We report preparation of stimuli-responsive bicomponent polymeric Janus particles and investigation of their switching behavior. The first polymer was immobilized on one side of silica particles using the surface-initiated atom transfer radical polymerization, “grafting from” approach. The second polymer was immobilized using the “grafting to” method by reaction between reactive terminating groups of polymer chains and functional groups on the particle surface. On the example of mixed oppositely charged polyelectrolyte Janus particles decorated with poly(acrylic acid) and poly(2-vinylpyridine) chains, we demonstrate stimuli-responsive aggregation/disaggregation behavior upon pH changes.

Surface heterogeneities, including roughness, significantly affect the adsorption, motion and interactions of particles at fluid interfaces. However, a systematic experimental study, linking surface roughness to particle wettability at a microscopic level, is currently missing. Here we synthesize a library of all-silica microparticles with uniform surface chemistry, but tuneable surface roughness and study their spontaneous adsorption at oil–water interfaces. We demonstrate that surface roughness strongly pins the particles' contact lines and arrests their adsorption in long-lived metastable positions, and we directly measure the roughness-induced interface deformations around isolated particles. Pinning imparts tremendous contact angle hysteresis, which can practically invert the particle wettability for sufficient roughness, irrespective of their chemical nature. As a unique consequence, the same rough particles stabilize both water-in-oil and oil-in-water emulsions depending on the phase they are initially dispersed in. These results both shed light on fundamental phenomena concerning particle adsorption at fluid interfaces and indicate future design rules for particle-based emulsifiers.

The AFM colloidal probe technique was used to measure the interaction between microsized silica spheres and annealed polyelectrolyte brushes made of poly(acrylic acid) (PAA) and poly(2-vinyl pyridine) (P2VP) in KCl solutions of various pH values and salt concentrations. The interaction energy showed a distance dependence that was affected strongly by the swelling and the electric properties of the brushes. Between PAA brushes and silica particles, a repulsive interaction has been observed for all pH values and salt concentrations reflecting the swelling of the brush with varying pH value and the transition from osmotic to salted brush regime with increasing KCl concentration. Force measurements between P2VP brushes and silica particles revealed a much more complex behavior: a steric repulsion by the swollen brush at low pH values, a complex interplay of attractive and repulsive forces at intermediate pH values and a short-ranged attraction between the collapsed brush and the silica particle at basic pH values and high salt concentrations. The results are interpreted in comparison with the Alexander de Gennes model and zeta potential and ellipsometric measurements.

Janus particles are a unique class of multifunctional patchy particles combining two dissimilar chemical or physical functionalities at their opposite sides. The asymmetry characteristic for Janus particles allows them to self-assemble into sophisticated structures and materials not attainable by their homogeneous counterparts. Significant breakthroughs have recently been made in the synthesis of Janus particles and the understanding of their assembly. Nevertheless, the advancement of their applications is still a challenging field. In this Review, we highlight recent developments in the use of Janus particles as building blocks for functional materials. We provide a brief introduction into the synthetic strategies for the fabrication of JPs and their properties and assembly, outlining the existing challenges. The focus of this Review is placed on the applications of Janus particles for active interfaces and surfaces. Active functional interfaces are created owing to the stabilization efficiency of Janus particles combined with their capability for interface structuring and functionalizing. Moreover, Janus particles can be employed as building blocks to fabricate active functional surfaces with controlled chemical and topographical heterogeneity. Ultimately, we will provide implications for the rational design of multifunctional materials based on Janus particles.