Nanoparticles shape-specific emergent behaviour on liquid crystal droplets

Z Sumer and A Striolo, MOLECULAR SYSTEMS DESIGN & ENGINEERING, 5, 449-460 (2020).

DOI: 10.1039/c9me00153k

Self-assembly attracts enormous research attention because it is at the core of important applications ranging from medical treatments to renewable energy production. Among several classes of self-assembling materials, liquid crystals (LCs) and nanoparticles yield ordered structures under well-defined thermodynamic conditions and could yield supra-molecular aggregates, respectively. In this work, nanoparticle self-assembly on LC nano-droplets is investigated. The LC nano-droplets act as templating agents on which homogeneous and Janus nanoparticles of various geometrical features are adsorbed. LC mesogens and water have low mutual solubility, and under the conditions chosen the LCs yield bipolar nano-droplets. Particle self-assembly on oil nano-droplets is also considered for comparison. Our results reveal that the mesogens can direct the assembly of the nanoparticles. This effect is mainly governed by the nanoparticle size and shape. In some cases, strong evidence of emergent behaviour is observed depending on entropic forces that arise because of the shape and patchiness of the nanoparticles. For example, while one small spherical homogeneous particle does not show preferential adsorption on specific LC nano-droplet locations, 100 spherical nanoparticles preferentially agglomerate at the nano-droplet boojums, providing evidence of emergent behaviour. On the contrary, Janus spherical nanoparticles do not show such a strong emergent behaviour. Cylindrical NPs manifest the opposite trend: while homogeneous nano-cylinders do not exhibit orientational order on the LC nano-droplet, Janus ones either locate at the LC nano-droplet boojums or orient towards the direction vector of bipolar droplets. Quantification of the orientational order within the LC nano-droplets suggests that the self-assembly of the LC mesogens does not significantly change upon nanoparticle adsorption. These simulations clearly suggest an interplay between nanoparticle size, shape and chemical composition upon their self-assembly on LC nano-droplets. The results could be helpful for the design of new sensors and for the directed self-assembly of advanced materials. Design, System, Application Self-assembly of liquid crystals (LCs) is crucial for a variety of high-end applications ranging from sensors to displays. Towards designing supra-molecular aggregates for such applications, it is crucial to better understand nanoparticle (NP)-LC interactions. Yet, this area is not yet fully explored by either theoretical or experimental studies. To aid future system design and engineering for such systems, we conducted computational studies to quantify the molecular driving forces responsible for the collective behaviour of LC molecules in the presence of NPs. We focused on NPs with different shapes, sizes, and chemical functionalities. LC droplets dispersed in aqueous medium were considered as templates for guiding the self-assembly of the NPs. The LC droplets were prepared in the bipolar nematic phase. Our results yield a comprehensive map of NP-LC interactions at the nanoscale, revealing that the NP shape could be equally important as NP size in driving assembly. In addition, our results show that the chemical functionality, in particular homogeneous vs. Janus NPs, have strong effects on the NP assembly on the LC droplets. The results suggest that a balance among different driving forces is essential to develop advanced supra-molecular devices.

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