Wetting of a drop on a sphere

In this work, the equilibrium morphology of a drop on a sphere is analyzed as a function of the contact angle and drop volume experimentally and with analytical effective interfacial energy calculations. Experimentally, a drop on a sphere geometry is realized in an oil bath by placing a water drop on a sphere coated with a dielectric, of which the radii of curvature are comparable with that of the drop. Electrowetting (EW) is used to change the contact angle of the water drop on the sphere. To validate the applicability of EW and the Lippman-Young equation on nonflat surfaces, we systematically investigate the response of the contact angle to the applied voltage (EW response) for various drop volumes and compared the results with the case of a planar surface. The effective interfacial energy of two competing morphologies, namely, the spherically symmetric "completely engulfing" and "partially engulfing" morphologies are calculated analytically. The analytical calculations are then compared to the experimental results to confirm which morphology is energetically more favored for a given contact angle and drop volume. Our findings indicate that the "partially engulfing" morphology is always the energetically more favorable morphology.     

Hydrophilicity and the viscosity of interfacial water

We measure the viscosity of nanometer-thick water films at the interface with an amorphous silica surface. We obtain viscosity values from three different measurements: friction force in a water meniscus formed between an oxide-terminated W tip and the silica surface under ambient conditions; similar measurements for these interfaces under water; and the repulsive "drainage" force as the two surfaces approach at various speeds in water. In all three cases, we obtain effective viscosities that are approximately 10(6) times greater than that of bulk water for nanometer-scale interfacial separations. This enhanced viscosity is not observed when we degrade the hydrophilicity of the surface by terminating it with -H or -CH3. In view of recent results from other interfaces, we conclude that the criterion for the formation of a viscous interphase is the degree of hydrophilicity of the interfacial pair.     

Nanoscale electrowetting effects observed by using friction force microscopy

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.     

Relation between two-photon absorption and dipolar properties in a series of fluorenyl-based chromophores with electron donating or electron withdrawing substituents

We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ～ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent.     

Relationship between interfacial forces measured by colloid-probe atomic force microscopy and protein resistance of poly(ethylene glycol)-grafted poly(L-lysine) adlayers on niobia surfaces

Adsorbed layers of "comb-type" copolymers consisting of PEG chains grafted onto a poly(l-lysine) (PLL) backbone on niobium oxide substrates were studied by colloid-probe AFM in order to characterize the interfacial forces associated with coatings of varying architectures (PEG/PLL ratios and PEG chain lengths) and their relevance to protein resistance. The steric and electrostatic forces measured varied substantially with the architecture of the PLL-g-PEG copolymers. Varying the ionic strength of the buffer solutions enabled discrimination between electrostatic and steric-entropic contributions to the net interfacial force. For high PEG grafting densities the steric component was most prominent, but at low ionic strengths and high grafting densities, a repulsive electrostatic surface force was also observed; its origin was assigned to the niobia charges beneath the copolymer, as insufficient protonated amine groups in the PLL backbone were available for compensation of the oxide surface charges. For lower grafting densities and lower ionic strengths there was a substantial attractive electrostatic contribution arising from interaction of the electrical double layer arising from the protonated amine groups, with that of the silica probe surface (as under low ionic strength conditions, the electrical double layer was thicker than the PEG layer). For these PLL-g-PEG coatings the net interfacial force can thus be a markedly varying superposition of electrostatic and steric-entropic contributions, depending on various factors. The force curves correlate with protein adsorption data, demonstrating the utility of AFM colloid-probe force measurements for quantitative analysis of surface forces and how they determine interfacial interactions with proteins. Such characterization of the net interfacial forces is essential to elucidate the multiple types of interfacial forces relevant to the interactions between PLL-g-PEG coatings and proteins and to advance interpretation of protein adsorption or repellence beyond the oversimplified steric barrier model; in particular, our data demonstrate the importance of an ionic-strength-dependent minimum PEG layer thickness to screen the electrostatic interactions of charged interfaces.     

Surface forces and friction tuned by thermo-responsive polymer films

Thermo-responsive polymer films have enabled the development of various functional surfaces with switchable interfacial properties. Assessing the surface forces and friction on such films is of paramount importance. On the one hand, it allows us to extract a great deal of information on the interfacial properties of the films, e.g., adhesiveness and lubricity, and how they could be tuned using different stimuli. On the other hand, surface force measurements complement other thin-film analysis methods, e.g., ellipsometry, to better perceive the correlation between the molecular properties of the polymer chains and the interfacial properties of the film. On this basis, we will, herein, provide a concise review of some recent studies on surface forces and friction tuned by thermo-responsive polymer films. This outline comprises a summary of several research works addressing the effects of temperature, solvent composition, and salts on surface forces and friction. In the end, we briefly discuss a few select studies in which the regulation of surface forces by thermo-responsive polymers is examined with an emphasis on the potential applications.     

A Novel Pyrene‐Based Fluorescing Amphiphile with Unusual Bulk and Interfacial Properties

We have synthesised a new, pyrene‐based, low‐molecular‐mass, amphiphilic molecule that displays a wealth of properties of potential interest for aggregation and interfacial applications. In order to elucidate some of the key properties of this molecule, which consists of a pyrene‐containing hydrophobic head and a short PEG‐based hydrophilic tail, we investigate herein some aspects of its concentration‐dependent behaviour in aqueous solutions. We show that the inclusion of the hydrophobic pyrene group not only provides the molecule with intriguing bulk and interfacial properties down to low concentrations, but also with various means of assessing its aggregation behaviour by means of its well‐characterised fluorescence properties. Combining a range of fluorescence techniques with microscopic imaging (optical and Cryo‐TEM), interfacial tension measurements and foaming studies, we have been able to identify and characterise three concentration‐dependant regimes. At low concentrations, the molecule is dissolved in monomeric form. At intermediate concentrations, labile aggregates are formed, which, at higher concentrations, give way to aggregates containing pre‐associated pyrenes. Our measurements strongly imply that the latter aggregates are hexagonally close‐packed tubular micelles. In this latter regime we also find a range of micron‐sized precipitates. Additionally, the molecule displays strong interfacial activity, yet a surprisingly slow dynamics of interfacial adsorption. Finally, we demonstrate the possibility of using it to visualize interfaces and also create reasonably stable (1 hour) and fluorescing foams.     

Effects of ionic strength and surface charge on protein adsorption at PEGylated surfaces

PEGylated Nb2O5 surfaces were obtained by the adsorption of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) copolymers, allowing control of the PEG surface density, as well as the surface charge. PEG (MW 2 kDa) surface densities between 0 and 0.5 nm(-2) were obtained by changing the PEG to lysine-mer ratio in the PLL-g-PEG polymer, resulting in net positive, negative and neutral surfaces. Colloid probe atomic force microscopy (AFM) was used to characterize the interfacial forces associated with the different surfaces. The AFM force analysis revealed interplay between electrical double layer and steric interactions, thus providing information on the surface charge and on the PEG layer thickness as a function of copolymer architecture. Adsorption of the model proteins lysozyme, alpha-lactalbumin, and myoglobin onto the various PEGylated surfaces was performed to investigate the effect of protein charge. In addition, adsorption experiments were performed over a range of ionic strengths, to study the role of electrostatic forces between surface charges and proteins acting through the PEG layer. The adsorbed mass of protein, measured by optical waveguide lightmode spectroscopy (OWLS), was shown to depend on a combination of surface charge, protein charge, PEG thickness, and grafting density. At high grafting density and high ionic strength, the steric barrier properties of PEG determine the net interfacial force. At low ionic strength, however, the electrical double layer thickness exceeds the thickness of the PEG layer, and surface charges "shining through" the PEG layer contribute to protein interactions with PLL-g-PEG coated surfaces. The combination of AFM surface force measurements and protein adsorption experiments provides insights into the interfacial forces associated with various PEGylated surfaces and the mechanisms of protein resistance.     

Electrowetting-based control of static droplet states on rough surfaces

Electrowetting (EW) is a powerful tool to control fluid motion at the microscale and has promising applications in the field of microfluidics. The present work analyzes the influence of an electrowetting voltage in determining and altering the state of a static droplet resting on a rough surface. An energy-minimization-based modeling approach is used to analyze the influence of interfacial energies, surface roughness parameters, and electric fields in determining the apparent contact angle of a droplet in the Cassie and Wenzel states under the influence of an EW voltage. The energy-minimization-based approach is also used to analyze the Cassie-Wenzel transition under the influence of an EW voltage and estimate the energy barrier to transition. The results obtained show that EW is a powerful tool to alter the relative stabilities of the Cassie and Wenzel states and enable dynamic control of droplet morphology on rough surfaces. The versatility and generalized nature of the present modeling approach is highlighted by application to the prediction of the contact angle of a droplet on an electrowetted rough surface consisting of a dielectric layer of nonuniform thickness.     

Evanescent Wave-Guided Growth of an Organic Supramolecular Nanowire Array

The ordered assembly of molecules within a specific space of nanoscale, such as a surface, holds great promise in advanced micro-/nanostructure fabrication for various applications. Herein, we demonstrate the evanescent wave (EW)-guided organization of small molecules into a long-range ordered nanowire (NW) array. Experiment and simulation revealed that the orientation and periodicity of the NW array were feasibly regulated by altering the propagation direction and the wavelength of EW. The generality of this approach was demonstrated by using different molecule precursors. While existing studies on EW often took advantages of its near-field property for optical sensing, this work demonstrated the photochemical power of EW in the guided-assembly of small molecules for the first time. It also provides an enlightening avenue to periodic structure with fluorescence, promising for super-resolution microscopy and important devices applicable to optical and bio-related fields.     

Mechanism underlying bioinertness of self-assembled monolayers of oligo(ethyleneglycol)-terminated alkanethiols on gold: protein adsorption, platelet adhesion, and surface forces

The mechanism underlying the bioinertness of the self-assembled monolayers of oligo(ethylene glycol)-terminated alkanethiol (OEG-SAM) was investigated with protein adsorption experiments, platelet adhesion tests, and surface force measurements with an atomic force microscope (AFM). In this work, we performed systematic analysis with SAMs having various terminal groups (-OEG, -OH, -COOH, -NH(2), and -CH(3)). The results of the protein adsorption experiment by the quartz crystal microbalance (QCM) method suggested that having one EG unit and the neutrality of total charges of the terminal groups are essential for protein-resistance. In particular, QCM with energy dissipation analyses indicated that proteins absorb onto the OEG-SAM via a very weak interaction compared with other SAMs. Contrary to the protein resistance, at least three EG units as well as the charge neutrality of the SAM are found to be required for anti-platelet adhesion. When the identical SAMs were formed on both AFM probe and substrate, our force measurements revealed that only the OEG-SAMs possessing more than two EG units showed strong repulsion in the range of 4 to 6 nm. In addition, we found that the SAMs with other terminal groups did not exhibit such repulsion. The repulsion between OEG-SAMs was always observed independent of solution conditions [NaCl concentration (between 0 and 1 M) and pH (between 3 and 11)] and was not observed in solution mixed with ethanol, which disrupts the three-dimensional network of the water molecules. We therefore concluded that the repulsion originated from structured interfacial water molecules. Considering the correlation between the above results, we propose that the layer of the structured interfacial water with a thickness of 2 to 3 nm (half of the range of the repulsion observed in the surface force measurements) plays an important role in deterring proteins and platelets from adsorption or adhesion.     

Interaction between silica nanoparticles in binary liquid mixtures: the effect of polarity on adhesion

In order to explore the relationship between selective liquid sorption and the stability of dispersed silica particles, we carried out studies in ethanol–cyclohexane binary mixtures as well as in partially miscible 1-butanol–water mixtures. In ethanol–cyclohexane, adsorption excess isotherms were determined on hydrophilic and hydrophobic aerosil particles. The volume and the thickness of the adsorption layer were derived from the isotherms. Knowing the layer thickness and Hamaker constants, interparticle interaction potentials were calculated at various mixture compositions. At low ethanol concentration, where hydrophilic surfaces are considerably enriched in ethanol, interparticle interaction is enhanced and the high shear stress calculated from rheological measurements indicates the development of a three-dimensional network of aggregated particles. In contrast, hydrophobic aerosil particles in ethanol–cyclohexane approach each other without ensuing aggregation because interparticle interactions are weak, a fact well demonstrated by rheological measurements. It was also established that interactions between silica particles with hydrophilic surfaces are weak in butanol–water mixtures. Since water is preferentially adsorbed on the surface of hydrophilic particles to the azeotropic composition (\( x_{\rm{1}}^{\rm{a}} \)=0.62), within this wide composition range the Hamaker constant of the interfacial layer is identical with that of water.     

Surfactant-mediated formation of alginate layers at the water-air interface

The self-organization process of polysaccharide alginate with different cationic surfactants at the water-air interface was investigated over a wide concentration regime. The changes of surface properties determined by surface tension measurements, surface rheology, and X-ray reflectivity are correlated with changes of bulk properties measured by turbidity, light scattering, and zeta potential measurements. We demonstrate that the interactions between the alginate and cationic surfactants result in significant changes of bulk and interfacial properties. The results of surface shear experiments point to the existence of highly viscoelastic interfacial films. In combination with X-ray reflectivity, we demonstrate that these rheological features are related to polymer-surfactant associations at the interface. In the regime of high surfactant concentrations, we observed the existence of multilayer structures.     

Human immunoglobulin adsorption investigated by means of quartz crystal microbalance dissipation, atomic force microscopy, surface acoustic wave, and surface plasmon resonance techniques

Time-resolved adsorption behavior of a human immunoglobin G (hIgG) protein on a hydrophobized gold surface is investigated using multitechniques: quartz crystal microbalance/dissipation (QCM-D) technique; combined surface plasmon resonance (SPR) and Love mode surface acoustic wave (SAW) technique; combined QCM-D and atomic force microscopy (AFM) technique. The adsorbed hIgG forms interfacial structures varying in organization from a submonolayer to a multilayer. An "end-on" IgG orientation in the monolayer film, associated with the surface coverage results, does not corroborate with the effective protein thickness determined from SPR/SAW measurements. This inconsistence is interpreted by a deformation effect induced by conformation change. This conformation change is confirmed by QCM-D measurement. Combined SPR/SAW measurements suggest that the adsorbed protein barely contains water after extended contact with the hydrophobic surface. This limited interfacial hydration also contributed to a continuous conformation change in the adsorbed protein layer. The viscoelastic variation associated with interfacial conformation changes induces about 1.5 times overestimation of the mass uptake in the QCM-D measurements. The merit of combined multitechnique measurements is demonstrated.     

Non-Fickian water vapor sorption dynamics by Nafion membranes

Dynamics of water absorption from a saturated vapor and water desorption into dry air for Nafion 1100 EW ionomers have been measured for film thicknesses between 51 and 606 microm and at temperatures ranging from 30 to 90 degrees C. Water absorption and desorption exhibit two distinct non-Fickian characteristics: (1) desorption is 10 times faster than absorption and (2) the normalized mass change does not collapse to a single master curve when plotted against time normalized by membrane thickness squared, t/l2, for either absorption or desorption. Water desorption data were fit well by a model in which diffusion is rapid and interfacial mass transport resistance is the rate-limiting process for water loss. Water absorption is described by a two-stage process. At early times, interfacial mass transport controls water absorption, and at longer times, water absorption is controlled by the dynamics of polymer swelling and relaxation.     

Characterization of acid-base properties of two gibbsite samples in the context of literature results

Two different gibbsites, one commercial and one synthesized according to a frequently applied recipe, were studied in an interlaboratory attempt to gain insight into the origin of widely differing reports on gibbsite acid-base surface properties. In addition to a thorough characterization of the two solids, several methods relevant to the interfacial charging were applied to the two samples: potentiometric titrations to obtain the "apparent" proton related surface charge density, zeta-potential measurements characterizing the potential at the plane of shear, and Attenuated Total Reflection Infrared Spectroscopy (ATR-IR) to obtain information on the variation of counter-ion adsorption with pH (using nitrate as a probe). Values of the IEP at 9-10 and 11.2-11.3 were found for the commercial and synthesized sample, respectively. The experimental observations revealed huge differences in the charging behavior between the two samples. Such differences also appeared in the titration kinetics. A detailed literature review revealed similar disparity with no apparent systematic trend. While previously the waiting time between additions had been advocated to explain such differences among synthesized samples, our results do not support such a conclusion. Instead, we find that the amount of titrant added in each aliquot appears to have a significant influence on the titration curves. While we can relate a number of observations to others, a number of open questions and contradictions remain. We suggest various processes, which can explain the observed behavior.     

Acid treatment for functionalizing polyethylene fiber surfaces for enhanced adhesion to epoxy resins

The utility of high-strength, high-modulus polyethylene fibers in fiber-reinforced composites is limited due to its poor interfacial adhesion to various polymeric matrices. One way to overcome this limitation is to introduce reactive functionalities on the fiber surface capable of covalently bonding to matrix resins. Ultra high-strength polyethylene (UHSPE) fibers were treated with chlorosulfonic acid. The surface acid groups were found to considerably improve the interfacial adhesion between polyethylene fibers and epoxy resins as shown by the microbond test. These surface functionalities were found to improve the fiber wettability, as shown by contact angle measurements using the Wilhelmy balance method. Colorimetric measurements of methylene blue absorption were used to quantify the surface concentrations of the acid groups. It was possible to functionalize the UHSPE fiber surfaces using this method to obtain fibers that formed a stronger adhesive bond with epoxy resins; this was achievable without sacrificing other fiber mechanical properties.     

Microemulsion microstructure and interfacial curvature

The typical phase behavior of microemulsion systems undergoing phase inversion is briefly reviewed. As a model system H₂O-n-octane-C₁₂E₅ is studied with various experimental techniques. The occurring microstructures are visualized by freeze fracture electron microscopy and the corresponding domain sizes are quantified by small-angle neutron scattering. From the variations of the domain sizes the mean and Gaussian curvatures of the interfacial film with temperature are determined. It is found that the mean interfacial curvatureH changes gradually and nearly linearly with temperature from positive (Winsor I) to negative (Winsor II), passing through zero for bicontinuous microemulsions where these contain exactly equal volume fractions of water and oil. There the interfacial tension between bulk water-and oil-rich phases passes through an extreme minimum. Quantitative knowledge of the curvatures permits the measurements of interfacial tensions between the bulk phases to be discussed in terms of the relative contributions of bending energy and entropy of dispersion.     

Erythemally Weighted Radiometers in Solar UV Monitoring: Results from the WMO/STUK Intercomparison

The first international intercomparison of erythemally weighted (EW) broadband radiometers was arranged in 1995 to improve the accuracy and comparability of the measurements carried out by solar UV monitoring networks. The intercomparison was arranged at the Radiation and Nuclear Safety Authority in Helsinki, Finland, in cooperation with the University of Innsbruck and with support from the World Meteorological Organization. Altogether 20 EW meters of six different types from 16 countries were (1) tested in the laboratory by measuring the spectral and angular responsivities and (2) calibrated in solar radiation against two reference spectroradiometers. Calibration factors (CFs) for the EW meters were determined by using simultaneously measured EW solar UV spectra as a calibration reference. The CFs averaged over solar elevations higher than 35° varied from 0.87 to 1.75, with the estimated uncertainty being ±10%. As a result of this intercomparison, for the first time the calibrations of more than 100 EW radiometers around the world are possible to trace to the same origin. The present experience indicates that the accuracy of temperature-controlled EW radiometers is not significantly lower than the accuracy of spectroradiometers provided that strict quality assurance/quality control procedures are followed.     

Use of IR thermography to investigate heated droplet evaporation and contact line dynamics

In this paper we present the results of an experimental study investigating interfacial properties during the evaporation of sessile water droplets on a heated substrate. This study uses infrared thermography to map the droplet interfacial temperature. The measurements evidence nonuniform temperature and gradients that evolve in time during the evaporation process. A general scaling law for the interfacial temperature is deduced from the experimental observations. A theoretical analysis is performed to predict the local evaporation rates and their evolution in time. The use of energy conservation laws enabled us to deduce a general expression for the interfacial temperature. The comparison between the theory and experiments shows good agreement and allows us to rationalize the experimental observations. The thermography analysis also enabled the detection of the three-phase contact line location and its dynamics. To our knowledge, such measurements are performed for the first time using thermography.