Abstract
The quest for designing new self-propelled colloids is fuelled by the demand for simple experimental models to study the collective behaviour of their more complex natural counterparts. Most synthetic self-propelled particles move by converting the input energy into translational motion. In this work we address the question if simple self-propelled spheres can assemble into more complex structures that exhibit rotational motion, possibly coupled with translational motion as in flagella. We exploit a combination of induced dipolar interactions and a bonding step to create permanent linear bead chains, composed of self-propelled Janus spheres, with a well-controlled internal structure. Next, we study how flexibility between individual swimmers in a chain can affect its swimming behaviour. Permanent rigid chains showed only active rotational or spinning motion, whereas longer semi-flexible chains showed both translational and rotational motion resembling flagella like-motion, in the presence of the fuel. Moreover, we are able to reproduce our experimental results using numerical calculations with a minimal model, which includes full hydrodynamic interactions with the fluid. Our method is general and opens a new way to design novel self-propelled colloids with complex swimming behaviours, using different complex starting building blocks in combination with the flexibility between them.
Highlights
The quest for designing new self-propelled colloids is fuelled by the demand for simple experimental models to study the collective behaviour of their more complex natural counterparts
The rotating movement can be attributed to the fact that the propulsion force on each Janus particle is acting in opposite directions in alternative fashion along the length of the chain, resulting in a constant torque acting on the chain, which induces a rotational motion of the chain
We note that the angular velocity obtained from the mean-squared angular displacement (MSAD) fitting (ω = 0.48 ± 0.03 rad/s) is in quantitative agreement with the value obtained from the mean-squared displacement (MSD) fitting (ω = 0.42 ± 0.09 rad/s)
Summary
The quest for designing new self-propelled colloids is fuelled by the demand for simple experimental models to study the collective behaviour of their more complex natural counterparts. Externally powered propellers such as helix-shaped[27,28,29,30] particles using rotating magnetic fields, and DNA-linked assemblies of magnetic particles tethered to a red blood cell[31] in biaxial magnetic fields have shown this motion These systems are not suitable for collective behaviour studies because the long-ranged magnetic interactions between individual units are difficult to minimize. Several synthesis routes have been reported for synthesizing ‘passive’ colloidal molecules, complex-shaped particles[33,34,35,36,37,38], chains of particles using different starting building blocks, e.g., isotropic spherical particles using various linking mechanisms[34,39,40,41,42], Janus particles[43], and flattened particles[44] Making these model systems with self-propelling capabilities remains a challenge.
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