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SUMMARY:Collective motion and hydrodynamic instabilities in a sheet of mic
 roswimmers - Joakim Stenhammar (Lund University)
DTSTART:20240612T135000Z
DTEND:20240612T142000Z
UID:TALK214705@talks.cam.ac.uk
DESCRIPTION:The collective dynamics of swimming microorganisms is often di
 ctated by long-ranged hydrodynamic interactions (HIs). One example is the 
 collective motion of swimming\, rear-actuated (&ldquo\;pusher&rdquo\;) bac
 teria that interact through their long-ranged dipolar flow fields to creat
 e a state of so-called bacterial turbulence with chaotic\, collective swim
 ming with long-ranged correlations. This phenomenology contrasts with the 
 behaviour of front-actuated (&ldquo\;puller&rdquo\;) organisms such as cer
 tain algae\, that do not exhibit any collective motion in unbounded bulk s
 ystems. For pushers\, this state of collective motion is widely understood
  as a manifestation of a hydrodynamic instability due to the mutual alignm
 ent of pusher microswimmers due to HIs. Unlike this idealised bulk setting
 \, many experimental realisations of microswimmers instead involves swimmi
 ng close to a solid surface or air-liquid interface\, which effectively co
 nfines the swimmers&rsquo\; motion to a 2D plane. As I will show in this t
 alk\, this restriction of the dynamics qualitatively changes the collectiv
 e motion compared to bulk systems. For pushers\, the long-ranged hydrodyna
 mic instability leading to bacterial turbulence in 3D is instead rendered 
 short-ranged\, with a collective state characterised by vortices of the or
 der of the swimmer size. Additionally\, we demonstrate a previously unknow
 n density instability of confined puller microswimmers\, which has no coun
 terpart in unbounded systems. This instability is driven by swimmer advect
 ion and leads to phase separation into dense puller clusters for high enou
 gh densities. Our results thus highlight that accounting the experimental 
 geometry can have a crucial impact on collective phenomena in active matte
 r dominated by hydrodynamic interactions.
LOCATION:Seminar Room 1\, Newton Institute
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