Soft Matter and Complex Systems Seminar

Complex liquids occur in various problems in soft condensed matter, such as colloidal suspensions, liquid crystals, and the cytoplasm of a living cell. In these systems, ions are often present, and they usually play an important role by screening electrostatic interactions or expressing a biological function. In this talk, I will highlight two recent cases where ions in complex liquids lead to unusual phenomena. First, I will show how electrostatics can lead to unusual density sorting in binary mixtures of charged colloids, where heavier particles float on top of the lighter particles. Unlike the granular or active analogue of this system, this colloidal Brazil-nut effect is purely driven by Brownian motion and electrostatics [1]. In the second part of my talk, I will discuss the role of ions in nematic liquid crystals. In our work, we found that ions in liquid crystals can lead to charge separation in the bulk around topological defects [2], surface charge manipulation [3], and anisotropic screening [4]. I will highlight our first efforts to extend such notions to the dynamical case by deriving the Onsager reciprocal coupling between ionic currents and nematic texture [5]. The dynamics are similar to how an Archimedes’ screw works: winding of nematic texture can generate an ionic current, and conversely, ions can push nematic texture, similar to spin pumping and spin torque in spintronics, respectively. Our results suggest that electric currents can facilitate topological defect transport and that defects can carry an inductance, paving the way for using defects as electric circuitry.

[1] M. N. van der Linden, J. C. Everts, R. van Roij, and A. van Blaaderen, submitted, arXiv:2208.11102 (2022).
[2] J. C. Everts and M. Ravnik, Phys. Rev. X 11, 011054 (2021).
[3] M. Ravnik and J. C. Everts, Phys. Rev. Lett. 125, 037801 (2020).
[4] J. C. Everts, B. Senyuk, H. Mundoor, M. Ravnik and I. I. Smalyukh, Sci. Adv. 7, eabd0662 (2021).
[5] C. Dao, J. C. Everts, M. Ravnik, and Y. Tserkovnyak, submitted, arXiv:2210.17116 (2022).