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We describe how a long-range repulsive interaction can surreptitiously modify the effective geometry of approaching compliant surfaces, with significant consequences on friction. We investigated the behavior under shear and compression of mica surfaces coated with poly(N-isopropylacrylamide) pNIPAM-based cationic microgels. We show that local surface deformations as small as a few nanometers must be considered to understand the response of such surfaces under compression and shear, in particular when the range of action of normal and friction forces are significantly different, as is often the case for macromolecular lubrication. Under these conditions, a subtle interplay between normal forces and surface compliance may significantly reduce friction increment by limiting the minimum approach of the surfaces under pressure. We found that stiffening of compressed microgels confined in the region of closest approach make it increasingly difficult to reduce the gap between the mica surfaces, limiting the defo...

International audience; As double stranded, single stranded siRNA (ss-siRNA) has demonstrated gene silencing activity but still requires efficient carriers to reach its cytoplasmic target. To better understand the fundamental aspect driving the complexation of ss-siRNA with nanocarriers, the interactions between surfaces of various compositions across a ss-siRNA solution were investigated using the Surface Forces Apparatus. The results show that ss-siRNA can adsorb onto hydrophilic (positively and negatively charged) as well as on hydrophobic substrates suggesting that the complexation can occur through hydrophobic interactions and hydrogen bonding in addition to electrostatic interactions. Moreover, the binding strength and the conformation of ss-siRNA depend on the nature of the interactions between the ss-siRNA and the surfaces. The binding of ss-siRNA with nanocarriers, such as micelles or liposomes through non-electrostatic interactions was also evidenced by a SYBR ® Gold cyanine dye. We evidenced the presence of interactions between the dye and oligonucleotides already complexed to non-cationic nanovectors biasing the quantification of the encapsulation. These results suggest that non-electrostatic interactions could be exploited to complement electrostatic interactions in the design of nanocarriers. In particular, the different highlighted interactions can be used to complex ss-siRNA with uncharged or anionic carriers which are related to lower toxicity compared to cationic carriers.

14 pages; International audience; We have studied the adsorption and lubricant properties of a multifunctional triblock copolymer poly(L-lysine)-b-poly(acrylic acid)-bpoly(L-lysine). In particular, we investigated the nature of the layer adsorbed under different conditions of polymer and salt concentration and the lubricant properties of the polymer layer before and after its chemical crosslinking by bridging the poly(acrylic acid) blocks. We found that the amount of polymer adsorbed is controlled by the ionic strength and the polymer concentration in the solution. In all cases, the self-assembled polymer layer is a poor lubricant before cross-linking, but the cohesion and load-carrying ability of the layer are substantially improved by this reaction. However, the chemically cross-linked coating has a limited deformation capacity as a consequence of its permanent network nature, and irreversible damage is observed after excessive strain of the film.

A new 3D sensor-actuator attachment for a Surface Forces Apparatus (SFA) allows measurements of friction forces between surfaces in all 3 orthogonal directions. The anisotropic friction forces when shearing two mica surfaces separated by a nanometer-thin film of hexadecane can induce complex transient and steady-state motions involving displacements perpendicular to the applied force and nonzero velocity at all stages of back-and-forth sliding. Refereed/Peer-reviewed

Static and dynamic interaction forces between two thermosensitive polymeric nanoparticle monolayers grafted onto mica surfaces and immersed in water were studied using a surface forces apparatus. The polymeric nanoparticles (NPs) were made of N,N-diethylacrylamide and had a hydrodynamic diameter of ca. 780 nm at 20 °C in aqueous suspension. They were irreversibly grafted onto chemically modified mica surfaces at a constant surface coverage of 2.6 NPs/µm2. The measured normal forces between two opposing NP monolayers were found to be strongly dependent on the temperature. At temperatures lower than the lower critical solution temperature (LCST), the grafted NPs were swollen, and the normal interaction forces between the two NP monolayers were repulsive. Above the LCST, the NPs collapsed, and attractive forces between the NP layers were measured. The swollen NPs were found to exhibit very low friction forces compared to the collapsed ones. The effect of the sliding velocity on the shear stress was investigated, and the results are in agreement with the so-called adhesive friction model developed for rubber friction. Our results suggest that the water content in the contact area and the interdiffusion of polymer chains are important parameters in determining the friction between polymer-bearing surfaces. Refereed/Peer-reviewed

In contrast to non-associating liquids such as oils or organic solvents, whose viscosity diverges when they are confined by solid surfaces to films thinner than about ten molecular diameters, recent studies reveal that salt-free water remains fluid, with a viscosity close to its bulk value, even when confined to films down to only one or two monolayers thick. For the case of high concentration aqueous salt solutions compressed down to subnanometre films between confining planar surfaces, the hydration sheaths about the ions (trapped between the oppositely charged surfaces) also remain extremely fluid: this behaviour is attributed to the tenacity of water molecules in the hydration layers together with their rapid relaxation/exchange time. Related experiments on highly compressed, polyelectrolyte brushes in aqueous media reveal a remarkable lubricity which is in large measure attributed to similar hydration layers about the charged segments: this water of hydration strongly resists being squeezed out, but at the same time it may rapidly exchange with adjacent water molecules, thereby remaining quite fluid and acting as a molecular lubricant.