Drop evaporation on superhydrophobic PTFE surfaces driven by contact line dynamics.

In the present study, we experimentally study the evaporation modes and kinetics of sessile drops of water on highly hydrophobic surfaces (contact angle ∼160°), heated to temperatures ranging between 40° and 70 °C. These surfaces were initially constructed by means of controlled tailoring of polytetrafluoroethylene (PTFE) substrates. The evaporation of droplets was observed to occur in three distinct phases, which were the same for the different substrate temperatures. The drops started to evaporate in the constant contact radius (CCR) mode, then switched to a more complex mode characterized by a set of stick-slip events accompanied by a decrease in contact angle, and finally shifted to a mixed mode in which the contact radius and contact angle decreased simultaneously until the drops had completely evaporated. It is shown that in the case of superhydrophobic surfaces, the energy barriers (per unit length) associated with the stick-slip motion of a drop ranges in the nJ m(-1) scale. Furthermore, analysis of the evaporation rates, determined from experimental data show that, even in the CCR mode, a linear relationship between V(2/3) and the evaporation time is verified. The values of the evaporation rate constants are found to be higher in the pinned contact line regime (the CCR mode) than in the moving contact line regime. This behavior is attributed to the drop's higher surface to volume ratio in the CCR mode.

Adhesion forces between AFM tips and superficial dentin surfaces.

In this work, we study the adhesion forces between atomic force microscopy (AFM) tips and superficial dentin etched with phosphoric acid. Initially, we quantitatively analyze the effect of acid etching on the surface heterogeneity and the surface roughness, two parameters that play a key role in the adhesion phenomenon. From a statistical study of the force-distance curves, we determine the average adhesion forces on the processed substrates. Our results show that the average adhesion forces, measured in water, increase linearly with the acid exposure time. The highest values of such forces are ascribed to the high density of collagen fibers on the etched surfaces. The individual contribution of exposed collagen fibrils to the adhesion force is highlighted. We also discuss in this paper the influence of the environmental medium (water/air) in the adhesion measurements. We show that the weak forces involved require working in the aqueous medium.

Dentin wetting by three adhesive systems: influence of etching time, temperature and relative humidity.

OBJECTIVES: The purpose of this study was to investigate the wetting properties of three adhesives systems on etched dentin surfaces submitted to a brief air-drying procedure, as in clinical conditions. The influence of two clinical parameters (surface temperature and surrounding hygrometry) on wetting was investigated. METHODS: Two total-etch adhesives (Excite Vivapen and Adper Scothbond 1XT) and one self-etching adhesive (GC-G Bond) were used. Superficial and deep dentin slice were obtained from six caries-free extracted third molars. Before the adhesive deposition, the dentin samples were etched with 37% phosphoric acid for different time durations. The surface topography was characterised by atomic force microscopy (AFM) observations and the wetting properties of dentin were studied by contact angle measurements. RESULTS: Unlike clinical expectations, the contact angles increased with the acid exposure time, and consequently with both surface roughness and the organic-mineral ratio of the dentin components. The wetting properties were clearly more sensitive to the surrounding relative humidity than the temperature. The contact angles values were reduced by about 50% when the relative humidity increased from 37% to 80% and by about 15% when the temperature was raised from 25 degrees C to 37 degrees C. CONCLUSION: Acid-etched dentin exposed to a brief air-drying appears as the key parameter to control the wetting properties of total-etch adhesive-dentin systems. The experimental results seem to be well described by the Cassie-Baxter approach, and suggest that dentinal tubules through an adhesive repellent contribution should account for the observed wettability decreases.

Dentinal tubules driven wetting of dentin: Cassie-Baxter modelling.

We investigate the wetting properties of dentin surfaces submitted to a phosphoric acid etching followed by an air drying procedure, as in clinical situations of adhesive dentistry. The surface topography of the etched surfaces was characterized by AFM, and the wetting properties of water on these rough and heterogeneous surfaces were studied, by contact angle measurements. We showed that the contact angle increases with the acid exposure time and consequently with both surface roughness and the organicmineral ratio of the dentin components. From the whole results, obtained on dentin and also on synthesized hydroxyapatites samples, we inferred a water contact angle of ~133° on the dentinal tubule. These experimental results may be described by the Cassie-Baxter approach, and it is suggested that small air pockets could be formed inside the dentinal tubules.

Wetting on nanorough surfaces.

We investigate the wetting properties of random nanostructured surfaces, with particular attention devoted to the phenomenon of contact angle hysteresis. For this purpose, solid substrates were initially tailored at a nanometric scale by using swift heavy ion irradiation which produced a random distribution of defects. We characterize the wetting properties of water on these heterogeneous surfaces by an average spreading parameter and by the contact angle hysteresis. For weak values of the areal density of defects phi(d), the hysteresis grows linearly with phi(d), indicating that the defects pin the contact line individually. However, at higher values of phi(d), collective pinning effects appear and the hysteresis decreases with increasing phi(d). We show that in the linear regime our experimental results are in good quantitative agreement with theoretical predictions for contact angle hysteresis induced by a single isolated defect on a solid surface.