Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays.

Normal and friction forces between immobilized two-dimensional (2D) homogeneous non-close-packed colloidal arrays made of different particles are compared in aqueous media. Soft pH-responsive (microgels) and nonresponsive hard (silica) particles of different sizes were used to create the 2D arrays. The results show that the friction of soft responsive structured layers can be successfully modulated by varying the pH, with a friction coefficient varying by nearly 3 orders of magnitude (10-2 to 1). This important change in lubricating properties is mainly correlated with the particle swelling behavior, i.e., the friction coefficient decreasing exponentially with an increase in the swelling ratio regardless of the size, surface coverage, and degree of ionization of the particles. In addition, the robustly attached microgel particles were able to sustain high pressure (up to 200 atm) without significant surface damage. The 2D arrays of nonresponsive hard particles also gave rise to a very low friction coefficient (μ ≈ 10-3 ) under similar experimental conditions and could sustain a larger pressure without damage (≤600 atm). The low friction dissipation observed between the hard arrays resulted from a rolling mechanism. Even though rolling requires nonimmobilized particles on the substrates, the results show the importance of attaching a certain proportion of particles on the surfaces to reduce friction.