Glassy Dynamics in Chiral Fluids Vincent E. Debets1Hartmut L owen2and Liesbeth M.C. Janssen1 1Department of Applied Physics Eindhoven University of Technology

2025-04-29 0 0 516.4KB 6 页 10玖币

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Glassy Dynamics in Chiral Fluids

Vincent E. Debets,1Hartmut L¨owen,2and Liesbeth M.C. Janssen1

1Department of Applied Physics, Eindhoven University of Technology,

P.O. Box 513, 5600 MB Eindhoven, The Netherlands

2Institut f¨ur Theoretische Physik II: Weiche Materie,

Heinrich-Heine-Universit¨at D¨usseldorf, D-40225 D¨usseldorf, Germany

Chiral active matter is enjoying a rapid increase of interest, spurred by the rich variety of asymme-

tries that can be attained in e.g. the shape or self-propulsion mechanism of active particles. Though

this has already led to the observance of so-called chiral crystals, active chiral glasses remain largely

unexplored. A possible reason for this could be the naive expectation that interactions dominate the

glassy dynamics and the details of the active motion become increasingly less relevant. Here we show

that quite the opposite is true by studying the glassy dynamics of interacting chiral active Brownian

particles (cABPs). We demonstrate that when our chiral ﬂuid is pushed to glassy conditions, it

exhibits highly nontrivial dynamics, especially compared to a standard linear active ﬂuid such as

common ABPs. Despite the added complexity, we are still able to present a full rationalization for

all identiﬁed dynamical regimes. Most notably, we introduce a new ’hammering’ mechanism, unique

to rapidly spinning particles in high-density conditions, that can ﬂuidize a chiral active solid.

Introduction.— Inspired by its omnipresence in biol-

ogy, as well as its growing relevance in condensed mat-

ter and materials science, active matter has proven to

be one of the prevailing subjects in biological and soft

matter physics [1–3]. Active or self-propelled particle

systems are intrinsically far from equilibrium, giving rise

to a myriad of surprising features that are inaccessible

to conventional passive matter. Well-known examples

include motility induced phase separation (MIPS) [4–7],

accumulation around repulsive obstacles [8], spontaneous

velocity alignment [9], and active turbulence [10,11]. In-

terestingly, so-called linear swimmer models such as ac-

tive Brownian particles (ABPs) [12–17], active Ornstein

Uhlenbeck particles (AOUPs) [18], and run-and-tumble

particles (RTPs) [19,20] have already been remarkably

successful in theoretically describing a signiﬁcant number

of these non-equilibrium features. Members of this class

of particles are typically endowed with a constant (av-

erage) self-propulsion whose direction changes randomly

via some form of rotational diﬀusion (often thermal ﬂuc-

tuations). However, due to for instance an asymmetric

shape [21–23], mass distribution [24], or self-propulsion

mechanism [25,26], active particles also frequently self-

rotate which is not included in the aformentioned mod-

els. This leads to chiral-symmetry breaking of the corre-

sponding active motion and, at small enough densities,

circular (2D) or helical trajectories (3D). A collection of

these spinning particles is usually referred to as an ac-

tive chiral ﬂuid and has been shown to exhibit many in-

teresting collective phenomena in both simulations and

experiments [23,27–38]. Understanding the inﬂuence of

chirality on active matter is therefore enjoying growing

attention [39,40], but at the same time requires more

involved modelling eﬀorts to fully comprehend.

Initial chiral active matter studies have focused pri-

marily on the low to moderate density regime [21,26,41–

43], but interest is now increasingly shifting towards high

densities. This has already yielded several seminal works

in the context of so-called chiral crystals [27,32,44,45].

At the same time, their disordered counterpart, i.e., an

active chiral glass, still remains largely unexplored. A

possible reason for this could be that one naively ex-

pects active motion, at least to a large degree, to be

impeded by interactions. As a result, the speciﬁc details

of the active motion, whether chiral or nonchiral, should

become increasingly less relevant upon approaching dy-

namical arrest. Here we demonstrate that in fact quite

the opposite is true and that chiral active motion can

certainly inﬂuence glassy dynamics in highly surprising

ways. We, for the ﬁrst time, delve into the unique physics

that emerges when a chiral ﬂuid ventures into the glassy

regime. Most notably, we introduce a new ’hammering’

mechanism (see Fig. 1), unique to rapidly spinning parti-

cles in high-density conditions, that can ﬂuidize a chiral

active solid.

In short, we explore the dynamics of interacting chi-

ral active Brownian particles (cABPs) [28,41] and show

that when pushed to glassy conditions our chiral ﬂuid ex-

hibits highly nontrivial dynamics, particularly compared

to standard linear active glassy matter (that is, conven-

tional ABPs), which has already been extensively stud-

ied in theory [46–55] and simulation [56–68]. Despite the

added complexity, we are still able to present a full ra-

tionalization for all identiﬁed dynamical regimes, includ-

ing the emergence of a complex reentrant behavior which

we explain by invoking the aforementioned ’hammering’

mechanism.

Simulation Details.— As our model chiral ﬂuid we

consider a two-dimensional (2D) Kob-Andersen mixture

which consists of NA= 650 and NB= 350 self-propelling

quasihard disks of type A and B, respectively. We assume

that the self-propulsion dominates over thermal ﬂuctua-

tions so that we can neglect passive diﬀusion and the

equation of motion for the position riof each particle i

arXiv:2210.03196v1 [cond-mat.soft] 6 Oct 2022

is given by [49]

ri=ζ−1Fi+vi.(1)

Here, ζrepresents the friction constant and vithe self-

propulsion velocity acting on particle i. The interaction

force Fi=−Pj6=i∇iVαβ (rij ) is obtained from a quasi-

hard sphere power law potential Vαβ (r) = 4αβ σαβ

r36

[69,70] and the interaction parameters, i.e., AA =

1, AB = 1.5, BB = 0.5, σAA = 1, σAB = 0.8, σBB =

0.88, are, in combination with setting ζ= 1, chosen

to frustrate crystallization and allow for glassy behav-

ior [71,72]. The choice of parameters also implies that

we use reduced units where σAA,AA,AA/kB, and

ζσ2

AA/AA represent the units of length, energy, temper-

ature, and time respectively [73]. For the self-propulsion

of each particle we employ the cABP scheme [28,41].

That is, the magnitude of the self-propulsion or active

speed v0is assumed to remain constant in time tso that

vi=v0ei=v0[cos(θi),sin(θi)], while the orientation an-

gle of the active velocity θievolves in time according to

θi=χi+ωs,(2)

with ωsa constant spinning frequency, χia Gaus-

sian noise process with zero mean and variance

hχi(t)χj(t0)inoise = 2Drδij δ(t−t0), and Drthe rotational

diﬀusion coeﬃcient. As our control parameters we take

ωs, the persistence time τp=D−1

r, and a so-called spin-

ning temperature Tωs=v2

0/2ωswhich represents (up to

a prefactor 4πζ) a measure for the amount of energy that

is dissipated by a single cABP during one circle motion.

Simulations are performed by solving the overdamped

equation of motion [Eq. (1)] via a forward Euler scheme

using LAMMPS [74]. We set the cutoﬀ radius at rc=

2.5σαβ , ﬁx the size of the periodic square simulation box

to ensure that the number density equals ρ= 1.2, run

the system suﬃciently long (typically between 500 and

10000 time units) to prevent aging, and afterwards track

the particles over time for at least twice the initialization

time. To correct for diﬀusive center-of-mass motion all

particle positions are retrieved relative to the momentary

center of mass [73].

Nonmonotonic Dynamics.— We are primarily inter-

ested in characterizing how the interplay between rota-

tional diﬀusion and spinning motion inﬂuences the ac-

tive glassy dynamics. Therefore, we have calculated the

long-time diﬀusion coeﬃcient D= limt→∞ ∆r2

i(t)/4t

of our chiral ﬂuid for several set spinning frequencies

ωs= 10,100,200 (keeping a ﬁxed value Tωs= 4 to en-

sure moderately supercooled behavior), while varying the

persistence time. The results are plotted as a function of

ωsτpin Fig. 2and show remarkably rich dynamics. In

particular, we ﬁnd initial nonmonotonic behavior with a

maximum at ωsτp∼1. This is followed by a form of reen-

trant behavior which becomes much more pronounced for

higher spinning frequencies. For example, at ωs= 200

𝜔!

𝐷"= 𝜏#

𝐯&

(a)

(d)

(b)

FIG. 1. (a) Visualization of a chiral active Brownian particle

(cABP). (b) Example short-time trajectories (total time is

equal to three spinning periods) of cABPs at large spinning

frequency and persistence exhibiting the ’hammering’ eﬀect

by undergoing circular motion inside their cage of surrounding

particles. (c-e) Schematic depiction of the ’hammering’ eﬀect.

(c-d) For large enough persistence and spinning frequency,

particles undergo back-and-forth motion inside their cage and

systematically collide with the same particle whose motion is

slightly altered by the collision. (e) After repeated collisions

the cage of a particle is suﬃciently remodelled such that the

particle can break out and migrate through the material.

the dynamics reaches a minimum with D∼10−4, which

is practically a frozen system like a glass, that is seen to

increase with orders of magnitude. Finally, in the limit of

large persistence diﬀerent asymptotic values ranging from

signiﬁcantly enhanced to zero dynamics are reached. To

contrast these complex dynamics, we emphasize that a

glassy liquid of standard ABPs at constant active speed

v0would only show a monotonic enhancement of the dy-

namics for increasing persistence time [46,57]. Thus, at

large densities chirality has a highly nontrivial impact on

active particle motion.

Moreover, we have veriﬁed that the same qualitative

behavior is observed for both a diﬀerent model glass-

former and a diﬀerent set of parameters where we have

ﬁxed the active speed v0instead of the spinning temper-

ature Tωs(see Figs. S1 and S2). We also mention that

the nontrivial change of the dynamics and in particular

the reentrant behavior are equally visible in the struc-

ture factor, self-intermediate scattering function, and the

non-Gaussian parameter (see Figs. S3, S4, and S5). The

latter being a measure for dynamical heterogeneity.

cABP in a Harmonic Trap.— Our aim now is to bet-

ter understand the complex dynamics, which, for con-

venience, we will separate in three distinct regimes (see

roman numerals in Fig. 2). We ﬁrst turn our attention to-

wards regime I. Here, the persistence of particles is still

relatively weak and we therefore expect that especially

in this regime the local environment of particles (or their

cage) acts primarily as an eﬀective conﬁning potential.

This then motivates a comparison of our simulation re-

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GlassyDynamicsinChiralFluidsVincentE.Debets,1HartmutLowen,2andLiesbethM.C.Janssen11DepartmentofAppliedPhysics,EindhovenUniversityofTechnology,P.O.Box513,5600MBEindhoven,TheNetherlands2InstitutfurTheoretischePhysikII:WeicheMaterie,Heinrich-Heine-UniversitatDusseldorf,D-40225Dusseldorf,GermanyChi...

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Glassy Dynamics in Chiral Fluids Vincent E. Debets1Hartmut L owen2and Liesbeth M.C. Janssen1 1Department of Applied Physics Eindhoven University of Technology

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