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SUMMARY:DFD Practice Talks - Maria Tatulea-Codrean\, Steven Htet\, Weida L
 iao
DTSTART:20221110T123000Z
DTEND:20221110T133000Z
UID:TALK184985@talks.cam.ac.uk
CONTACT:Raymond E. Goldstein
DESCRIPTION:a) Maria Tatulea-Codrean\nOptimal swimming of multi-flagellate
 d bacteria\n\nAn important characteristic of motile multi-flagellated bact
 eria is their variable number of flagella\, with some bacteria having only
  one\, while others have a few dozen. The number of flagella in a cell is 
 difficult to control in experiments\, but it can be changed easily in simu
 lations. This has motivated several theoretical investigations into the li
 nk between the swimming of bacteria and their number of flagella [1\,2]. H
 ow does the number of flagella affect the swimming speed and efficiency of
  a bacterium? We revisit this open question using slender-body theory simu
 lations\, where we include the full hydrodynamic interactions inside a bun
 dle of parallel helical filaments that rotate and translate in synchrony. 
 In contrast to previous studies\, we incorporate the full torque-speed rel
 ationship of the bacterial flagellar motor [3]. This enables us to obtain 
 novel and surprising predictions on the swimming speed of multi-flagellate
 d bacteria. Our observations are relevant to bacteria with a small number 
 of flagella\, such as the model organism Escherichia coli\, and we hope wi
 ll inspire new experiments to address this question.\n\nb) Steven (Pyae He
 in Htet)\nLoad-dependent resistive-force theory\n\nThe passive rotation of
  rigid helical filaments is the strategy employed by flagellated bacteria 
 (and some artificial microswimmers) to swim at low Reynolds numbers. In hi
 s classical 1976 paper\, Lighthill calculated\, for the force-free swimmin
 g of a rotating helix with no load attached (e.g. with no cell body)\, the
  'optimal' resistance coefficients that\, in a local resistive-force theor
 y\, most closely reproduce predictions from the nonlocal slender-body theo
 ry. These resistance coefficients have since been used ubiquitously in the
  literature\, regardless of whether the conditions under which they were o
 riginally derived hold. Here we revisit the problem in the case where a lo
 ad is attached to the rotating helical filament. We show that the optimal 
 resistance coefficients depend in fact on the size of the load\, and highl
 ight and improve upon the growing inaccuracy of Lighthill's coefficients a
 s the load increases. We also provide a physical explanation for the origi
 n of this surprising load-dependence.\n\nc) Weida Liao\nArtificial cytopla
 smic streaming\n\nRecent experiments in cell biology have probed the impac
 t of artificially-induced intracellular flows and transport in cell divisi
 on. Using focused light localised in a small region of the cell\, a global
  thermo-viscous flow was induced inside the cell in these studies\; this i
 s known as focused-light-induced cytoplasmic streaming (FLUCS). Here we pr
 esent an analytical\, theoretical model of FLUCS. The focused light induce
 s a small\, local temperature change\, causing a small change in the densi
 ty and viscosity of the fluid locally. This heat spot translates along a f
 inite scan path. We show that the leading-order instantaneous flow results
  from thermal expansion and depends linearly on the heat-spot amplitude. T
 he net displacement of a passive tracer after a full scan period is quadra
 tic in the heat-spot amplitude and is due to both thermal expansion and th
 ermal viscosity changes. The far-field average velocity of tracers is a so
 urce dipole\, showing excellent agreement with recent experimental data. O
 ur quantitative model will enable future work on artificial cytoplasmic st
 reaming.\n
LOCATION:MR15\,  Centre for Mathematical Sciences\, Wilberforce Road\, Cam
 bridge
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