Miguel A. Rodríguez-Valverde

Contact angle measurements on two (wood and stone) non-ideal surfaces

Abstract Surface properties of wood and stones are very important in influencing the bonding and finishing of wood and for road construction. Two important surface properties are wettability and surface free energy. Contact angle measurement is a simple, useful and sensitive tool for quantifying the wettability and the surface energy of different materials in contact with pure water and/or aqueous surfactant solutions. Nevertheless, a sessile drop on a real surface shows different contact angle values due to its lack of symmetry or to the loss of volume by capillary action. For this reason alternative techniques which supply an average contact angle value are required. In view of this fact, we applied axisymmetric drop shape analysis-diameter (ADSA-D) using a top view of a sessile drop and axisymmetric drop shape analysis-profile (ADSA-P) with a side view of a captive bubble. Both techniques calculate the contact angle by solving the Young–Laplace equation although with different algorithms, the former needs the maximum drop diameter while the last makes use of the complete bubble profile. ADSA-D was applied to study two wood species (eucalyptus and pine) and ADSA-P with polished rocks of two different compositions (silicate and calcite). The maximum contour of the drop was fitted to an ellipse after being detected and extracted by means of a semi-automatic procedure of image processing. Thus, two different contact angles were found out. The captive bubble method in conjunction with the current ADSA-P technique allows to obtain comfortable, automatic and reproducible measurements of contact angle on porous stones.

Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments

A new methodology capable of providing reliable and reproducible contact angle (theta) data has been employed to study the effect of clinical treatments grinding, acid etching, and deproteinization on medial dentin tissue. It is based on the application of the ADSA-CD algorithm to the determination of low-rate dynamic contact angles, obtained from slowly growing drops, and on contact angle measurement, as well as spreading behavior analysis, during the relaxation of the system (water on treated dentin) after initial drop growth. The theta data obtained were substantially more reproducible than those obtained with classical methods. A net effect of the treatment on theta was found, increasing dentin wettability: theta (polished) >theta (etched) >theta (deproteinized). The spreading rates correlate with the angles and are adequate for the dentin surface characterization. ANOVA and SNK tests show that for advancing contact angles the means corresponding to all treatments are significantly different. In the relaxing phase, mean angle and spreading rates on polished dentin differ significantly from those on etched and deproteinized dentin, but the latter do not differ significantly from each other.

Wettability and bonding of self-etching dental adhesives. Influence of the smear layer.

OBJECTIVES To evaluate dentin wettability and bonding of self-etching and total-etch adhesives on smear layer-covered and smear layer-free dentin. METHODS Three self-etching adhesives (Clearfil SE Bond, AdheSE and Xeno III) and one total-etch adhesive (SingleBond) were evaluated. The substrates were mid coronal smear layer-covered and smear layer-free dentin. Dentin wettability by resins was studied from contact angle measurement using sessile drop method and Axisymmetric Drop Shape Analysis (ADSA). Shear-bond strength was evaluated using a push-out technique (ad hoc design). Data were analysed with two-way ANOVA and Tukeys test. RESULTS Similar values of dentin wettability were obtained for all adhesives tested regardless the presence of smear layer. Even though, Xeno III and AdheSE exhibited slightly lower wettability (higher contact angles values) on smear layer-free dentin. Likewise, the presence of smear layer did not affect the shear-bond strength. Total-etch adhesive obtained higher shear-bond strength than self-etching adhesives, which obtained similar values. SIGNIFICANCE Wettability is similar between self-etching and total-etch adhesives. The smear layer affects slightly the wettability of self-etching adhesives. Shear-bond strength is not sensitive to the smear layer presence. Total-etch adhesion is stronger than self-etching adhesion. There is no clear relationship between wettability and bond strength.

Enzymatically-tailored pectins differentially influence the morphology, adhesion, cell cycle progression and survival of fibroblasts

Improved biocompatibility and performance of biomedical devices can be achieved through the incorporation of bioactive molecules on device surfaces. Five structurally distinct pectic polysaccharides (modified hairy regions (MHRs)) were obtained by enzymatic liquefaction of apple (MHR-B, MHR-A and MHR-alpha), carrot (MHR-C) and potato (MHR-P) cells. Polystyrene (PS) Petri dishes, aminated by a plasma deposition process, were surface modified by the covalent linking of the MHRs. Results clearly demonstrate that MHR-B induces cell adhesion, proliferation and survival, in contrast to the other MHRs. Moreover, MHR-alpha causes cells to aggregate, decrease proliferation and enter into apoptosis. Cells cultured in standard conditions with 1% soluble MHR-B or MHR-alpha show the opposite behaviour to the one observed on MHR-B and -alpha-grafted PS. Fibronectin was similarly adsorbed onto MHR-B and tissue culture polystyrene (TCPS) control, but poorly on MHR-alpha. The Fn cell binding site (RGD sequence) was more accessible on MHR-B than on TCPS control, but poorly on MHR-alpha. The disintegrin echistatin inhibited fibroblast adhesion and spreading on MHR-B-grafted PS, which suggests that MHRs control fibroblast behaviour via serum-adhesive proteins. This study provides a basis for the design of intelligently-tailored biomaterial coatings able to induce specific cell functions.

Comparison of the Interfacial Activity between Homogeneous and Janus Gold Nanoparticles by Pendant Drop Tensiometry

The interfacial activity of 3.5 nm homogeneous (HPs) and amphiphilic Janus gold nanoparticles (JPs) was characterized by pendant drop tensiometry for water/air and water/decane interfaces. This technique requires a smaller quantity of nanoparticles than the traditional Langmuir balance technique. The direct deposition at the interface of the nanoparticles dispersed in a spreading solvent also requires smaller quantities of sample than does adsorption from the bulk. From the growing and shrinking of the pendant drops, the interfacial activity of the nanoparticles can be evaluated and compared within a wide range of area per particle. In this work, the JPs exhibited a higher interfacial activity than did the HPs in all cases. A hard disk model fits the piecewise compression isotherm of the HPs, yet this model underestimates the interactions between the JPs adsorbed at the interface.

A new method for evaluating the most-stable contact angle using mechanical vibration

We propose a new method for the direct measurement of the most-stable contact angle, using the mechanical vibration of sessile drops from different metastable states. We relaxed sessile drops of identical volume but with different stable contact angles between advancing and receding configurations. Before the vibration, we were able to scan the range of experimentally-accessible drop configurations. In this manner, the most-stable contact angle was experimentally recognized as the observable contact angle unchanged after vibration independent of the previous history (initial state) of the system. We applied this novel strategy to paraffin wax surfaces with a wide range of roughness degree.

Interfacial Activity and Contact Angle of Homogeneous, Functionalized, and Janus Nanoparticles at the Water/Decane Interface

Surface heterogeneity affects the behavior of nanoparticles at liquid interfaces. To gain a deeper understanding on the details of these phenomena, we have measured the interfacial activity and contact angle at water/decane interfaces for three different types of nanoparticles: homogeneous poly(methyl methacrylate) (PMMA), silica functionalized with a capping ligand containing a methacrylate terminal group, and Ag-based Janus colloids with two capping ligands of different hydrophobicity. The interfacial activity was analyzed by pendant drop tensiometry, and the contact angle was measured directly by freeze-fracture shadow-casting cryo-scanning electron microscopy. The silver Janus nanoparticles presented the highest interfacial activity, compared to the silica nanoparticles and the homogeneous PMMA nanoparticles. Additionally, increasing the bulk concentration of the PMMA and silica nanoparticles up to 100-fold compared to the Janus nanoparticles led to silica particles forming fractal-like structures at the interface, contrary to the PMMA particles that did not show any spontaneous adsorption.

A new method for evaluating the most stable contact angle using tilting plate experiments

We propose to measure the most accurate contact angle for calculation of solid surface tensions from the most mechanically stable drop. From tilting plate experiments, we probed the sliding behavior of sessile drops with equal volume but different contact angle. We measured two critical tilt angles for each drop. When the surface was tilted beyond the first critical inclination, the drop began to slide down at the uphill or downhill point of its contact line. The second critical tilt angle occurred with the global motion of the sliding drop. We were able to identify the most stable drop when the first critical tilt angle was maximum, and it was then when both critical tilt angles agreed. We validated this methodology with the most stable contact angle values of polymer surfaces evaluated using mechanical vibrations.

Comparison of sessile drop and captive bubble methods on rough homogeneous surfaces: a numerical study.

Quasi-static experiments using sessile drops and captive bubbles are the most employed methods for measuring advancing and receding contact angles on real surfaces. These observable contact angles are the most easily accessible and reproducible. However, some properties of practical surfaces induce certain phenomena that cause a built-in uncertainty in the estimation of advancing and receding contact angles. These phenomena are well known in surface thermodynamics as stick-slip phenomena. Following the work of Marmur (Marmur, A. Colloids Surf., A 1998, 136, 209-215), where the stick-slip effects were studied with regard to sessile drops and captive bubbles on heterogeneous surfaces, we developed a novel extension of this study by adding the effects of roughness to both methods for contact angle measurement. We found that the symmetry between the surface roughness problem and the chemical heterogeneity problem breaks down for drops and bubbles subjected to stick-slip effects.

Modeling the corrugation of the three-phase contact line perpendicular to a chemically striped substrate.

We model an infinitely long liquid bridge confined between two plates chemically patterned by stripes of the same width and different contact angle, where the three-phase contact line runs, on average, perpendicular to the stripes. This allows us to study the corrugation of a contact line in the absence of pinning. We find that, if the spacing between the plates is large compared to the length scale of the surface patterning, the cosine of the macroscopic contact angle corresponds to an average of cosines of the intrinsic angles of the stripes, as predicted by the Cassie equation. If, however, the spacing becomes on the order of the length scale of the pattern, there is a sharp crossover to a regime where the macroscopic contact angle varies between the intrinsic contact angle of each stripe, as predicted by the local Young equation. The results are obtained using two numerical methods, lattice Boltzmann (a diffuse interface approach) and Surface Evolver (a sharp interface approach), thus giving a direct comparison of two popular numerical approaches to calculating drop shapes when applied to a nontrivial contact line problem. We find that the two methods give consistent results if we take into account a line tension in the free energy. In the lattice Boltzmann approach, the line tension arises from discretization effects at the diffuse three phase contact line.