Amoeboid propulsion of active solid bodies, vesicles and droplets: a comparison

Area of Science:

• Physics
• Fluid Dynamics
• Biophysics

Background:

• Microscopic swimmers utilize various mechanisms for propulsion.
• Understanding the hydrodynamics of microswimmers is crucial for applications in targeted drug delivery and diagnostics.
• Previous studies have analyzed individual microswimmer types, but a unified comparison is lacking.

Purpose of the Study:

• To present a unified discussion and comparison of three near-spherical amoeboid microswimmer types: solid deformable bodies, vesicles, and droplets.
• To analyze their swimming velocities, dissipated power, and Lighthill efficiencies based on boundary conditions and deformation amplitudes.
• To compare their performance under different swim stroke conditions and identify optimal strategies.

Main Methods:

• Utilized minimal models characterized by boundary conditions for each swimmer type.
• Employed a second-order perturbation expansion in small deformation amplitudes.
• Calculated swimming velocities, dissipated power, and Lighthill efficiencies.
• Analyzed swim strokes composed of spherical harmonics up to order 4, respecting volume- and surface-incompressibility.

Main Results:

• In a speed-optimization race, droplets consistently outperformed vesicles and solid bodies.
• Solid body microswimmers exhibited efficiencies two orders of magnitude lower than vesicles and droplets.
• Optimizing for Lighthill efficiency and swimming velocity yielded different optimal swim strokes for each type.

Conclusions:

• The droplet microswimmer demonstrates superior speed and efficiency compared to vesicles and solid bodies when strokes are optimized individually.
• The choice of swim stroke significantly impacts the relative performance and efficiency of different microswimmer types.
• A unified theoretical framework facilitates direct comparison and understanding of microswimmer hydrodynamics.