3.7 Active Suspensions
63
Fig. 3.28 (a) Snapshot of sedimented Janus particles; scale bar 40 μm. (b) Two overlaid snapshots
of the same cluster shifting by a distance Δd and rotating by an angle Δθ. (c) Series of snapshots
of a cluster rolling over another cluster (Ginot et al, 2018)
shows that a gas-like state prevails at high Pe values and not too strong a chemotactic
response.
Chemotactic clustering may also involve passive particles (Stürmer et al, 2019).
With increasing diffusiophoretic intensity, the system evolves from the gas-like state
to intermittent clustering to collapse into a single cluster. Passive particles tend to
gather around active ones, and the fledgeling cluster stabilizes if it reaches some
critical size. A ripening process may follow, as clusters merge, as shown in Fig. 3.26,
which may eventually lead to a total collapse.
Although either kind of interaction may induce clustering, the results are different.
When only chemotactic interactions are present, there is no collective motion within
a collapsed cluster, which performs at most slow diffusive motion (Fig. 3.27a). This
changes dramatically if, in addition to the chemotactic coupling, velocity alignment
is introduced (Romanczuk et al, 2012). Increasing the alignment strengths leads first
to collective rotation within a stationary cluster. A further increase in alignment
strength enables the particles to escape collectively out of the stationary maximum
of the chemical attractant. As the particles keep on producing the attractant, they drag
its cloud of chemoattractant around them, creating the compact chemically-bound
moving cluster shown in Fig. 3.27b.
Fig. 3.29 (a) Rotating cluster of active particles with trajectories shown by green lines. (b)–(d)
Spontaneous reversal of rotation direction (color coded). Scale bar 30 μm (Bäuerle et al, 2020)
63
Fig. 3.28 (a) Snapshot of sedimented Janus particles; scale bar 40 μm. (b) Two overlaid snapshots
of the same cluster shifting by a distance Δd and rotating by an angle Δθ. (c) Series of snapshots
of a cluster rolling over another cluster (Ginot et al, 2018)
shows that a gas-like state prevails at high Pe values and not too strong a chemotactic
response.
Chemotactic clustering may also involve passive particles (Stürmer et al, 2019).
With increasing diffusiophoretic intensity, the system evolves from the gas-like state
to intermittent clustering to collapse into a single cluster. Passive particles tend to
gather around active ones, and the fledgeling cluster stabilizes if it reaches some
critical size. A ripening process may follow, as clusters merge, as shown in Fig. 3.26,
which may eventually lead to a total collapse.
Although either kind of interaction may induce clustering, the results are different.
When only chemotactic interactions are present, there is no collective motion within
a collapsed cluster, which performs at most slow diffusive motion (Fig. 3.27a). This
changes dramatically if, in addition to the chemotactic coupling, velocity alignment
is introduced (Romanczuk et al, 2012). Increasing the alignment strengths leads first
to collective rotation within a stationary cluster. A further increase in alignment
strength enables the particles to escape collectively out of the stationary maximum
of the chemical attractant. As the particles keep on producing the attractant, they drag
its cloud of chemoattractant around them, creating the compact chemically-bound
moving cluster shown in Fig. 3.27b.
Fig. 3.29 (a) Rotating cluster of active particles with trajectories shown by green lines. (b)–(d)
Spontaneous reversal of rotation direction (color coded). Scale bar 30 μm (Bäuerle et al, 2020)