Understanding the interactions between porous materials and biosystems is of great important in biomedical and environmental sciences. Upon atomic force microscopy (AFM) adhesion measurement, a new experimental approach was presented here to determine the molecular interaction force between proteins and mesoporous TiO of various surface roughnesses. The interaction force between each protein molecule and the pure anatase TiO surface was characterized by fitting the adhesion and adsorption capacity per unit contact area, and it was found that the adhesion forces were approximately 0.86, 2.63, and 4.41 nN for lysozyme, myoglobin, and BSA, respectively. Moreover, we reported that the molecular interaction force was independent of the surface topography of the material but the protein type is a factor of the interaction. These experimental results on the molecular level provide helpful insights for stimulating model calculation and molecular simulation studies of protein interaction with surfaces.

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Our atomic force microscopy (AFM) experiments and non-equilibrium molecular dynamics (NEMD) simulations have demonstrated a negative friction-load dependence to ionic liquid-glycol ether mixtures, that is, the friction decreases as the normal load increases. NEMD simulations revealed a structural reorientation of the studied ionic liquid (IL): as the normal load increases, the cation alkyl chains of ILs change the orientation to preferentially parallel to the tip scanning path. The flat-oriented IL structures, similar to the 'blooming lotus leaf', produce a new sliding interface and reduce the friction. A further molecular dynamics simulation was carried out by adopting slit pore models to mimic the tip approaching process to confirm the dynamics of ILs. We observed a faster diffusion of ILs in the smaller slit pore. The faster diffusion of ILs in the more confined slit pore, facilitates the structural reorientation of ILs. The resulted new sliding surface is responsible for the observed smaller friction at higher loads, also known as the negative friction-load dependence.These findings provide a fundamental explanation to the role of ILs in interfacial lubrications. They help to understand liquid flow properties under confinement, with implications for the development of better nanofluidic devices.

Using atomic force microscopy, the nanofriction coefficient was measured systematically for a series of liquids on planar graphite, silica and mica surfaces. This allows us to explore the quantitative interplay between nanofriction at liquid-solid interfaces and interfacial wetting. A corresponding states theory analysis shows that the nanofriction coefficient, μ = dF(F)/dF(N), where FF is the friction force and FN is the normal force, is a function of three dimensionless parameters that reflect the intermolecular forces involved and the structure of the solid substrate. Of these, we show that one parameter in particular, β = ρ(s)Δ(s)σ(ls)(2), where ρ(s) is the atomic density of the solid, Δ(s) is the spacing between layers of solid atoms, and σ(ls) is the molecular diameter that characterizes the liquid-substrate interaction, is very important in determining the friction coefficient. This parameter β, which we term the structure adhesion parameter, provides a measure of the intermolecular interaction between a liquid molecule and the substrate and also of the surface area of contact of the liquid molecule with the substrate. We find a linear dependence of μ on the structure adhesion parameter for the systems studied. We also find that increasing β leads to an increase in the vertical adhesion forces FA (the attractive force exerted by the solid surface on the liquid film). Our quantitative relationship between the nanofriction coefficient and the key parameter β which governs the vertical adhesive strength, opens up an opportunity for describing liquid flows on solid surfaces at the molecular level, with implications for the development of membrane and nanofluidic devices.