Hauptseminar Active Matter SS 2017/Microswimmers under Confinement

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Date

2017-06-14

Time

17:30

Topic

Microswimmers under Confinement

Speaker

Christoph Lohrmann

Tutor

Will Uspal

Handout

[1]

Contents

The presence of confining surfaces can significantly affect the dynamics of microswimmers. Individually, microswimmers interact with surfaces through their self-generated fields, including hydrodynamic and chemical fields. Additionally, surfaces can significantly affect the interactions between microswimmers.

This topic mainly focuses on how surfaces affect the motion of individual microswimmers, including the possibility of "self-trapping" of microswimmers near surfaces. Microswimmer/surface hydrodynamic interactions provide a natural starting point, since all microswimmers, regardless of the mechanism of propulsion, create disturbance flows in the suspending fluid. The effects of the leading order singularities in a series expansion of the microswimmer-generated flow field are isolated and characterized [1,2]. Truncation of this expansion provides a "minimalistic" description of hydrodynamic interactions. Predictions of the minimalistic approach, including phase portraits for swimmer dynamics, are compared against more detailed numerical calculations obtained within the "squirmer" model [3,4].

Beyond hydrodynamic interactions, catalytically active particles have additional interactions with surfaces, which can significantly enrich swimmer dynamics. For example, the presence of a hard planar surface distorts the distribution of the product molecules. The effect of this "chemical interaction" is examined in detail [5]. As with hydrodynamic interactions, simple expressions, obtained by considering the leading order singularities in the solute number density field, are compared with detailed numerical calculations.

Finally, the effect of surfaces on interactions between microswimmers are briefly introduced. Surfaces can "screen" particle/particle interactions. In screening, the decay and tensorial form of the interaction is changed by confinement. As one example, the dipolar form of hydrodynamic interactions in a parallel-plate geometry is introduced and justified by simple considerations of mass and momentum conservation [6].

Literature

1. S. E. Spagnolie and E. Lauga, "Hydrodynamics of self-propulsion near a boundary: predictions and accuracy of far-field approximations", J. Fluid Mech. 700, 105 (2012).
2. A. P. Berke, L. Turner, H. C. Berg, E. Lauga, "Hydrodynamic attraction of swimming microorganisms by surfaces", Phys. Rev. Lett. 101, 038102 (2008).
3. K. Ishimoto and E. A. Gaffney, "Squirmer dynamics near a boundary", Phys. Rev. E 88, 062707 (2013).
4. J. S. Lintuvuori, A. T. Brown, K. Stratford, and D. Marenduzzo, "Hydrodynamic oscillations and variable swimming speed in squirmers close to repulsive walls", Soft Matter 12, 7959 (2016).
5. W.E. Uspal, M.N. Popescu, S. Dietrich, and M. Tasinkevych, "Self-propulsion of a catalytically active particle near a planar wall: from reflection to sliding and hovering", Soft Matter 11, 434 (2015).
6. H. Diamant,"Hydrodynamic Interaction in Confined Geometries", J. Phys. Soc. Jpn. 78, 041002 (2009).