No abstract

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 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) 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.

No abstract

Cross-linked polystyrene microspheres bearing positive and negative charges have been prepared by surfactant-free emulsion polymerization by the use of various amounts of vinylbenzyltrimethylammonium chloride and sodium styrenesulfonate as comonomers, respectively. Increasing the amount of the ionic comonomers tends to decrease the particle size due to better surface stabilization, but a high concentration of an ionic comonomer leads to a competitive mechanism that increases the polydispersity of the particle size. The amount of cationic and anionic comonomers leads to differences in size, shape and uniformity of the particles. The cationic particles can self-assemble into crystalline colloidal arrays with intense visible light diffraction just like the anionic ones. It is particularly interesting to observe that good packing can be obtained even for particles not quite uniform in size. To better understand the packing behavior, the properties and stability of the colloidal crystals have been studied as a function of the particle concentration and ionic strength of the media. The presence of charges helps in the formation of periodic structure over a wide range of particle concentrations at low ionic strength.

Understanding the thermo-sensitivity of crystalline colloidal arrays formed by poly(styrene-co-N-isopropylacrylamide) core-shell microspheres Gw ena€ elle Bazin and X. X. Zhu *