Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays

Multiresponsive Microgels: Toward an Independent Tuning of Swelling and Surface Properties

Dual-responsive poly-(N-isopropylacrylamide) (PNIPAM) microgels surface-functionalized with polyethylene glycol (PEG) or poly-2-dimethylaminoethyl methacrylate (PDMAEMA) were developed to enable the swelling behavior and surface properties of the microgels to be tuned independently. The thermo-triggered swelling and pH-triggered surface properties of the microgels were investigated in aqueous suspensions using dynamic light scattering and on substrates using the surface forces apparatus. Grafting polymer chains on the microgel surface did not impede the thermo-triggered swelling behavior of the microgels in suspensions and immobilized on substrates. An unprecedented decoupling of the swelling behavior and surface properties could be obtained. More particularly, the thermo-triggered swelling behavior of the PNIPAM underlying microstructure could be tuned below and above the phase transition temperature with no change in the surface potential and adhesion provided by the surface non-responsive PEG.

Effect of Dispersity on the Conformation of Spherical Polymer Brushes

We show that dispersity (D̵) markedly alters the conformation of spherical polymer brushes. The average lengths (l_b) of poly(tert-butyl acrylate) (PtBA) brushes of varying D̵ grafted to nanoparticles were measured using dynamic light scattering. In the semidilute polymer brush (SDPB) regime, the l_b of PtBA and polymers from earlier studies of various D̵ could be cleanly collapsed onto a master curve as a function of the scaling variable N_wσ^1/3, where N_w is the weight-average degree of polymerization and σ is the grafting density. In the concentrated polymer brush (CPB) regime, however, l_b collapsed onto a bifurcated curve as a function of the scaling variable N_wσ^1/2, indicating D̵ more strongly affects the average length of brushes with low N_wσ^1/2. We propose that the stretching of the stem near the particle surface due to interchain interactions in the CPB regime leads to greater l_b in broad dispersity brushes of low but not high N_wσ^1/2.

Recent Progress in Cartilage Lubrication

Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently-lubricated surface known in nature, with friction coefficients as low as 0.001 up to physiologically high pressures. Such low friction is indeed essential for its well-being. It minimizes wear-and-tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage-embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio-inspired lubricants is reviewed.

The wormlike micelles formed using an ionic liquid surfactant and polar organic solvents at low temperature without additives and their lubricant properties

Wormlike micelles (or reverse wormlike micelles) are flexible cylindrical chains that are normally formed in water (or a nonpolar organic solvent) at 25.0 °C or above; the formation of wormlike micelles at lower temperatures is rare. Here, we have reported wormlike micelles formed at low temperature using an ionic liquid surfactant (1-octadecyl-3-nonyl imidazolium bromide) in polar organic solvents (including 1,3-propanediol, 1,2-propylene glycol, N,N-dimethylformamide, and glycerol/1,2-propylene glycol mixture) in the absence of any additives. The viscoelasticity and morphology of the wormlike micelles were studied using rheology, small-angle X-ray scattering, and cryo-transmission electron microscopy. The viscoelastic properties of the wormlike micelles in polar solvents are affected by the solvent type (or the weight ratio of glycerol to 1,2-propylene glycol), surfactant concentration, and temperature. Moreover, the G' and G'' crossover twice in the dynamic curves, which is different from the case in water. The first crossover (at low frequency) corresponds to the relaxation time for the alkyl chains to disentangle from the transient network, and the second crossover (at high frequency) is related to the segmental motion of the chains. Furthermore, the tribological performance of these wormlike micelles is investigated at low temperature. It is found that the protective film (formed by the physical adhesion of the wormlike micelles on the surface of friction disk pair) and the tribochemical reaction together lead to good antifriction and antiwear performance, which indicates the application prospects of these wormlike micelles in low-temperature lubrication.

Designer Nanoparticles as Robust Superlubrication Vectors

Phosphatidylcholine lipid bilayers or liposomes at interfaces in aqueous environments can provide extremely efficient lubrication. This is attributed to the hydration lubrication mechanism acting at the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each bilayer. Micelles exposing such phosphocholine groups could be an attractive alternative to liposomes due to their much easier preparation and structure control, but all studies to date of surfactant micelles have revealed that at relatively low normal stresses the surface layers rupture and friction increases abruptly. Here, we examine surface interactions between three kinds of phosphocholine-exposing micelles with different designed structures: single-tail surfactant micelles, homo-oligomeric micelles, and block copolymer micelles. Normal and shear forces between mica surfaces immersed in solutions of these micelles were measured using a surface force balance. The adsorbed layers on the mica were imaged using atomic force microscope, revealing surface structures ranging from wormlike to spherical micelles. The block copolymer micelles showed relatively low coverage arising from their stabilizing corona and consequently poor lubrication (μ ∼ 10-1). In contrast, the surfactant and homo-oligomeric micelles fully covered the mica surface and demonstrated excellent lubrication (μ ∼ O(10-3)). However, while the boundary layer of single-tailed surfactant micelles degraded under moderate pressure, the homo-oligomeric micellar boundary layer was robust at all applied contact pressures in our study (up to about 5 MPa). We attribute the difference to the much greater energy required to remove a homo-oligomeric molecule from its micelle, resulting in far greater stability under pressure and shear.

Interaction between Compliant Surfaces: How Soft Surfaces Can Reduce Friction

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 deformation of microgels distant from the contact apex and their contribution to global friction while increasing the effective contact radius. These findings reveal a simple mechanism for a robust control of lubrication: by properly tuning the stiffness and geometry of the interacting bodies, for an ad hoc long-range interaction, the growth of friction with applied normal load can be significantly hindered. Thus, substrate compliance is as significant as surface interaction in the design of low friction, long life tribological systems.

Tribological Behavior of Surface-Immobilized Novel Biomimicking Multihierarchical Polymers: The Role of Structure and Surface Attachment

The tribological properties of two novel biomimetic multihierarchical polymers, synthesized by covalently linking single bottlebrush polymers onto a hyaluronic acid (HA) backbone, were investigated in the boundary lubrication regime using the surface forces apparatus. The polymers were immobilized on flat substrates, and their lubrication properties and wear resistance were investigated in aqueous media in the absence of a polymer reservoir (i.e., no free polymer chains in the surrounding medium) in order to better reveal the underlying mechanism of surface-attached biomimetic polymers. The effects of composition, structure, and, more particularly, surface attachment (physisorbed vs chemisorbed) on the tribological properties were investigated and compared with other biomimicking systems reported in the literature. The covalently surface attached bottlebrushes allowed wear resistance between sliding surfaces to be significantly improved, compared to physisorbed bottlebrushes, with a constant coefficient of friction (10^-1) of up to few tens of MPa. The results confirm that surface-attached bottlebrushes on their own are not responsible for the extremely low friction often reported in the literature or found in articular joints. Moreover, the study confirmed that the irreversible attachment of bottlebrushes, or multihierarchical polymer layers, to surfaces is crucial to improving wear resistance between sliding surfaces in aqueous media.

Enhanced biolubrication on biomedical devices using hyaluronic acid-silica nanohybrid hydrogels

In this work, highly lubricous hyaluronic acid-silica (HA-SiO₂) nanohybrid coatings were fabricated through a sequential process consisting of a sol-gel followed by electrophoretic deposition (EPD). SiO₂ nanoparticles were uniformly distributed in the coating layers, and the coating thickness was identified as approximately 1-2 μm regardless of the amount of SiO_2. Incorporation of SiO₂ into the HA polymer matrix enhanced the mechanical stability of the nanohybrid coatings, indicating greater interfacial bonding strength compared to HA coating layers alone. In addition, due to improved stability, the nanohybrid coatings showed excellent biolubrication properties, which were evaluated with a tribological experiment. These results indicate that the nanohybrid coatings have great potential to be used in biomedical applications that require superior biolubrication properties.