Field generated nematic microflows via backflow mechanism

Abstract

Generation of flow is an important aspect in microfluidic applications and generally relies on external pumps or embedded moving mechanical parts which pose distinct limitations and protocols on the use of microfluidic systems. A possible approach to avoid moving mechanical parts is to generate flow by changing some selected property or structure of the fluid. In fluids with internal orientational order such as nematic liquid crystals, this process of flow generation is known as the backflow effect. In this article, we demonstrate the contact-free generation of microfluidic material flows in nematic fluids -including directed contact-free pumping- by external electric and optical fields based on the dynamic backflow coupling between nematic order and material flow. Using numerical modelling, we design efficient shaping and driving of the backflow-generated material flow using spatial profiles and time modulations of electric fields with oscillating amplitude, rotating electric fields and optical fields. Particularly, we demonstrate how such periodic external fields generate efficient net average nematic flows through a microfluidic channel, that avoid usual invariance under time-reversal limitations. We show that a laser beam with rotating linear polarization can create a vortex-like flow structure and can act as a local flow pump without moving mechanical parts. The work could be used for advanced microfluidic applications, possibly by creating custom microfluidic pathways without predefined channels based on the adaptivity of an optical set-up, with a far reaching unconventional idea to realize channel-less microfluidics.

Active nematic flows confined in a two-dimensional channel with hybrid alignment at the walls: A unified picture

“…relax to equilibrium for a 3D Q. For |α| = 10 −12 the 3D n-profile shows a deviation of ≈0.015% from the analytical profile (17), while the variation of q 0 , as well as the degree of biaxiality estimated as the difference between the two lowest eigenvalues, are approximately ≈0.01%. Therefore, we conclude that the 2D Q solution is a very good approximation for the 3D Q case.…”

“…In nematic liquid crystals the coupling between the orientational order of the molecules and the flow is controlled by several material and flow parameters and the nematic configuration is highly sensitive to geometrical constraints. The emerging complex dynamics of these active liquids is of great promise for microfluidic applications since it provides a means to control and finely tune the flow overcoming the intrinsic difficulties of directing and pumping isotropic fluids at the microscale [15][16][17]. Devices that direct and sort nano and micro-particles have already been presented in the literature: some exploit the anisotropic nature of the fluid to control the flow resistance and streamlines through the application of external electrical fields [18], some use defect lines as rails to transport colloids [19,20] in what is referred to generically as topological microfluidics.…”

Generation and focusing of a second-order vector beam using a subwavelength optical element

“…Также ранее было показано [46,47], что обратный поток энергии может быть получен путем острой фокусировки радиальнополяризованных лазерных пучков более высокого порядка. Существует ряд приложений, основанных на явлениях обратного потока энергии [48,49], среди которых следует отметить острую фокусировку оптических вихрей [51], а также оптический захват и манипулирование [52].…”

Fluctuation-induced dynamics of nematic topological defects

“…The elastic stress, Π elastic , shows a dependence on the order parameter, Q, and derived quantities. This coupling, known as backflow [36], has been numerically and experimentally shown to be relevant to ±1/2 defect annihilation dynamics in passive nematics [37,38]. For the rotational fluctuations, white noise with the strength Q κ B T is added to the two independent elements of the nematic tensor, Q xx and Q yx , at the beginning of each LB step.…”