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-Non-neutrally buoyant soft particles in vertical microflows are investigated. We find, soft particles lighter than the liquid migrate to off-center streamlines in a downward Poiseuille flow (buoyancy-force antiparallel to flow). In contrast, heavy soft particles migrate to the center of the downward (and vanishing) Poiseuille flow. A reversal of the flow direction causes in both cases a reversal of the migration direction, i. e. heavier (lighter) particles migrate away from (to) the center of a parabolic flow profile. Non-neutrally buoyant particles migrate also in a linear shear flow across the parallel streamlines: heavy (light) particles migrate along (antiparallel to) the local shear gradient. This surprising, flow-dependent migration is characterized by simulations and analytical calculations for small particle deformations, confirming our plausible explanation of the effect. This density dependent migration reversal may be useful for separating particles.Introduction. -Microfluidics is a rapidly evolving cross-disciplinary field, ranging from basic physics to a great variety of applications in life science and technology [1][2][3][4][5][6][7][8][9]. The blooming subfield of the dynamics of neutrally buoyant soft particles in suspension and their crossstreamline migration (CSM) in rectilinear shear flows, plays a central role for cell and DNA sorting, blood flow, polymer processing and so on [6,[10][11][12][13]. In contrast, little is known about the dynamics of non-neutrally buoyant soft particles in rectilinear flows, but we show in this work for such particles a novel migration reversal.Segre and Silberberg reported in 1961 about CSM of neutrally buoyant rigid particles at finite Reynolds numbers in flows through pipes [14]. When particles and channels approach the micrometer scale, fluid inertia does not matter and particles follow the Stokesian dynamics. In this limit CSM occurs only for soft particles but in curvilinear [15][16][17] as well as in rectilinear flows [18][19][20], whereby in rectilinear flows, the flows fore-aft symmetry is broken, requiring intra-particle hydrodynamic interaction [18,19]. Such symmetry breaking occurs also near boundaries via wall-induced lift forces [20][21][22][23][24] or by space-dependent shear rates, so that dumbbells [18,19], droplets [25,26], vesicles and capsules [27][28][29] exhibit CSM even in unbounded flow. Such parity breaking mechanisms may be also accompanied by a viscosity contrast [30] or chirality [31]. Recently was found, that CSM takes place also for asymmetric soft particles in time-dependent linear

The locomotion and design of microswimmers are topical issues of current fundamental and applied research. In addition to numerous living and artificial active microswimmers, a passive microswimmer was identified only recently: a soft, Λ-shaped, non-buoyant particle propagates in a shaken liquid of zero-mean velocity (Jo et al 2016 Phys. Rev. E 94 063116). We show that this novel passive locomotion mechanism works for realistic non-buoyant, asymmetric Janus microcapsules as well. According to our analytical approximation, this locomotion requires a symmetry breaking caused by different Stokes drags of soft particles during the two half periods of the oscillatory liquid motion. It is the intrinsic anisotropy of Janus capsules and Λ-shaped particles that break this symmetry for sinusoidal liquid motion. Further, we show that this passive locomotion mechanism also works for the wider class of symmetric soft particles, e.g. capsules, by breaking the symmetry via an appropriate liquid shaking. The swimming direction can be uniquely selected by a suitable choice of the liquid motion. Numerical studies, including lattice Boltzmann simulations, also show that this locomotion can outweigh gravity, i.e. non-buoyant particles may be either elevated in shaken liquids or concentrated at the bottom of a container. This novel propulsion mechanism is relevant to many applications, including the sorting of soft particles like healthy and malignant (cancer) cells, which serves medical purposes, or the use of non-buoyant soft particles as directed microswimmers.

We present a deformation-dependent propulsion phenomenon for soft particles such as cells in microchannels. It is based on a broken time reversal symmetry generated by a fast forward and slow backward motion of a fluid which does not progress on average. In both sections, soft particles deform differently and thus progress relatively to the liquid. We demonstrate this by using Lattice-Boltzmann simulations of ubiquitous red blood cells in microchannels, as well as simulations for capsules and minimal soft tissue models in unbounded Poiseuille flows. The propulsion of the soft particles depends besides the oscillation asymmetry on their size, deformation type and elasticity. This is also demonstrated by analytical calculations for a minimal model. Our findings may stimulate a rethinking of particle sorting methods. For example, healthy and malignant cells often differ in their elasticity. With the proposed method, several cell types with different deformability can be separated simultaneously without labeling or obstacles in a microfluidic device.