Reversible Microwheel Translation Induced by Polymer Depletion

Area of Science:

• Soft Matter Physics
• Micro-robotics
• Colloidal Science

Background:

• Microscopic robots (μbots) require motility for in vivo applications, with designs like helical swimmers translating through bulk fluids.
• Previously, we demonstrated reversible assembly of superparamagnetic colloidal particles into μbots that translate along surfaces via wet friction under rotating magnetic fields.

Purpose of the Study:

• To investigate the impact of high-molecular-weight polymers on the surface transport of microbots (μbots).
• To determine how polymer exclusion and its effect on local viscosity influence μbot locomotion and interactions.

Main Methods:

• Utilized xanthan gum as a model high-molecular-weight polymer in aqueous systems.
• Applied rotating magnetic fields to assemble and actuate superparamagnetic colloidal particles into μbots.
• Analyzed μbot translation along surfaces, focusing on the effects of polymer depletion and local viscosity changes.

Main Results:

• High-molecular-weight polymers, approaching the microbot-surface gap size, are excluded from the interface.
• Polymer depletion showed a weak effect on colloid-surface interactions but significantly influenced local viscosity.
• The alteration in local viscosity was sufficient to induce a reversal in the μbot translation direction.

Conclusions:

• The presence of high-molecular-weight polymers can critically impact microbot (μbot) transport dynamics along surfaces.
• Local viscosity modulation by polymer depletion is a key factor, overriding direct surface interaction effects.
• This finding is crucial for controlling microbot navigation in complex biological environments.