Investigating the relationship between the mechanical properties of plasma polymer-like thin films and their glass transition temperature.

This work aims at understanding the influence of the substrate temperature (Ts) on the viscoelastic properties of propanethiol plasma polymer films (PPFs). By means of state-of-the-art AFM characterization-based techniques including peak force quantitative nanomechanical mapping (PFQNM), nano dynamic mechanical analysis (nDMA) and "scratch" experiments, it has been demonstrated that the mechanical behaviour of PPFs is dramatically affected by the thermal conditions of the substrate. Indeed, the material behaves from a high viscous liquid (i.e. viscosity ∼ 106 Pa s) to a viscoelastic solid (loss modulus ∼ 1.17 GPa, storage modulus ∼ 1.61 GPa) and finally to an elastic solid (loss modulus ∼ 1.95 GPa, storage modulus ∼ 8.51 GPa) when increasing Ts from 10 to 45 °C. This behaviour is ascribed to an increase in the surface glass transition temperature of the polymeric network. The latter has been correlated with the chemical composition through the presence of unbound molecules acting as plasticizers and the cross-linking density of the layers. In a second step, this knowledge is exploited for the fabrication of a nanopattern by generating surface instabilities in the propanethiol PPF/Al bilayer system.

Combining Ellipsometry and AFM To Probe Subnanometric Precursor Film Dynamics of Polystyrene Melts.

We investigate the evolution over time of the space profiles of precursor films spreading away from a droplet of polymer in the poorly explored pseudo-partial wetting case. We use polystyrene melt droplets on oxidized silicon wafers. Interestingly, the film thicknesses measured by ellispometric microscopy are found in the 0.01 to 1 nm range. These thicknesses were validated by atomic force microscopy measurements performed on the textured film obtained after quenching at room temperature. From this, an effective thickness is obtained and compares well to the thicknesses measured by ellipsometry, validating the use of an optical method in this range of thickness. Ellipsometric microscopy provides a height resolution below the ångström with lateral resolution, image size, and framerate well adapted to spreading precursor films. From this, we demonstrate that precursor films of polystyrene consist of polymer chains with a surface density decreasing to zero away from the droplet. We further find that the polymer chains follow a simple diffusive law with the diffusion coefficient independent of density. This demonstrates that polystyrene chains spread independently in precursor films in pseudo-partial wetting condition. This behavior differs significantly from the case of chains spreading in total wetting for which the diffusion coefficient was found in the literature to depend on surface density or thickness.

Effect of geometry on the dewetting of granular chains by evaporation.

Understanding evaporation or drying in granular media still remains complex despite recent advancements. Evaporation depends on liquid transport across a connected film network from the bulk to the surface. In this study, we investigate the stability of film networks as a function of the geometry of granular chains of spherical grains. Using a controlled experimental approach, we vary the grain arrangement or packing and measure the height of the liquid film network during evaporation as packing shifts from loose-packed to close-packed arrangement. This height can be calculated from an equilibrium between hydrostatic pressure and the capillary pressure difference in the vertical film network. Following a simulation approach using Surface Evolver, we evaluate the pressure variation due to dewetting of the meniscus volume in the grains in both the percolating front and evaporating front within the two-phase zone of air/water mixture. Results show good agreement between model and experiment. We find that above a "critical" packing angle, the liquid continuity is broken and films connections fragment into separate, isolated capillary bridges.

Lithography by Scanning Electrochemical Microscopy with a multiscaled electrode.

A multiscaled electrochemical probe is presented for Scanning Electrochemical Microscopy (SECM) experiments. It is fabricated by wet chemical etching followed by sputter-coating of an ordered optical fiber bundle. Owing to the optical fiber bundle preparation, the global electrode may present different shapes. After the chemical etching step, each one of these shapes is conserved and finally decorated with 6000 nanotips. Numerical simulations and approach curves are used to study the probe properties and the influence of the global shape and of the nanotips. The numerical simulations show that the approach curves do not depend on the shape of the electrode but rather on the total height of the protuberance of its electroactive part. Such new SECM probes are then used to pattern a Teflon surface. Indeed, by controlling the time scale of the applied potential pulses, the thickness of the reaction layer is confined at each nanotip, and the nanotip pattern is electrochemically transferred onto the non-conductive surface. Both scales (i.e., global electrode shape and nanotip array) thus show distinct and complementary features for positioning the probe and for the subsequent electrochemical patterning.

Preferential Root Tropisms in 2D Wet Granular Media with Structural Inhomogeneities.

We investigate certain aspects of the physical mechanisms of root growth in a granular medium and how these roots adapt to changes in water distribution induced by the presence of structural inhomogeneities in the form of solid intrusions. Physical intrusions such as a square rod added into the 2D granular medium maintain robust capillary action, pumping water from the more saturated areas at the bottom of the cell towards the less saturated areas near the top of the cell while the rest of the medium is slowly devoid of water via evaporation. The intrusion induces "preferential tropism" of roots by first generating a humidity gradient that attracts the root to grow towards it. Then it guides the roots and permits them to grow deeper into more saturated regions in the soil. This further allows more efficient access to available water in the deeper sections of the medium thereby resulting to increased plant lifetime.

MAS NMR spectra of quadrupolar nuclei in disordered solids: the Czjzek model.

Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters upsilon(Q) and eta from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.

Contact of a spherical probe with a stretched rubber substrate.

We report on a theoretical and experimental investigation of the normal contact of stretched neo-Hookean substrates with rigid spherical probes. Starting from a published formulation of surface Green's function for incremental displacements on a prestretched, neo-Hookean, substrate [J. Mech. Phys. Solids 56, 2957 (2008)JMPSA80022-509610.1016/j.jmps.2008.07.002], a model is derived for both adhesive and nonadhesive contacts. The shape of the elliptical contact area together with the contact load and the contact stiffness are predicted as a function of the in-plane stretch ratios λ_{x} and λ_{y} of the substrate. The validity of this model is assessed by contact experiments carried out using an uniaxally stretched silicone rubber. For stretch ratio below about 1.25, a good agreement is observed between theory and experiments. Above this threshold, some deviations from the theoretical predictions are induced as a result of the departure of the mechanical response of the silicone rubber from the neo-Hokeean description embedded in the model.

Experimental investigation of water distribution in a two-phase zone during gravity-dominated evaporation.

We characterize the water repartition within the partially saturated (two-phase) zone (PSZ) during evaporation from mixed wettable porous media by controlling the wettability of glass beads, their sizes, and as well the surrounding relative humidity. Here, capillary numbers are low and under these conditions, the percolating front is stabilized by gravity. Using experimental and numerical analyses, we find that the PSZ saturation decreases with the Bond number, where packing of smaller particles have higher saturation values than packing made of larger particles. Results also reveal that the extent (height) of the PSZ, as well as water saturation in the PSZ, both increase with wettability. We also numerically calculate the saturation exclusively contained in connected liquid films and results show that values are less than the expected PSZ saturation. These results strongly reflect that the two-phase zone is not solely made up of connected capillary networks but also made of disconnected water clusters or pockets. Moreover, we also find that global saturation (PSZ + full wet zone) decreases with wettability, confirming that greater quantity of water is lost via evaporation with increasing hydrophilicity. These results show that connected liquid films are favored in more-hydrophilic systems while disconnected water pockets are favored in less-hydrophilic systems.

Multiscale Surface-Attached Hydrogel Thin Films with Tailored Architecture.

A facile route for the fabrication of surface-attached hydrogel thin films with well-controlled chemistry and tailored architecture on wide range of thickness from nanometers to micrometers is reported. The synthesis, which consists in cross-linking and grafting the preformed and ene-reactive polymer chains through thiol-ene click chemistry, has the main advantage of being well-controlled without the addition of initiators. As thiol-ene click reaction can be selectively activated by UV-irradiation (in addition to thermal heating), micropatterned hydrogel films are easily synthesized. The versatility of our approach is illustrated by the possibility to fabricate various chemical polymer networks, like stimuli-responsive hydrogels, on various solid substrates, such as silicon wafers, glass, and gold surfaces. Another attractive feature is the development of new complex hydrogel films with targeted architecture. The fabrication of various architectures for polymer films is demonstrated: multilayer hydrogel films in which single-networks are stacked one onto the other, interpenetrating networks films with mixture of two networks in the same layer, and nanocomposite hydrogel films where nanoparticles are stably trapped inside the mesh of the network. Thanks to its simplicity and its versatility this novel approach to surface-attached hydrogel films should have a strong impact in the area of polymer coatings.

Kinetics of gravity-driven water channels under steady rainfall.

We investigate the formation of fingered flow in dry granular media under simulated rainfall using a quasi-two-dimensional experimental setup composed of a random close packing of monodisperse glass beads. Using controlled experiments, we analyze the finger instabilities that develop from the wetting front as a function of fundamental granular (particle size) and fluid properties (rainfall, viscosity). These finger instabilities act as precursors for water channels, which serve as outlets for water drainage. We look into the characteristics of the homogeneous wetting front and channel size as well as estimate relevant time scales involved in the instability formation and the velocity of the channel fingertip. We compare our experimental results with that of the well-known prediction developed by Parlange and Hill [D. E. Hill and J. Y. Parlange, Soil Sci. Soc. Am. Proc. 36, 697 (1972)]. This model is based on linear stability analysis of the growth of perturbations arising at the interface between two immiscible fluids. Results show that, in terms of morphology, experiments agree with the proposed model. However, in terms of kinetics we nevertheless account for another term that describes the homogenization of the wetting front. This result shows that the manner we introduce the fluid to a porous medium can also influence the formation of finger instabilities. The results also help us to calculate the ideal flow rate needed for homogeneous distribution of water in the soil and minimization of runoff, given the grain size, fluid density, and fluid viscosity. This could have applications in optimizing use of irrigation water.

Simple analytical model of evapotranspiration in the presence of roots.

Evaporation of water out of a soil involves complicated and well-debated mechanisms. When plant roots are added into the soil, water transfer between the soil and the outside environment is even more complicated. Indeed, plants provide an additional process of water transfer. Water is pumped by the roots, channeled to the leaf surface, and released into the surrounding air by a process called transpiration. Prediction of the evapotranspiration of water over time in the presence of roots helps keep track of the amount of water that remains in the soil. Using a controlled visual setup of a two-dimensional model soil consisting of monodisperse glass beads, we perform experiments on actual roots grown under different relative humidity conditions. We record the total water mass loss in the medium and the position of the evaporating front that forms within the medium. We then develop a simple analytical model that predicts the position of the evaporating front as a function of time as well as the total amount of water that is lost from the medium due to the combined effects of evaporation and transpiration. The model is based on fundamental principles of evaporation fluxes and includes empirical assumptions on the quantity of open stomata in the leaves, where water transpiration occurs. Comparison between the model and experimental results shows excellent prediction of the position of the evaporating front as well as the total mass loss from evapotranspiration in the presence of roots. The model also provides a way to predict the lifetime of a plant.

Friction of viscoelastic elastomers with rough surfaces under torsional contact conditions.

Frictional properties of contacts between a smooth viscoelastic rubber and rigid surfaces are investigated using a torsional contact configuration where a glass lens is continuously rotated on the rubber surface. From the inversion of the displacement field measured at the surface of the rubber, spatially resolved values of the steady state frictional shear stress are determined within the nonhomogeneous pressure and velocity fields of the contact. For contacts with a smooth lens, a velocity-dependent but pressure-independent local shear stress is retrieved from the inversion. On the other hand, the local shear stress is found to depend on both velocity and applied contact pressure when a randomly rough (sand-blasted) glass lens is rubbed against the rubber surface. As a result of changes in the density of microasperity contacts, the amount of light transmitted by the transparent multicontact interface is observed to vary locally as a function of both contact pressure and sliding velocity. Under the assumption that the intensity of light transmitted by the rough interface is proportional to the proportion of area into contact, it is found that the local frictional stress can be expressed experimentally as the product of a purely velocity-dependent term, k(v), by a term representing the pressure and velocity dependence of the actual contact area, A/A(0). A comparison between k(v) and the frictional shear stress of smooth contacts suggests that nanometer scale dissipative processes occurring at the interface predominate over viscoelastic dissipation at microasperity scale.

Contact compliance effects in the frictional response of bioinspired fibrillar adhesives.

The shear failure and friction mechanisms of bioinspired adhesives consisting of elastomer arrays of microfibres terminated by mushroom-shaped tips are investigated in contact with a rigid lens. In order to reveal the interplay between the vertical and lateral loading directions, experiments are carried out using a custom friction set-up in which normal stiffness can be made either high or low when compared with the stiffness of the contact between the fibrillar adhesive and the lens. Using in situ contact imaging, the shear failure of the adhesive is found to involve two successive mechanisms: (i) cavitation and peeling at the contact interface between the mushroom-shaped fibre tip endings and the lens; and (ii) side re-adhesion of the fibre's stem to the lens. The extent of these mechanisms and their implications regarding static friction forces is found to depend on the crosstalk between the normal and lateral loading directions that can result in contact instabilities associated with fibre buckling. In addition, the effects of the viscoelastic behaviour of the polyurethane material on the rate dependence of the shear response of the adhesive are accounted for.

Two beam surface fluctuation specular reflection spectroscopy.

In surface fluctuation specular reflection spectroscopy (SFSRS) deflections of a specularly reflected laser beam are used to characterize thermally excited surface waves. Here we report on a new two beam version of SFSRS in which the deflections of two reflected laser beams from separate locations on a surface are correlated. We demonstrate that this new two beam SFSRS technique can be used to determine directly the power spectrum of height fluctuation of thermally excited surface waves over a large range of both frequencies and wavevectors. In addition, we show that the technique is well suited for materials ranging from simple liquids to complex liquids and soft solids, including turbid materials.

Friction and shear fracture of an adhesive contact under torsion.

The shear failure or stiction of an adhesive contact between a poly(dimethylsiloxane) (PDMS) rubber and a glass lens has been investigated using a torsional contact configuration. As compared to linear sliding, torsion presents the advantage of inducing a shear failure under a pure mode III condition, while preserving the cylindrical symmetry of the contact. The surface of the transparent PDMS substrate was marked using a network of dots in order to monitor continuously the in-plane surface displacements during the stiction process. Using a previously developed inversion procedure (A. Chateauminois and C. Fretigny, Eur. Phys. J. E 27, 221 (2008)), the corresponding surface shear stress distributions were obtained from the displacement fields. Stiction was found to involve the progressive shrinkage of a central adhesive zone surrounded by an annular microslip region. Adhesion effects were especially evidenced from a stress overshoot at the boundary of the adhesive zone. The experimental data were analysis using an extension to torsional contact of the Maugis-Dugdale approach's to adhesive contacts which takes into account frictional effects. This model allowed to extract an effective adhesion energy in the presence of friction, which dependence on kinetics effect is briefly discussed.

Reduced viscosity in thin polymer films.

The dewetting of thin polystyrene films (20-500 nm) on a liquid substrate is studied at time scales that are long compared to the reptation time. It is shown that the kinetics correspond to those of purely viscous flow and that the viscosity measured by this technique is, for the thickest films, consistent with bulk measurements. Films on the order of the coil size are then studied. The effective viscosity of these films displays a large decrease when the film thickness h is below several radius of gyration, R(g). This viscosity reduction is found to depend only on the ratio h/R(g).