Electrokinetic effects in nematic suspensions: Single-particle electro-osmosis and interparticle interactions

Phenomena of electrically driven fluid flows, known as electro-osmosis, and particle transport in a liquid electrolyte, known as electrophoresis, collectively form a subject of electrokinetics. Electrokinetics shows a great potential in microscopic manipulation of matter for various scientific and technological applications. Electrokinetics is usually studied for isotropic electrolytes. Recently it has been demonstrated that replacement of an isotropic electrolyte with an anisotropic, or liquid crystal (LC), electrolyte, brings about entirely new mechanisms of spatial charge formation and electrokinetic effects. This review presents the main features of liquid crystal-enabled electrokinetics (LCEK) rooted in the field-assisted separation of electric charges at deformations of the director that describes local molecular orientation of the LC. Since the electric field separates the charges and then drives the charges, the resulting electro-osmotic and electrophoretic velocities grow as the square of the applied electric field. We describe a number of related phenomena, such as alternating current (AC) LC-enabled electrophoresis of colloidal solid particles and fluid droplets in uniform and spatially-patterned LCs, swarming of colloids guided by photoactivated surface patterns, control of LCEK polarity through the material properties of the LC electrolyte, LCEK-assisted mixing at microscale, separation and sorting of small particles. LC-enabled electrokinetics brings a new dimension to our ability to manipulate dynamics of matter at small scales and holds a major promise for future technologies of microfluidics, pumping, mixing, sensing, and diagnostics.

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“…For an extended theoretical treatment of LCEK involving colloidal particles, see Refs. [36,44,45,46,47].…”

Section: Lc-enabled Electrophoresismentioning

confidence: 99%

“…As a result, the LCEO flows in a patterned director field can be driven by an alternating current (AC) field (Figure 9e). Dielectric anisotropy supplements the mechanism of LCEO with an additional source of space charge [26,46,47].…”

Section: Lc-enabled Electrokinetics In Patterned Cellsmentioning

confidence: 99%

Liquid Crystals-Enabled AC Electrokinetics

Peng

Lavrentovich

2019

Micromachines

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“…Over the past two decades, the properties and behaviors of colloids-suspended NLC have attracted considerable interest and triggered a wide range of promising practical applications, such as in new display and topological memory devices [1][2][3], new materials [4], report external triggers and release microcargo [5], and biological detectors [6,7]. To better understand the physics of colloidal particles embedded within an NLC cell, much effort has been made by means of experiment, theoretical modeling, and computer simulation so far [8][9][10][11][12][13][14][15][16][17][18][19][20].…”

Section: Introductionmentioning

confidence: 99%

“…In the presence of an external electric field, additionally, fascinating physical phenomena such as levitation, lift, bidirectional motion, aggregation, electrokinetic and superdiffusion have been found for colloids dispersed in NLCs [20,[25][26][27]. In most cases the inclusion of particles into an NLC cell tends to create LC alignment singularities around the suspended substances, which in general are determined by boundary conditions such as surface anchoring features, particle size and shape, LC elasticity, and external fields etc [13,[28][29][30][31].…”

Section: Introductionmentioning

confidence: 99%

Formation of a fast "lane" for positional transition in a microparticle-suspended nematic liquid crystal cell

Xiao,

Chen,

Cao

et al. 2020

Preprint

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In this paper, based on the numerical calculation of total energy utilizing the Green's function method, we found that the external electric field applied to a microparticle-suspended nematic liquid crystal cell, if reaching a critical value, combined with its direction, surface anchoring feature and molecular dielectric anisotropy, is possible to create an anisotropic "bubble" around the microparticle with a vertical fast "lane", in which the microparticle can, driven by the asymmetric buoyant force, vertically move swiftly from the cell's midplane to a new equilibrium position, triggering a positional transition discovered by the author previously. Such a new equilibrium position is decided via a competition between the buoyant force and the effective force built upon the microparticle by the elastic energy gradient along the "lane". The threshold value of external field, depends on thickness L and Frank elastic constant K and slightly on the microparticle size and density, in a Fréedericksz-like manner, but by a factor. For a nematic liquid crystal cell with planar surface alignment, a bistable equilibrium structure for the transition is found when the direction of the applied electric field is (a) perpendicular to the two plates of the cell with positive molecular dielectric anisotropy, or (b) parallel to the two plates and the anchoring direction of the cell with negative molecular dielectric anisotropy. Except for the formation of a vertical fast "lane", when the electric field applied is parallel to both the two plates and perpendicular to the anchoring direction, the microparticle suspended in the nematic liquid crystal tends to be trapped in the midplane, regardless of the sign of the molecular dielectric anisotropy. Such phenomenon also occurs for negative molecular dielectric anisotropy while the external is applied perpendicular to the two plates. Explicit formulae proposed for the critical electric field agrees extremely well with the numerical calculation.

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“…The near field does not seem to affect the assembly, and we include hard-core repulsion, U hc , with a hydrodynamic diameter of the particles slightly larger than their real size. Indeed, in the experiment the particles in a cluster do not come into contact, which can be due to local defects or thin boundary layerlike vortex flows attached to the surface of the particles [17,44].…”

mentioning

confidence: 99%