Particle accumulation and depletion in a microfluidic Marangoni flow

Abstract

Thermosolutal and thermocapillary Marangoni convection at a liquid-gas interface in a microchannel structure of approximately 100 × 90 μm cross section creates a localized vortex that acts as a trap for micrometer and sub-micrometer sized tracer particles. Next to the vortex, depleted volumes appear that are entirely cleared of particles. This particle redistribution is caused by collisions of the tracers with the meniscus, which push the particles back onto the critical streamline with one particle radius distance to the meniscus. The streamlines between the meniscus and the critical streamline feed the depleted regions. Since the critical streamline depends on the particle radius, the effect leads to a particle fractionation according to their size. Diffusion allows only small particles to escape from the trap. Larger particles are permanently confined and their diffusion is rectified after every revolution at the meniscus, which produces a ratchet effect and increases the particle localization within the vortex.

Attractors for the motion of a finite-size particle in a two-sided lid-driven cavity

“…Under the idealising assumption that the particle is perfectly advected in the bulk, it may not, however, populate those chaotic streamlines which originate from the layer on the (moving) walls which is inaccessible for the particle centroid. This depletion effect, which has been found in other systems as well Kuhlmann et al 2014;Orlishausen et al 2017;, is visible in figure 22(a) on the walls at y = ±0.5 just upstream of the moving walls and results from the deceleration, thus widening, of the layer of streamlines from the inaccessible layers on the moving walls. Regardless of the depletion effect in the region of chaotic streamlines, the particles do not enter the regions occupied by KAM tori.…”

“…For the present cm size flow and particles in the mm range the restrictions on the density matching are quite severe due to the large buoyancy forces. Buoyancy becomes less important for particles in millimetric thermocapillary flow, and even less in micrometre-scale flows (Orlishausen et al 2017). If, furthermore, the particle size a is small, FSCS can be expected to play an important role in incompressible multiphase micro-flows which are often laminar and thus can exhibit KAM structures.…”

“…The breakdown of the Maxey-Riley model is frequently met for particles suspended in millimetric thermocapillary flows (Schwabe et al 2007;, in micro-flows (Wang, Jalikop & Hilgenfeldt 2011;Karimi, Yazdi & Ardekani 2013) or even in nano-flows (Orlishausen et al 2017), because the particle size relative to the domain size can be much larger than, e.g. 0.1 %.…”

“…2007; Romanò & Kuhlmann 2019), in micro-flows (Wang, Jalikop & Hilgenfeldt 2011; Karimi, Yazdi & Ardekani 2013) or even in nano-flows (Orlishausen et al. 2017), because the particle size relative to the domain size can be much larger than, e.g. 0.1 %.…”

Elastohydrodynamic Lift at a Soft Wall

“…With g = 9.81 m.s −2 and a density difference of ∆ρ = 40 kg.m −3 between the beads and the fluid [29], we compute F g = 3.2 pN. This corresponds to an interaction energy per unit area F g /R ≃ 2.5 × 10 −7 N.m −1 at which we anticipate the brushes to be essentially uncompressed [27].…”