Navas Salman Fariz, Klapp Sabine H L
Institute for Theoretical Physics, Technical University of Berlin, Hardenbergstr. 36, 10623 Berlin, Germany.
J Chem Phys. 2024 Aug 7;161(5). doi: 10.1063/5.0214730.
Non-reciprocal (NR) effective interactions violating Newton's third law occur in many biological systems, but can also be engineered in synthetic, colloidal systems. Recent research has shown that such NR interactions can have tremendous effects on the overall collective behavior and pattern formation, but can also influence aggregation processes on the particle scale. Here, we focus on the impact of non-reciprocity on the self-assembly of a colloidal system (originally passive) with anisotropic interactions whose character is tunable by external fields. In the absence of non-reciprocity, that is, under equilibrium conditions, the colloids form square-like and hexagonal aggregates with extremely long lifetimes yet no large-scale phase separation [Kogler et al., Soft Matter 11, 7356 (2015)], indicating kinetic trapping. Here, we study, based on Brownian dynamics simulations in 2D, an NR version of this model consisting of two species with reciprocal isotropic, but NR anisotropic interactions. We find that NR induces an effective propulsion of particle pairs and small aggregates ("active colloidal molecules") forming at the initial stages of self-assembly, an indication of the NR-induced non-equilibrium. The shape and stability of these initial clusters strongly depend on the degree of anisotropy. At longer times, we find, for weak NR interactions, large (even system-spanning) clusters where single particles can escape and enter at the boundaries, in stark contrast to the small rigid aggregates appearing at the same time in the passive case. In this sense, weak NR shortcuts the aggregation. Increasing the degree of NR (and thus, propulsion), we even observe large-scale phase separation if the interactions are weakly anisotropic. In contrast, systems with strong NR and anisotropy remain essentially disordered. Overall, the NR interactions are shown to destabilize the rigid aggregates interrupting self-assembly and phase separation in the passive case, thereby helping the system to overcome kinetic barriers.
违反牛顿第三定律的非互易（NR）有效相互作用存在于许多生物系统中，但也可以在合成胶体系统中构建。最近的研究表明，这种NR相互作用不仅会对整体集体行为和图案形成产生巨大影响，还会在粒子尺度上影响聚集过程。在这里，我们关注非互易性对具有各向异性相互作用的胶体系统（原本是被动的）自组装的影响，这种相互作用的性质可由外部场调节。在不存在非互易性的情况下，即在平衡条件下，胶体形成具有极长寿命但没有大规模相分离的方形和六边形聚集体[Kogler等人，《软物质》11, 7356 (2015)]，这表明存在动力学捕获。在这里，我们基于二维布朗动力学模拟，研究了该模型的NR版本，该版本由两种具有互易各向同性但NR各向异性相互作用的物种组成。我们发现，NR会诱导在自组装初始阶段形成的粒子对和小聚集体（“活性胶体分子”）产生有效推进，这表明NR诱导了非平衡态。这些初始团簇的形状和稳定性强烈依赖于各向异性程度。在更长的时间尺度上，我们发现，对于弱NR相互作用，会形成大的（甚至跨越整个系统的）团簇，单个粒子可以在边界处逃逸和进入，这与被动情况下同时出现的小的刚性聚集体形成鲜明对比。从这个意义上说，弱NR缩短了聚集过程。如果相互作用是弱各向异性的，增加NR程度（从而推进力），我们甚至会观察到大规模相分离。相比之下，具有强NR和各向异性的系统基本上保持无序状态。总体而言，NR相互作用被证明会破坏刚性聚集体的稳定性，中断被动情况下的自组装和相分离，从而帮助系统克服动力学障碍。
