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Colloidal Janus microparticles can be propelled by controlled chemical reactions on their surfaces. Such microswimmers have been used as model systems for the behavior on the microscale and as carriers for cargo to well-defined positions in hard-to-access areas. Here we demonstrate the propagation motion of clusters of magnetic Janus particles driven by the catalytic decomposition of $$\hbox {H}_2\hbox {O}_2$$ on their metallic caps. The magnetic moments of their caps lead to certain spatial arrangements of Janus particles, which can be influenced by external magnetic fields. We investigate how the arrangement of the particles and caps determines the driven motion of the particle clusters. In addition, we show the influence of confining walls on the cluster motion, which will be encountered in any real-life biological system.

The shape of objects has a strong influence on their dynamics. Here, we present comparative studies of two different motile objects, spherical Ag/AgCl Janus particles and polystyrene Janus nanorods, that move due to an ionic self-diffusiophoretic propulsion mechanism when exposed to blue light. In this paper, we propose a method to fabricate Janus rodlike particles with high aspect ratios and hemispherical tip shapes. The inherent asymmetry due to the ratio between capped and uncapped parts of the particles as well as the shape anistropy of Janus nanorods enables imaging and quantification of rotational dynamics. The dynamics of microswimmers are compared in terms of velocities and diffusion coefficients. We observe that despite a small amount of the Ag/AgCl reagent on the surface of rodlike objects, these new Janus micromotors reveal high motility in pure water. While the velocities of spherical particles reach 4.2 μm/s, the single rodlike swimmers reach 1.1 μm/s, and clusters reach 1.6 μm/s. The effect of suppressed rotational diffusion is discussed as one of the reasons for the increased velocities. These Janus micro- and nanomotors hold the promise for application in light-controlled propulsion transport.

We designed spongy monoliths allowing liquid delivery to their surfaces through continuous nanopore systems (mean pore diameter ∼40 nm). These nanoporous monoliths were flat or patterned with microspherical structures a few tens of microns in diameter, and their surfaces consisted of aprotic polymer or of TiO2 coatings. Liquid may reduce adhesion forces FAd; possible reasons include screening of solid-solid interactions and poroelastic effects. Softening-induced deformation of flat polymeric monoliths upon contact formation in the presence of liquids enhanced the work of separation WSe. On flat TiO2-coated monoliths, WSe was smaller under wet conditions than under dry conditions, possibly because of liquid-induced screening of solid-solid interactions. Under dry conditions, WSe is larger on flat TiO2-coated monoliths than on flat monoliths with a polymeric surface. However, under wet conditions, liquid-induced softening results in larger WSe on flat monoliths with a polymeric surface than on flat monoliths with an oxidic surface. Monolithic microsphere arrays show antiadhesive properties; FAd and WSe are reduced by at least 1 order of magnitude as compared to flat nanoporous counterparts. On nanoporous monolithic microsphere arrays, capillarity (WSe is larger under wet than under dry conditions) and solid-solid interactions (WSe is larger on oxide than on polymer) dominate contact mechanics. Thus, the microsphere topography reduces the impact of softening-induced surface deformation and screening of solid-solid interactions associated with liquid supply. Overall, simple modifications of surface topography and chemistry combined with delivery of liquid to the contact interface allow adjusting WSe and FAd over at least 1 order of magnitude. Adhesion management with spongy monoliths exploiting deployment (or drainage) of interfacial liquids as well as induction or prevention of liquid-induced softening of the monoliths may pave the way for the design of artificial surfaces with tailored contact mechanics. Moreover, the results reported here may contribute to better understanding of the contact mechanics of biological surfaces.

Bioinspired artificial surfaces with tailored adhesive properties have attracted significant interest. While fibrillar adhesive pads mimicking gecko feet are optimized for strong reversible adhesion, monolithic microsphere arrays mimicking the slippery zone of the pitchers of carnivorous plants of the genus Nepenthes show anti-adhesive properties even against tacky counterpart surfaces. In contrast to the influence of topography, the influence of relative humidity (RH) on adhesion has been widely neglected. Some previous works deal with the influence of RH on the adhesive performance of fibrillar adhesive pads. Commonly, humidity-induced softening of the fibrils enhances adhesion. However, little is known on the influence of RH on solid anti-adhesive surfaces. We prepared polymeric nanoporous monolithic microsphere arrays (NMMAs) with microsphere diameters of a few 10 {\mu}m to test their anti-adhesive properties at RHs of 2 % and 90 %. Despite the presence of continuous nanopore systems through which the inner nanopore walls were accessible to humid air, the topography-induced anti-adhesive properties of NMMAs on tacky counterpart surfaces were retained even at RH = 90 %. This RH-independent robustness of the anti-adhesive properties of NMMAs significantly contrasts the adhesion enhancement by humidity-induced softening on nanoporous fibrillar adhesive pads made of the same material.

Bioinspired surface topographies showing generic anti-adhesive behaviour by minimization of the real contact area not only with rigid, but also with soft and compliant counterpart surfaces recently attracted increasing attention. In the present study, we show that such generic anti-adhesive surfaces, which moreover demonstrate anti-fouling behaviour, can be produced on a large scale by a simple double replication of monolayers of microspheres with diameters of a few 10 mu m. Thus, we obtained mechanically stable monolithic arrays of microspheres tightly connected to a support of the same material. Adhesion of these microsphere arrays to sticky and compliant counterpart surfaces was one order of magnitude weaker than that of flat control samples of the same material. The generation of nanorod arrays with nanorod diameters of a few 100 nmas the second hierarchical structure level on monolithic microsphere arrays did not significantly affect the adhesion force. The experimental data on anti-adhesive behaviour were modelled using a modified Johnson-Kendall-Roberts theoretical approach that also provided general design criteria for topographic adhesion minimization to sticky counterpart surfaces.