Jan-Timm
Kuhr
*,
Felix
Rühle
and
Holger
Stark
*
Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany. E-mail: jan-timm.kuhr@tu-berlin.de; holger.stark@tu-berlin.de
First published on 21st June 2019
We present a hydrodynamic study of a monolayer of squirmer model microswimmers confined to a boundary by strong gravity using the simulation method of multi-particle collision dynamics. The squirmers interact with each other via their self-generated hydrodynamic flow fields and thereby form a variety of fascinating dynamic states when density and squirmer type are varied. Weak pushers, neutral squirmers, and pullers have an upright orientation. With their flow fields they push neighbors away and thereby form a hydrodynamic Wigner fluid at lower densities. Furthermore, states of fluctuating chains and trimers, of kissing, and at large densities a global cluster exist. Finally, pushers at all densities can tilt against the wall normal and their in-plane velocities align to show swarming. It turns into chaotic swarming for strong pushers at high densities. We characterize all these states quantitatively.
An ubiquitous example of an external field is gravity, which affects every swimmer that is not neutrally buoyant. Already a range of diverse phenomena has been observed such as bound swimmer states,26 polar order in sedimenting swimmers, which in turn can be described by an effective temperature,27–29 gravitaxis of asymmetric swimmers,30 and inverted sedimentation profiles of bottom-heavy swimmers.31 Very appealing patterns occur during bioconvection32 and more recent work addresses the formation of thin phytoplankton layers in the coastal ocean33 or rafts of active emulsion droplets, where the role of phoretic interactions has to be clarified.34,35 In realistic settings non-buoyant swimmers will naturally interact with bounding surfaces. Their impact has already been studied in several works.36–45
In recent articles we focussed on single microswimmers in moderate gravitational fields42 and on their collective sedimentation.46 For our studies we used squirmers as model microswimmers,47–50 the swimmer type of which can be continuously tuned from pushers over neutral swimmers to pullers. To fully integrate hydrodynamic interactions between squirmers and also between squirmers and bounding walls, we employed the method of multi-particle collision dynamics (MPCD), a particle-based solver of the Navier–Stokes equations.51,52
In this article we address a monolayer of squirmers that forms under strong gravity at the bottom surface of the simulation box by performing parallelized simulations with up to 108 fluid particles and up to several thousand squirmers. In contrast to our previous work42,46 where the squirmers can leave the bottom surface to swim upwards, in this article gravity is so strong that they are constrained to the bottom surface. Here, the squirmers either point upwards or tilt against the surface normal so that they move along the surface. They interact with each other via their self-generated flow fields and thereby induce an intriguing variety of dynamic states when density and squirmer type are varied. We categorize them in a state diagram. The most fascinating state is the hydrodynamic Wigner fluid. It is formed by weak pushers, neutral squirmers, and pullers at lower densities due to an effective hydrodynamic repulsion and it shows a glassy relaxation dynamics. We also observe states of fluctuating chains and trimers, of kissing, and at large densities a global cluster. Furthermore, pushers over the whole density range can tilt against the normal and the in-plane velocities align to show swarming, which turns into chaotic swarming for strong pushers at high densities.
The article is organized as follows. In Section 2 we present the squirmer model for microswimmers, introduce the MPCD simulation method, and give the relevant parameters of our simulations. In Section 3 we first present the state diagram for the squirmer monolayer and discuss the different states in subsequent subsections. Finally, in Section 4 we summarize our findings and conclude.
vs(rs) = B1(1 + βê·s)[(ê·s)s − ê], | (1) |
In our MPCD simulations the fluid is composed of up to 108 point particles of mass m0, which perform alternating streaming and collision steps. In the streaming step the position ri of each particle i is updated using its velocity vi and the time step Δt: ri(t + Δt) = ri(t) + viΔt. In the collision step the simulation volume is divided into cubical cells of edge length a0. All fluid particles within one cell exchange momentum according to the MPC-AT+a rule.61 It ensures linear and angular momentum conservation as well as a thermalization of particle velocities to temperature T. On average there are nfl = 10 fluid particles in each collision cell. We choose the duration of the streaming step as , with Boltzmann constant kB, which sets the shear viscosity to .63 Note that in a recent work the authors of ref. 64 used a larger number of fluid particles per unit cell, nfl = 80, where the MPCD fluid is less compressible. For neutral squirmers confined between two plates, they do not observe motility-induced phase separation (MIPS) into a dilute and a cluster phase, which occurs for nfl = 10.37,40 Since in our system MIPS does not occur and clustering at medium densities is only transient as in the simulations of ref. 64, we used the lower value for nfl = 10, which makes our simulations feasible.
In the streaming step momentum is transferred from the fluid particles to the squirmers. Fluid particles that enter a squirmer or a bounding surface are repositioned outside of the obstacle by updating their positions and velocities. We apply the “bounce-back rule”65 to make sure the updated velocity fulfills the surface flow field of eqn (1) and the no-slip boundary condition at bounding walls, respectively. Between squirmers and walls as well as between pairs of squirmers also steric interactions are implemented, which we take into account in a molecular dynamics step. Further details of our implementation are described in ref. 37 and 40. Since we simulate large systems with many squirmers, we employ the parallelized version of ref. 40.
As in earlier works,42,46 we are interested in squirmers moving under gravity but now make it so strong that all squirmers are constrained to the bottom surface with very little variation in z-direction as discussed in the beginning of Section 3.3. Gravity acts on each squirmer with a force F = −mgez. Here m is the squirmer mass and g the acceleration. Through g = g0(1 − ρf/ρs) it depends on the mismatch of fluid and squirmer densities (ρf and ρs) and the gravitational acceleration g0. The buoyant squirmer mass then is m(1 − ρf/ρs). The force F adds a contribution of F/(2m)Δt2 to the update of the squirmer's position during the streaming step. Since the influence of gravity on a fluid on the micron scale is negligible, the update of the fluid particles’ positions does not include F.
The MPCD method is known to reproduce analytic results, e.g., the flow field around passive colloids,60 fluid friction as a particle approaches a wall,66 the active velocity of squirmers,50 or hydrodynamic torques acting on them close to walls.39 Even in systems with many particles, such as a dense colloidal suspension under Poiseuille flow, it correctly predicts velocity oscillations and particle segregation.67,68
Finally, MPCD resolves flow fields on time and length scales large compared to the duration of the streaming step Δt and the mean free path of the fluid particles, respectively. Using a squirmer radius of R = 4a0, we are therefore able to resolve flow fields even if squirmers approach each other closely.
Similar to our earlier works42,46 there are two velocities that characterize squirmer dynamics. These are the bulk sedimentation velocity vg = mg/(6πηR) of a single squirmer, where m is the squirmer mass, and its swimming velocity v0 = 2/3B1. Their ratio α = v0/vg is a dimensionless number, which we fix throughout this work to α = 0.06 by setting and . Without activity these squirmers are equivalent to passive Brownian particles with the same mass m, for which we can compute the passive sedimentation length δ0 = kBT/(mg) = 7.5 · 10−4a0 = 1.9 · 10−5R. Hence squirmers experience only very little variability in their vertical position due to thermal fluctuations. Together with the small α this means that squirmers cannot escape from the bottom wall. The total number of squirmers is sufficiently small so that they form a monolayer with squirmer centers located close to z = R. We quantify their two-dimensional density by the area fraction ϕ = NπR2/L2. Note, with monolayer we do not mean a densely packed layer of squirmers but a collection of squirmers at z ≈ R with tunable density.
In our simulations the Péclet number Pe = v0R/D, where D = kBT/(6πηR) is the translational diffusion coefficient, takes the value Pe = 323. This means that thermal translational motion is negligible. Furthermore, we are within the Stokesian regime of hydrodynamics, where inertia is irrelevant, as evident from the low Reynolds number of Re = v0Rnfl/η = 0.17. Note that real microswimmers move at much smaller Reynolds numbers but the present value is widely deemed acceptable in particle-based hydrodynamic simulations.
Fig. 1 Top: Schematic state diagram illustrating the collective dynamics of a monolayer of squirmers confined to a bounding wall by gravity. The different states in the parameter space squirmer type β versus area fraction ϕ are discussed in the main text. Note, the lines separating the states are schematically drawn. In the shaded gray area local clusters such as pairs, trimers, and chains are observed. Bottom: Snapshots of the observed dynamic states as seen from above. The hydrodynamic Wigner fluid is represented by a Voronoi tessellation, where pentagon and heptagon defects are colored yellow and red, respectively. The kissing state is represented by a single kissing event. The red and yellow color in the dense packing of the cluster state indicate heptagon and pentagon defects. The ring in the fluctuating cluster state shows a trimer cluster. In the (chaotic) swarming state the arrows point along the orientation of the squirmers. The ESI† provides Videos S1–S6 of all the relevant states and Video S7, which shows a collection of the dynamic states in the whole state diagram. |
For all densities pushers (β < 0) exhibit swarming (Section 3.3). Their swimming direction tilts against the vertical so that they move in the horizontal plane. While strong pusher at low densities and weaker pushers at large densities swarm with a common direction, strong pushers at large densities show chaotic swarming. Adjacent in the state diagram is a region at small and medium densities, where neutral squirmers and weak pushers/pullers form pairs, trimers, larger clusters, and chains, which form and break up due to stochastic fluctuations (Section 3.2). In contrast, stronger squirmer pullers at medium densities perform a deterministic maneuver which we term “kissing” (Section 3.2). They approach each other, form pairs or trimers, then orient away from each other and thereby separate again. At large densities fluctuating and kissing clusters enter a state, where one global cluster forms in which squirmers hardly move or behave rather dynamic with increasing β. However, the most intriguing state is the hydrodynamic Wigner fluid, which weak pushers, pullers, and neutral squirmers form at low to medium densities (Section 3.1). Due to hydrodynamic repulsion they form local hexagonal order while long-range translational and orientational order does not occur. We start with describing this state in more detail in Section 3.1.
A first understanding of the observed phenomenology at low densities is provided by the probability distributions for the orientation of single squirmers and their mean orientation plotted in Fig. 2. Strong pushers (β = −4, −5) have a maximum of the orientational distribution at θ ≠ 0, where θ is the angle between the wall normal and the squirmer's orientation. Since they are tilted against the wall normal, they move in the horizontal plane and their collective motion initiates swarming. The tilted orientation is expected for pushers at heights between the validity of lubrication theory and far-field approximation.41,42 As we will discuss in the beginning of Section 3.3, strong pushers are not sitting directly on the surface. For all other β single squirmers are oriented on average along the wall normal in agreement with the fluctuating cluster state and the hydrodynamic Wigner fluid. The orientations of strong pullers (β = 4, 5), however, strongly fluctuate about the wall normal and instead of the Wigner fluid they form a state “tilted squirmers”, which we shortly introduce in the paragraph before Section 3.1.1.
When thermal fluctuations tilt the swimming direction, a squirmer moves towards its neighbors until their flow fields and the hydrodynamic interaction with the wall rotate the orientation back to normal. This causes the clearly visible fluctuations in Video S8 (ESI†). Assuming perfect hexagonal order, the squirmer distance in units of the squirmer radius is approximated as
(2) |
(3) |
To analyze the Voronoi tessellation further, we plot in Fig. 5 the standard deviation ΔNV from the mean number of Voronoi neighbors, which is six in all our simulations as required by the planar surface. The minimum at ϕ = 0.26 confirms the earlier observations that at this density local hexagonal order is most pronounced.
Fig. 5 The standard deviation from the mean number of Voronoi neighbors, ΔNV, plotted versus ϕ for β = 0, 1, and 2. Linear system size is L = 112. |
Ultimately, to probe the squirmer monolayers for long-range positional order, we determine the structure factor
(4) |
In Fig. 6 it is color-coded in the kx, ky plane for β = 0 at ϕ = 0.26. Clearly, the inner ring at |k*| and two weak larger rings indicate that long-range positional order does not exist although weak maxima are visible in the inner ring, which we attribute to the finite size of our simulations. This justifies the name hydrodynamic Wigner fluid. We also checked that a perfectly ordered squirmer monolayer is not stable but develops pentagon/heptagon defects in time and ultimately shows the same structure factor. For β = 1, 2 the weak maxima in S(k) are a bit more pronounced and seem to indicate the different hexagonal domains visible in the Voronoi tessellation of Video S9 (ESI†).
A two-dimensional system allows for a hexatic phase, which is fluid but shows long-range correlations in the 6-fold bond order as demonstrated in the theory of two-dimensional melting.69,72–75 The 6-fold bond-order correlation function,
G6(|r − r′|): = 〈q6(r)q6(r′)〉, | (5) |
(6) |
Interestingly, after some initial decay the self-intermediate scattering function, especially for β = 0, can be fitted by a stretched exponential,
f(t) = e−(t/τ)α. | (7) |
Typically, this indicates a more complex relaxation process. For β = 0 we find the exponent α = 0.66 as indicated in the inset of Fig. 8. Stretched exponentials are observed in the α-relaxation of (colloidal) glasses close to the glass transition.76–80 It is preceded by the β-relaxation‡, which enters a plateau before α-relaxation sets in. We do not observe such a plateau. However, already at β = 0 we can identify a different initial relaxation process in Fs(|k*|,t), which cannot be fit by the stretched exponential as the inset demonstrates. This process is even better visible for β = 1 and 2.
Close to the colloidal glass transition α-relaxation is associated with a colloid leaving the cage formed by surrounding colloids. We anticipate a similar dynamics as illustrated in Fig. 9, where we plot the mean squared displacement versus time. When calculating 〈|Δr|2〉 we first subtract any global drift motion of all squirmers. For β = 0 squirmers move beyond the squirmer-squirmer distance dhex leaving the cage formed by neighboring squirmers. For β = 1 and 2 this process is not completed in the available simulation time in agreement with Fig. 8. Interestingly, the mean-square displacement is (super)diffusive for small times, becomes subdiffusive at intermediate times, when the squirmer starts to feel its neighbors, and for large times for β = 0 approaches diffusive dynamics. Thus, squirmers in a hydrodynamic Wigner fluid perform glassy dynamics reminiscent of what is observed close to the glass transition.
The formation of clusters, their sizes and stability depend on the hydrodynamic interactions between the squirmers. For all β the probability of squirmers to touch each other and thereby form clusters grows with area density ϕ, as the mean cluster size 〈Ncl〉, plotted in Fig. 10, demonstrates. To acquire the statistical data for calculating mean values and also cluster-size distributions, we define squirmers to be in the same cluster if the gap between them is smaller than 0.1R, i.e., the distance of their centers is below 2.1R. For all squirmer types β the mean cluster size ultimately grows faster than exponentially with increasing ϕ. Of course, in the hydrodynamic Wigner fluid (β > −2) the mean cluster size is one and then increases beyond ϕ = 0.26 when the states of kissing or fluctuating clusters are entered. In the swarming state of strong pushers (see β = −5) and in the fluctuating cluster state of weaker pushers (see β = −2) the mean cluster size is larger than 1 even for small ϕ. Here, the squirmers are tilted against the wall normal so that they constantly move along the bottom wall. In this dynamic environment they bump into each other and form transient clusters.
We further analyzed the distribution P(Ncl) of cluster sizes for neutral squirmers and found several characteristic shapes (see Fig. 11 for two of them). Of course, for ϕ ≤ 0.26 all clusters have size one (distribution not shown). Above ϕ = 0.26 in the fluctuating cluster state the cluster-size distribution has exponential form (see Fig. 11, left), where the mean size grows with ϕ as discussed before. This shape persists until the largest cluster of squirmers is close to the percolation transition, where the cluster spans the whole system. Below but close to this transition (see Fig. 11, right) the cluster-size distribution takes the form of a power law with an exponential cutoff. The exponent of the power law P(Ncl) ∼ Ncl−τ has the value τ ≈ 2.0, which nicely agrees with the theoretical value 187/91 ≈ 2.05 of two-dimensional percolation.81,82 Finally, at high ϕ the distribution P(Ncl) is bimodal (not shown). It results from the dominant percolating cluster of fluctuating size, which scales with L2 at constant ϕ, and a distribution of smaller clusters, which break away from and merge with the dominant cluster dynamically.
In the fluctuating cluster state pairs form when the orientations of the squirmers are tilted towards each other so that they swim against each other (see Video S10 in the ESI†). The pairs break up again when nearby squirmers approach or when orientational fluctuations tilt the orientations away. The events of a breakup occur stochastically and seem to be independent from each other. Then, we expect them to follow a Poissonian process like radioactive decay. Indeed, the distribution of pair lifetimes is roughly exponential as the inset of Fig. 12 demonstrates.
In the kissing state the breakup of pairs and trimers follows a different process. This is obvious from the distribution of pair lifetimes plotted in Fig. 12 for puller squirmers (β = 2) at ϕ = 0.33. The distribution has a clear maximum at a non-zero lifetime Tp. Indeed, Video S2 in the ESI† and the graphic representation in Fig. 1, bottom for the formation and breakup of a pair hints to a mostly deterministic process. As squirmers approach, their orientations are tilted towards each other. However, this configuration is unstable. As soon as they touch, they turn to the side, pass each other, and thereby separate to find another nearby puller. Due to this scenario, we called the dynamical state “kissing”. Interestingly, similar behavior occurs for trimers (marked with a red circle in Video S2 in the ESI†).
Finally, we note that for the large density of ϕ = 0.68 and β = −1 we even observe symmetric clusters with seven squirmers as part of larger groupings (see, e.g., bottom left in the beginning of Video S11 in the ESI†). They spontaneously switch between rotating and non-rotating states.
Fig. 14 Mean collective horizontal speed plotted versus density ϕ for squirmer type β = −1, −2, and −5. Error bars indicate the temporal standard deviation of the recorded horizontal velocities. |
Fig. 14 quantifies the swarming by plotting the mean collective horizontal speed , where vhor:= (vx, vy). Here it is important that first the ensemble average of the in-plane velocity vector is taken before the absolute value in order to identify a collective swarming direction. Thus, swarming or collective motion in a preferred direction is measured. We consider the system to display swarming if . The mean collective horizontal speed reflects the different states in the state diagram of Fig. 1. Since the slope of the curves in Fig. 14 are rather steep around , changing the criterion will not strongly affect the schematic boundaries in the state diagram. For β = −1 it is zero, which corresponds to the Wigner fluid and the adjacent fluctuating cluster state. Only at the largest densities a noticeable collective horizontal speed indicates swarming. For β = −2 the in-plane velocities have random orientation at small densities and then for ϕ ≥ 0.3 align with each other in the swarming state. Interestingly, the in-plane velocity of strong pushers decreases with increasing ϕ above ϕ = 0.2. Above ϕ ≈ 0.58 a global direction in the collective horizontal velocity does no longer exist and the squirmers perform what we call chaotic swarming (see Video S6 in the ESI†). The flow field of strong pushers possesses vortices close to the squirmer surface,20 which rotate nearby pushers and thereby more and more destroy the alignment of the in-plane velocity for increasing ϕ. This randomization is also evident from the large error bars in Fig. 14, which indicate the strength of temporal fluctuations of . Finally, we note that in the vicinity of the wall swarming pushers can move faster than pushers in a bulk fluid.
If density is increased starting from the Wigner fluid, squirmers enter the state of fluctuating clusters or, for medium to strong pullers, the kissing state. Fluctuating clusters also exist for medium pushers at small to medium densities. Both states show an exponential cluster size distribution but they differ in the kinetics how they break up. Fluctuating pairs break up by stochastic events due to orientational fluctuations or due to approaching nearby squirmers, which roughly gives an exponential lifetime distribution. In contrast, the distribution in the kissing state is peaked at a finite time, which hints to a more deterministic process. At even larger densities the cluster size distribution becomes algebraic close to the percolation transition and ultimately reaches a bimodal shape in the global cluster state where most squirmers are part of a percolating cluster. Finally, the orientation of strong pushers and also weaker pusher at large densities tilts against the wall normal. The in-plane velocities align and thereby form the swarming state, which for very strong pushers at large densities becomes chaotic.
In a recent work hydrodynamically interacting squirmers experience an aligning torque towards a bounding wall.45 Similar to our results, the authors detect a variety of different states. So, it would be interesting to identify an experimental system to study structure formation close to a bounding wall under defined conditions, which in our case would mean strong gravity. Possible realizations of squirmers in such a system are Volvox algae and active emulsions.58
Footnotes |
† Electronic supplementary information (ESI) available: 11 videos and corresponding information. See DOI: 10.1039/c9sm00889f |
‡ Note that this has nothing to do with the squirmer parameter β. |
This journal is © The Royal Society of Chemistry 2019 |