Arrested coarsening in active colloidal suspensions driven by nonreciprocal electrohydrodynamic interactions

Abstract

Nonreciprocal interactions have recently attracted growing interest in nonequilibrium physics. In particular, breaking action-reaction symmetry has been proposed as a mechanism for collective motion, yet controlled experimental realizations remain scarce. Here we show that bidisperse colloidal suspensions driven by AC electric fields exhibit persistent active clusters sustained by nonreciprocal electrohydrodynamic interactions. Size-asymmetric particle pairs spontaneously self-propel due to imbalanced electrohydrodynamic attraction, producing clusters that continuously fragment and reorganize rather than coarsening into static aggregates as in monodisperse systems. Agent-based simulations reproduce the observed dynamics and identify nonreciprocal pair propulsion as the minimal ingredient for the persistent cluster dynamics. These results demonstrate that action-reaction symmetry breaking in electrohydrodynamic interactions can arrest coarsening and sustain dynamically reconfigurable collective states in dense colloidal suspensions.

Figures (14)

• Figure 1: (a) Experimental setup. An in-plane AC voltage was applied; PS particles sedimented onto ITO-coated glass plates. (b) EHD flow induces size-dependent attraction, leading to nonreciprocal interactions for unequal particles. (c) Self-propelled motion of an S-L pair driven by imbalanced attraction. Scale bar: $15µm$. (d) Velocity distribution $p(v_p)$ of 10 S-L pairs. The velocity $v_p$ was obtained with time interval $\delta t$=0.5 s. (e) Trajectories of the 10 S-L pairs (f) Mean square displacement $\langle \bm{x}^2 \rangle$ showing ballistic short-time behavior. (g,h) Spontaneous accumulation and division of clusters in a bidisperse suspension (equal S and L weight density). Scale bar: $50µm$.
• Figure 2: Dynamics of (a) monodisperse and (b) bidisperse suspensions. Snapshots at $t=300s$ and $600s$ are shown; spatio-temporal plots are taken along the indicated lines. Scale bar: $100µm$. (a) L particles form static aggregates. (b) Bidisperse mixtures exhibit persistent cluster rearrangement. (c) Extended time window showing persistent dynamics up to 3600 s. (d) Temporal evolution of the velocity variance $\sigma_v^2$ for monodisperse(L) and bidisperse(S&L) suspensions. Inset: velocity distributions $p(v_y)$ with Gaussian fits. (e) Speed distributions $p(v)$ with radial Gaussian fits. Fitted widths are $\sigma_v=0.054$ µms (monodisperse) and $\sigma_v=0.15$ µms (bidisperse).
• Figure 3: (a,b) Numerical simulations in a system of size 648µm$\times$360µm: (a) monodisperse ($N_L=10000$) and (b) bidisperse ($N_L=5000$, $N_S=17000$). Particle radii were set to $\ell_L=s_L=1.5µm$ and $\ell_S=s_S=1µm$. Spatio-temporal plots are taken along the indicated line. Scale bar: $100µm$. (c) Temporal evolution of the velocity variance $\sigma_v^2$ for monodisperse(L) and bidisperse(S&L) suspensions. Inset: velocity distributions $p(v_y)$. (d) Speed distributions $p(v)$ with Gaussian fits. Fitted widths are $\sigma_v=0.086$ µms (monodisperse) and $\sigma_v=0.22$ µms (bidisperse).
• Figure 4: (a,b) Numerical simulations with $L_x=L_y=216µm$, $N_\mathrm{I}=1000$, and $N_\mathrm{II}=3000$. (a) Head--large geometry ($\ell_\mathrm{I}=s_\mathrm{I}=1.5µm$, $\ell_\mathrm{II}=s_\mathrm{II}=1µm$). (b) Tail--large geometry (particle sizes exchanged). Scale bar: $100µm$. (c) Schematic polarity field $\bm{p}$ illustrating $\bm{\nabla}\cdot\bm{p}>0$ (head--large) and $\bm{\nabla}\cdot\bm{p}<0$ (tail--large).
• Figure 5: Interactions between particles $\hat{\bm{F}_{ij}}$ with different sizes. Here we set $\hat{s}_{\mathrm L}=\hat{\ell}_{\mathrm L}=1/6, \hat{s}_{\mathrm S}=\hat{\ell}_{\mathrm S}=1/9$.