon. In this work, we have used static and dynamic contact angle (CA) measurements to characterize both dioxide solid surfaces being treated by using the two procedures of cleaning and chemical activation. Qualitative and quantitative concepts of analysis, using the Van Oss approach, are based on the determination of dioxide surface hydrophilic and hydrophobic features and the thermodynamic parameters such as free energy, acid, base, and Lewis acid-base surface tension components. Electrochemical capacitance-potential measurements were carried out to study the reactivity of both silicon dioxide and silicon nitride surfaces for pH variation. Furthermore, the surface roughness of these insulators was examined by using the contact angle hysteresis (CAH) measurements and atomic force microscopy (AFM). It was concluded that CA technique can be used as a suitable and base method for the understanding of surface wettability and for the control of surface wetting behavior.

A new model to estimate the Young contact angle from contact angle hysteresis measurements

Beyond the linear model of Schulze et al. [ J. Adhes. Sci. Technol. 3 (1) (1989) 39–48], a more comprehensive model for predicting the Young contact angle of rough and chemically homogeneous surfaces from contact angle hysteresis measurements is proposed. This new model incorporates the barrier effect, described by Shuttleworth and Bailey, and the Wenzel model as well. Both the linear and nonlinear models were evaluated using a thermoplastic polyether-based elastomer and PDMS and two liquids. The elastomer surfaces were textured on purpose at differing degrees of roughness. Both models provided similar values of Young angle, although the nonlinear model also provided information about the distribution of practical metastable configurations around the most stable equilibrium configuration. Despite the linear model showed slightly better goodness-of-fit and minor uncertainties in its parameters, the nonlinear model is more reliable and theoretically consistent.

Combinatorial Materials Research Applied to the Development of New Surface Coatings XIII: An Investigation of Polysiloxane Antimicrobial Coatings Containing Tethered Quaternary Ammonium Salt Groups

High-throughput biological assays were used to develop structure - antimicrobial relationships for polysiloxane coatings containing chemically bound (tethered) quaternary ammonium salt (QAS) moieties. The QAS-functional polysiloxanes were derived from solution blends of a silanol-terminated polydimethylsiloxane, a trimethoxysilane-functional QAS (QAS-TMS), and methylacetoxysilane. Since the QAS moieties provide antimicrobial activity through interaction with the microorganism cell wall, most of the compositional variables that were investigated were associated with the chemical structure of the QAS-TMS. Twenty different QAS-TMS were synthesized for the study and the antimicrobial activity of sixty unique polysiloxane coatings derived from these QAS-TMS determined toward Escherichia coli , Staphylococcus aureus , and Candida albicans . The results of the study showed that essentially all of the compositional variables significantly influenced antimicrobial activity. Surface characterization of these moisture-cured coatings using atomic force microscopy as well as water contact angle and water contact angle hysteresis measurements indicated that the compositional variables significantly affected coating surface morphology and surface chemistry. Overall, compositional variables that produced heterogeneous surface morphologies provided the highest antimicrobial activity suggesting that the antimicrobial activity was primarily derived from the relationship between coating chemical composition and self-assembly of QAS moieties at the coating/air interface. Using data modeling software, a narrow region of the compositional space was identified that provided broad-spectrum antimicrobial activity.

Contact Angle and Contact Angle Hysteresis Measurements Using the Capillary Bridge Technique

A new experimental technique is proposed to easily measure both advancing and receding contact angles of a liquid on a solid surface, with unprecedented accuracy. The technique is based on the analysis of the evolution of a capillary bridge formed between a liquid bath and a solid surface (which needs to be spherical) when the distance between the surface and the liquid bath is slowly varied. The feasibility of the technique is demonstrated using a low-energy perfluorinated surface with two different test liquids (water and hexadecane). A detailed description of both experimental procedures and computational modeling are given, allowing one to determine contact angle values. It is shown that the origin of the high accuracy of this technique relies on the fact that the contact angles are automatically averaged over the whole periphery of the contact. This method appears to be particularly adapted to the characterization of surfaces with very low contact angle hysteresis.

Reversible Electrowetting on Superhydrophobic Double-Nanotextured Surfaces

The paper reports on wetting, electrowetting (EW), and systematic contact angle hysteresis measurements after electrowetting of superhydrophobic silicon nanowire surfaces (NWs). The surfaces consist of C4F8-coated silicon nanowires grown on Si/SiO2 substrate. Different surfaces modulating (i) the dielectric layer thickness and (ii) the nanotexturation were investigated in this study. It was found that the superhydrophobic NWs display different EW behaviors according to their double nanotexturation with varying droplet impalement levels. Some surfaces exhibited a total reversibility to EW with no impalement (contact angle variation of 35+/-2 degrees at 190 VTRMS with deionized water), whereas other surfaces showed nonreversible behavior to EW with partial droplet impalement. A scenario is proposed to explain the unique properties of these surfaces.

Synthesis studies of sputtering TiO 2 films on poly(dimethylsiloxane) for surface modification

Titanium dioxide (TiO 2) films were prepared on poly(dimethylsiloxane) (PDMS) substrate by direct current (dc) reactive sputtering in order to change surface physical properties of PDMS. The morphology of the films was investigated by scanned electron microscopy (SEM), SEM of films showed homogeneous distribution on the film surface, which consisted of sever 10 nanometer crystallites. The effects of the surface modification were investigated by using UV spectrum analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analysis, the contact angle hysteresis measurements and protein adsorption tests. Films characterization show the films were highly transparent, porous and covered by mixed oxidation states TiO 2. Improved wettability and reduced adsorption properties were observed on PDMS surface coated TiO 2 films.

Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis.

A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.

Effect of ageing on wettability of quartz surfaces modified by Ar implantation

Ion implantation is a powerful technique to modify the contact angles of surfaces. However, a change of the contact angle with the passage of time, called ageing, is observed on most surfaces. This paper aims to better understand the mechanism leading to the ageing of quartz surfaces implanted by argon ion. Coupons of quartz have been irradiated by 3 keV Ar ions to a fluence of 1.8×1016 Ar cm−2. Some implantations were performed under oxygen partial pressure (∼5×10−5 Torr). The samples have been characterized at different periods of time after implantation by means of contact angle hysteresis measurements, angle resolved x-ray photoelectron spectroscopy (ARXPS) and lateral force microscopy (LFM). A more hydrophilic surface was observed immediately after implantation. This effect was strongly enhanced by the use of an O2 partial pressure. With the passage of time, the contact angles increased both in ambient air as well as under an Ar atmosphere. Characterization by means of XPS has shown a decrease of the relative concentrations of the more polar molecules containing oxygen (C–O, C = O and (O–C = O)–O) which could partly explain the increase of the contact angles with time. On the other hand, a higher ratio of the signals SiO2(104 eV)/SiO2(103 eV) is observed for samples implanted with O2which could be responsible for their higher level of wettability. A model using the ARXPS measurements suggests that for the samples implanted without oxygen, the ageing could also be caused by the spreading of a natural carbonaceous surface layer dispersed into islands after the Ar implantation. This layer being more hydrophobic than the SiO2 surface, its spreading would increase the contact angles. Characterization by means of AFM–LFM shows that the surface is composed of two areas having different friction properties. Their distribution is in agreement with the covered surface fraction given by the model for the samples implanted without oxygen and aged in Ar.

Effect of aging on wettability of silicon surfaces modified by Ar implantation

The contact angles of most surfaces modified by ion implantation change with the passage of time. The aim of this experiment is to better understand the mechanism leading to the aging of argon implanted silicon oxide surfaces. Coupons of silicon with its native oxide layer have been irradiated by means of 3 keV Ar ions with a fluence of 1.8×1016 Ar/cm2. Some of the implantations have been performed under oxygen partial pressure (∼5×10−5 Torr). The samples have been characterized at different periods of time after the implantation, by means of contact angle hysteresis measurements and angle resolved x-ray photoelectron spectroscopy (ARXPS). Irradiation with Ar ions has produced a more hydrophilic surface immediately after implantation. With the passage of time, the contact angles have increased both in ambient air and in argon atmosphere. Characterization by means of XPS has shown that there is with time a small increase of the O concentration as well as a small decrease of the quantity of Ar in the silicon oxide surfaces. The shape of the C 1s and Si 2p spectra and their details are also slightly modified with the passage of time. A model using the ARXPS measurements suggests that for the samples implanted without oxygen, the aging would be principally due to the spreading of a natural carbonaceous surface layer dispersed into islands after the Ar implantation. The layer being more hydrophobic than the silicon oxide, its spreading would increase the apparent contact angles. On the other hand, the increase of the carbon concentration in the carbonaceous layer would produce a more hydrophobic surface and would be responsible for the aging of the samples implanted in O2 partial pressure. Results obtained by means of a simple model and the Cassie’s equation suggest that for the samples implanted without O2 gas, the contact angles of the substrate and the carbonaceous layer are 5° and 110°, respectively. The apparent contact angle obtained with the Cassie’s equation is in good agreement with the experimental measurements.

Modification of wetting properties of SiO x surfaces by Ar implantation

The aim of this experiment is to better understand the mechanism leading to the modification of the wetting properties of insulating (quartz) and non-insulating (Si) surfaces by ion and atom implantation. Coupons of quartz and silicon (with its native oxide layer) have been irradiated by means of 3 keV Ar ions and atoms with a fluence of 1.8×10 16 Ar/cm 2. Some exposures to Ar ions have been performed under oxygen partial pressure (∼5×10 −5 Torr). The samples have been characterized before and after implantation by means of contact angle hysteresis measurements, Rutherford backscattering spectroscopy (RBS) and angle resolved X-ray photoelectron spectroscopy (ARXPS). Irradiation with argon ions or atoms has produced a more hydrophilic surface immediately after implantation. Ar depth profiling by means of RBS has shown that 4% of implanted (atoms and ions) Ar has been retained in quartz, while 13% and 21% of Ar has been retained in silicon after Ar implantation with and without oxygen partial pressure, respectively. However, no difference in the depth distribution has been measured. Characterization by means of ARXPS has shown a noticeable change in the composition of the quartz and silicon oxide surfaces (implantation of Si under an O 2 partial pressure producing a thicker oxide layer) which can explain the differences in the reported Ar retention. In general, the irradiation have removed a large portion of oxygen present in the pre-existing carbonaceous layer on the surface of the samples. The ion beam irradiation has been more efficient than atom beam to both, increase the wettability of the quartz surfaces and enhance the concentration of the carbonaceous layer in “dispersed islands” on the surfaces.

Simple apparatus for contact angle measurements by the heavy drop method

An apparatus for measuring contact angles by the heavy drop method is proposed. After a few introductory considerations, the article reviews some experimental methods and shows their main advantages and drawbacks. In the heavy drop method, the mean contact angle of a liquid on a solid is calculated from a measurement of the height in the central zone of a large pancake-like drop. This measurement is based in turn on partial occultation of a parallel light beam, detected by a photodiode. Connected to a microcomputer for data storage and processing, the device allows investigations to be made of either static or dynamic phenomena by contact angle and/or contact angle hysteresis measurements; it is simple and well suited to various problems of surface characterization.