Optothermally driven colloidal transport in a confined nematic liquid crystal

Light-Induced Structures and Microparticle Transportation in a Free-Surface Frustrated Chiral Nematic Film

Local illumination with a light beam leads to thermo-orientational processes in a frustrated chiral nematic film with a free surface. Light-induced hydrodynamic flow and orientational structure create an adaptive platform for the collection, translation and rotation of suspended spherical microparticles. The demonstrated approach has potential applications in soft robotics, micro-object delivery systems, and other micro- and nanotechnologies.

Optothermal grid activation of microflow with magnetic nanoparticle thermophoresis for microfluidics

We report focused light-induced activation of intense magnetic microconvection mediated by suspended magnetic nanoparticles in microscale two-dimensional optothermal grids. Fully anisotropic control of microflow and mass transport fluxes is achieved by engaging the magnetic field along one or the other preferred directions. The effect is based on the recently described thermal diffusion-magnetomechanical coupling in synthetic magnetic nanofluids. We expect that the new phenomenon can be applied as an efficient all-optical mixing strategy in integrated microfluidic devices. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Field generated nematic microflows via backflow mechanism

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.

Thermal molecular focusing: tunable cross effect of phoresis and light-driven hydrodynamic focusing

The control of solute flux by either microscopic phoresis or hydrodynamic advection is a fundamental way to transport molecules, which are ubiquitously present in nature and technology. We study the transport of large solutes such as DNA driven by a time-dependent thermal field in a polymer solution. Heat propagation of a heat spot moving back and forth gives rise to the molecular focusing of DNA with frequency-tunable control. We develop a model where the viscoelastic expansion of a solution and the viscosity gradient of a smaller solute are coupled, which explains the underlying hydrodynamic focusing. This effect offers novel non-invasive manipulation of soft and biological materials in a frequency-tunable manner.

Rotation of a floating hydrophobic disk: influence of line tension

The rotation of a sub-millimeter size disk over a water bath is reported. The origin of the rotation arises from the transfer of angular momentum from a plane wave diffracted by an asymmetrical picture printed on the disk. Because of its hydrophobic character, the viscous friction contribution to the rotational motion of the object floating on the air/liquid interface is weak. From the driving optical torque and the steady-state rotation, we measure the contribution of the line tension in the femto-Newton range.