Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userElectric field driven controllable motility of metal-dielectric Janus particles with boojum defects in a nematic liquid crystal
109
0
0.0
(
0
)
Added by
Prof. Surajit Dhara
Publication date
2021
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
In a sharp contrast to the response of silica particles we show that the metal-dielectric Janus particles with boojum defects in a nematic liquid crystal are self-propelled under the action of an electric field applied perpendicular to the director. The particles can be transported along any direction in the plane of the sample by selecting the appropriate orientation of the Janus vector with respect to the director. The direction of motion of the particles is controllable by varying the field amplitude and frequency. The command demonstrated on the motility of the particles is promising for tunable transport and microrobotic applications.
rate research
Read More
We create controllable active particles in the form of metal-dielectric Janus colloids which acquire motility through a nematic liquid crystal film by transducing the energy of an imposed perpendicular AC electric field. We achieve complete command over trajectories by varying field amplitude and frequency, piloting the colloids at will in the plane spanned by the axes of the particle and the nematic. The underlying mechanism exploits the sensitivity of electro-osmotic flow to the asymmetries of the particle surface and the liquid-crystal defect structure. We present a calculation of the dipolar force density produced by the interplay of the electric field with director anchoring and the contrasting electrostatic boundary conditions on the two hemispheres, that accounts for the dielectric-forward (metal-forward) motion of the colloids due to induced puller (pusher) force dipoles. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly and collective dynamics.
A gold-capped Janus particle suspended in a near-critical binary liquid mixture can self-propel under illumination. We have immobilized such a particle in a narrow channel and studied the nonequilibrium dynamics of a binary solvent around it, using experiment and numerical simulations. For the latter we consider both a purely diffusive and a hydrodynamic model. All approaches indicate that the early time dynamics is purely diffusive and characterized by composition layers traveling with a constant speed from the surface of the colloid into the bulk. Subsequently, hydrodynamic effects set in and the transient state is destroyed by strong nonequilibrium concentration fluctuations, which arise as a result of the temperature gradient and the vicinity of the critical point of the binary liquid mixture. They give rise to a complex, permanently changing coarsening patterns. For a mobile particle, the transient dynamics results in propulsion in the direction opposite to that observed after the steady state is attained.
We consider a mathematical model that describes the flow of a Nematic Liquid Crystal (NLC) film placed on a flat substrate, across which a spatially-varying electric potential is applied. Due to their polar nature, NLC molecules interact with the (nonuniform) electric field generated, leading to instability of a flat film. Implementation of the long wave scaling leads to a partial differential equation that predicts the subsequent time evolution of the thin film. This equation is coupled to a boundary value problem that describes the interaction between the local molecular orientation of the NLC (the director field) and the electric potential. We investigate numerically the behavior of an initially flat film for a range of film heights and surface anchoring conditions.
The effect of superimposed ac and dc electric fields on the formation of electroconvection and flexoelectric patterns in nematic liquid crystals was studied. For selected ac frequencies an extended standard model of the electro-hydrodynamic instabilities was used to characterize the onset of pattern formation in the two-dimensional parameter space of the magnitudes of the ac and dc electric field components. Numerical as well as approximate analytical calculations demonstrate that depending on the type of patterns and on the ac frequency, the combined action of ac and dc fields may either enhance or suppress the formation of patterns. The theoretical predictions are qualitatively confirmed by experiments in most cases. Some discrepancies, however, seem to indicate the need to extend the theoretical description.
We report a dynamic light scattering study of the fluctuation modes in a thermotropic liquid crystalline mixture of monomer and dimer compounds that exhibits the twist-bend nematic ($mathrm{N_{TB}}$) phase. The results reveal a spectrum of overdamped fluctuations that includes two nonhydrodynamic and one hydrodynamic mode in the $mathrm{N_{TB}}$ phase, and a single nonhydrodynamic plus two hydrodynamic modes (the usual nematic optic axis or director fluctuations) in the higher temperature, uniaxial nematic phase. The properties of these fluctuations and the conditions for their observation are comprehensively explained by a Landau-deGennes expansion of the free energy density in terms of heliconical director and helical polarization fields that characterize the $mathrm{N_{TB}}$ structure, with the latter serving as the primary order parameter. A coarse-graining approximation simplifies the theoretical analysis, and enables us to demonstrate quantitative agreement between the calculated and experimentally determined temperature dependence of the mode relaxation rates.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
