OPTICS OF SOFT MATTER: Recent Selected Publications

Explosive electrostatic instability of ferroelectric liquid droplets on ferroelectric solid surfaces.

In this work, we show that when sessile droplets of the newly discovered ferroelectric nematic fluid phase are deposited on a ferroelectric solid substrate, they can become suddenly unstable and disintegrate though the emission of fluid jets. The instability is due to the coupling between the polarizations of the liquid and solid materials, which induces the accumulation of polarization charges on the droplet–air interface and thus, the buildup of a repulsion pressure that eventually overcomes the surface tension. This kind of polarization-induced Rayleigh instability crucially depends on the unique combination of polarization and fluidity of the ferroelectric nematic and might provide the basis for electrohydromechanical applications.

PNAS, 119 (32) e2207858119 2022 https://doi.org/10.1073/pnas.2207858119

Surface alignment of ferroelectric nematic liquid crystals

The coupling of the newly discovered ferroelectric liquid crystal with solid surface gives rise to a variety of orientation, defects and electric field response that are a consequence of the polar symmetry breaking and depend on the specific surface.

Soft Matter, 2021,17, 8130-8139 https://doi.org/10.1039/D1SM00734C

Pitch tuning induced by optical torque in heliconical cholesteric liquid crystals

Heliconical cholesteric liquid crystals are expected to be more sensitive to torque induced by light field since their structure allows both bend and twist in molecular orientations, differently from the conventional cholesterics in which only twist deformation is involved, requiring much higher fields. We report here a demonstration of tuning the helical pitch in heliconical cholesterics induced by an optical torque. Experimental observations are in agreement with expectations of the classical theory extended to include the effect of the optical field. A dual control of the helical pitch is achieved including both the low-frequency electric field applied along the helix axis and the optical field orthogonal to it.

Phys. Rev. Research 1, 033215 (2019)

Walking Ferroelectric Liquid Droplets with Light            

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. This behavior is understood 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.

Adv.Mater.2023, 35, 2212067

Elasticity and Viscosity of DNA Liquid Crystals

Concentrated solutions of blunt-ended DNA oligomer duplexes self-assemble in living polymers and order into lyotropic nematic liquid crystal phase. Using the optical torque provided by three distinct illumination geometries, we induce independent splay, twist, and bend deformations of the DNA nematic and measure the corresponding elastic coefficients K1, K2, and K3, and viscosities ηsplay, ηtwist, and ηbend. We find the viscoelasticity of the system to be remarkably soft, as the viscoelastic coefficients are smaller than in other lyotropic liquid crystals. We find K1 > K3 > K2, in agreement with the elasticity of the nematic phase of flexible polymers, and ηbend > ηsplay > ηtwist a behavior that is nonconventional in the context of chromonic, polymeric, and thermotropic liquid crystals, indicating a possible role of the weakness and reversibility of the DNA aggregates.

ACS Macro Lett. 2020, 9, 1034−1039

Optofluidic platform using liquid crystals in lithium niobite microchannel
SCIENTIFIC REPORTS vol.9, 1062 (2019)