arXiv subjects

Stefano Marni

Publications and source records attributed to Stefano Marni.

4 recordsLinked to original sources

arXiv2026

Polarization Plasticity of Ferroelectric Nematics: a case of Electrostatic Frustration in Simple Planar Electrode Cells

Because of their spontaneous bulk polarization, ferroelectric nematic liquid crystals can be easily brought by surface coupling or confinement in a state in which the accumulation of bound charges becomes incompatible with polar order, creating frustration. This condition also occurs in the simplest cells with parallel uncoated metal electrodes where we find that the polarization charge accumulating on the electrodes is proportional to the applied voltage for $\Delta V< V_{sat}$, a threshold value independent of cell thickness. We show that below $V_{sat}$, frustration drives the system into a regime of polarization plasticity, in which nematic ordering is preserved and the nematic director remains perpendicular to the electrodes, while instead the polarization is reduced to cancel the internal electric field. In this regime, the kinetics of bound charge reversal becomes independent from $\Delta V$. The observations are consistent with a model in which the system splits into antipolar tubular domains of mesoscale diameter extending from electrode to electrode, in which the polarization retains its unconstrained equilibrium value, separated by pure polarization-reversal walls.

cond-mat.soft↗

arXiv2023

Optical Control Of Mass Ejection From Ferroelectric Liquid Droplets: A Possible Tool For The Actuation Of Complex Fluids

We report on the optical control of the recently observed electromechanical instability of ferroelectric liquid droplets exposed to the photovoltaic field of a lithium niobate ferroelectric crystal substrate. The ferroelectric liquid is a nematic liquid crystal in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase, droplets irradiated by unfocused beam undergo an electromechanical instability and disintegrate by the explosive emission of fluid jets. We show here that the regions of jets emission can be controlled by focusing the light beam in areas close to the droplet's edge. Once emitted, the fluid jets can be walked by moving the beam up to millimeter distance from the mother droplet. Reverting the lithium niobate substrate, jets become thinner and show the tendency of being repelled by the beam instead of being attracted, thus offering an additional tool for their optical manipulation. These observations may pave the way to intriguing applications of ferroelectric nematic fluids related to manipulation, actuation, and control of soft, flexible materials.

cond-mat.soft↗

arXiv2022

Walk ferroelectric liquid droplets with light

We show that the motion of ferroelectric liquid sessile droplets deposited on a ferroelectric lithium niobate substrate can be controlled by a light beam of moderate intensity irradiating the substrate at a distance of several droplet diameters from the droplet itself. The ferroelectric liquid is a nematic liquid crystal in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase droplets are either attracted toward the center of the beam or repelled, depending on the side of the lithium niobate exposed to light irradiation. Moreover, moving the beam results in walking the ferroelectric droplet over long distances on the substrate. We understand this behavior as due to the coupling between the polarization of the ferroelectric droplet and the polarization photoinduced in the irradiated region of the lithium niobate substrate. Indeed, the effect is not observed in the conventional nematic phase, suggesting the crucial role of the ferroelectric liquid crystal polarization.

cond-mat.soft↗

arXiv2022

Explosive electrostatic instability of ferroelectric liquid droplets on ferroelectric solid surfaces

We investigated the electrostatic behavior of ferroelectric liquid droplets exposed to the pyroelectric field of a lithium niobate ferroelectric crystal substrate. The ferroelectric liquid is a nematic liquid crystal in which almost complete polar ordering of the molecular dipoles generates an internal macroscopic polarization locally collinear to the mean molecular long axis. Upon entering the ferroelectric phase by reducing the temperature from the nematic phase, the liquid crystal droplets become electromechanically unstable and disintegrate by the explosive emission of fluid jets. These jets are mostly interfacial, spreading out on the substrate surface, and exhibit fractal branching out into smaller streams to eventually disrupt, forming secondary droplets. We understand this behavior as a manifestation of the Rayleigh instability of electrically charged fluid droplets, expected when the electrostatic repulsion exceeds the surface tension of the fluid. In this case the charges are due to the bulk polarization of the ferroelectric fluid which couples to the pyroelectric polarization of the underlying lithium niobate substrate through its fringing field and solid-fluid interface coupling. Since the ejection of fluid does not neutralize the droplet surfaces, they can undergo multiple explosive events as the temperature decreases.

cond-mat.soft↗