Colloidal Micromotors: Controlled Directed Motion

Larysa Barabana, Christian Kreidler, Denys Makarov, Paul Leiderer, and Artur Erbe Department of Physics, University of Konstanz, Universitätstrasse 10, Konstanz, D- 78457, Germany

Here we demonstrate a synthetic micro-engine, based on long-range controlled movement of colloidal particles, which is induced by a local catalytic reaction. The directed motion at long timescales was achieved by placing specially designed magnetic capped colloids in a hydrogen peroxide solution at weak magnetic fields. The control of the motion of the particles was provided by changes of the concentration of the solution and by varying the strength of the applied magnetic field. Such synthetic objects can then be used not only to understand the fundamental driving processes but also be employed as small motors in biological environments, for example, for the transportation of molecules in a controllable way.
82.70.Dd, 82.39.-k

a To whom correspondence should be addressed; e-mail: Larysa.Baraban@uni-konstanz.de

2 Chemical processes, which make life of cells and bacteria possible, rely on the existence of directed transport of certain substances to relevant places. On the other hand, the behaviour of microscopic objects is governed by random Brownian motion. Biological engines combine random and directed contributions of motion. From this interplay of energy scales and the microscopic nature of the building blocks an exceptional complexity of the cellular driving mechanisms arises. The common functional principle of biological engines is the conversion of chemical energy of the environment into mechanical work [1, 5]. Prominent examples are cytoplasmic molecular motors [1 – 3] – the proteins, which use ATP hydrolysis [4] to perform movement. The complexity of all such structures often impedes fundamental understanding of the underlying processes. In contrast to molecular driving processes [1 – 5], artificial motors reveal simple working principles. Recently colloidal particles were suggested as appropriate objects for the development of synthetic nano- and micro-engines [6, 7]. However, studies of colloidal transport in fluids showed the necessity of an external macroscopic influence (e.g. gradient fields [8 – 11]) in order to generate the movement of particles. Consequently, in this approach a macroscopic response of a colloidal ensemble is achieved, without control of the motion of individual particles. In contrast to the previously mentioned macroscopic approach, another concept was applied, where every particle is driven by its own local motive force, excluding the necessity of the external influence [12, 13]. Although the directed motion at short timescales was successfully achieved, the control of motion at long timescales was not realized [13]. Here we demonstrate the concept of a combined local-macroscopic influence on colloidal particles, which explores the transition from the regime of Brownian random walk to fully directed motion. This is achieved by using silica spheres (diameter 4.75 μm)

3 with artificially designed magnetic moments. Magnetic properties were provided by deposition of a multilayer stack Pt(1 nm) / [Co(0.3 nm) / Pt(0.8 nm)]8 / Pt(5 nm) with out- of-plane magnetic anisotropy onto densely packed monolayer of particles, as it is described previously [14 – 16]. For the purpose of the current experiment, the sample after deposition was magnetically saturated in order to induce a non-zero out-of-plane remanent magnetic moment m, directed along the axis of the capped particles (Fig. 1 a). Finally, a top layer of 25 nm Pd was evaporated as a catalyst for the chemical reaction. After fabrication the capped colloids were detached from the surface [16] and suspended in an aqueous solution of H2O2 [17]. The suspension was placed into the measurement cell [14], where particles are settled on the glass substrate due to gravity. In the following we study the motion of the capped particles on the glass surface in the xy-plane (Fig. 1 b). Being mixed with solution of H2O2 these particles experience a driving force due to the chemical decomposition of hydrogen peroxide at the surface of the cap, which is catalyzed by Pd. The balance equation 2 H2O2 → 2 H2O + O2 gives the ratio of reactants to products of 2:3. Consequently a local gradient of the chemical potential around the particle is formed, resulting in the appearance of a difference in osmotic pressure Δp (Fig. 1 b). This imposes propulsion of the sphere from its Pd side (high pressure region) towards the uncoated silica part (low pressure region). A similar method to induce motion of macroscopic objects by chemical reaction was first suggested by Ismagilov et al. [18]. The recent experimental study by Howse et al. [13] revealed the possibility to obtain the directed motion of microscopic beads

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