Diffusiophoretic design of self-spinning microgears from colloidal microswimmers

Abstract

Design strategies to assemble dissipative building blocks are essential to create novel and smart materials and machines. We recently demonstrated the hierarchical self-assembly of phoretic microswimmers into self-spinning microgears and their synchronization by diffusiophoretic interactions [Aubret et al., Nature Physics, 2018]. In this paper, we adopt a pedagogical approach and expose our strategy to control self-assembly and build machines using phoretic phenomena. We notably introduce Highly Inclined Laminated Optical sheets microscopy (HILO) to image and quantify anisotropic and dynamic diffusiophoretic interactions, which could not be observed by standard fluorescence microscopy. The dynamics of a (haematite) photocalytic material immersed in (hydrogen peroxide) fuel under various illumination patterns is first described and quantitatively rationalized by a model of diffusiophoresis, the migration of a colloidal particle in a concentration gradient. It is further exploited to design phototactic microswimmers, that direct towards the high intensity of light, as a result of the the torque exerted by the haematite in a light gradient on a microswimmer. We finally demonstrate the assembly of self-spinning microgears from colloidal microswimmers by controlling dissipative diffusiophoretic interactions, that we characterize using HILO and quantitatively compare to analytical and numerical predictions. Because the approach described hereby is generic, this works paves the way for the rational design of machines by controlling phoretic phenomena.

Optimal slip velocities of micro-swimmers with arbitrary axisymmetric shapes

“…2017). The SEM image of a phototactic swimmer, which consists of a haematite particle extruded from a colloidal bead (Aubret & Palacci 2018). …”

“…(c) Scanning electron microscope (SEM) image of a single half-coated Janus particle; inset: the dark blue shows the location of the platinum (Pt) cap (Choudhury et al 2017). (d) The SEM image of a phototactic swimmer, which consists of a haematite particle extruded from a colloidal bead (Aubret & Palacci 2018).…”

Motility-Induced Temperature Difference in Coexisting Phases

“…Coined as "motility-induced phase separation", or MIPS, this phenomenon has advanced to a key paradigm in the physics of self-propelled particles. When the microparticles are overdamped, like microorganisms in a solvent [18] or active colloidal microswimmers [19][20][21][22][23], they are equally fast in both phases. Hence, despite the presence of active microparticles, liquid and gas as emerging from MIPS have identical kinetic temperatures, just like for liquid-gas phase separation in equilibrium.…”

Preparation, properties, and applications of magnetic hematite microparticles

“…This property has been recently successfully exploited for the design and production of well-defined colloidal swimmers with real scale-up production potential. [27][28][29][30][31] Hematite is a cheap and abundant material and its unique physicochemical and magnetic properties in the colloidal form provide the soft matter community with an exceptional system that possess all the features for the design of novel classes of active systems that can be chemically or magnetically actuated.…”