Abstract:Many biological microswimmers locomote by periodically beating the densely packed cilia on their cell surface in a wave-like fashion. While the swimming mechanisms of ciliated microswimmers have been extensively studied both from the analytical and the numerical point of view, optimisation of the ciliary motion of microswimmers has received limited attention, especially for non-spherical shapes. In this paper, using an envelope model for the microswimmer, we numerically optimise the ciliary motion of a ciliate… Show more
“…2021; Guo et al. 2021). Here, we adopt the widely used definition of swimming efficiency introduced by Lighthill (1975) for low-Reynolds-number swimmers, , to characterize the efficiency of swimming in a shear-thinning fluid.…”
Section: Resultsmentioning
confidence: 99%
“…In addition to propulsion speed, efficiency is another relevant performance measure of the swimming motion. Recent studies have investigated how the geometrical shapes of active particles influence their efficiency of swimming in a Newtonian fluid (Daddi-Moussa-Ider et al 2021;Guo et al 2021). Here, we adopt the widely used definition of swimming efficiency introduced by Lighthill (1975) for low-Reynolds-number swimmers, η = F • U/P, to characterize the efficiency of swimming in a shear-thinning fluid.…”
Section: Effect Of Active Surface Coverage On Self-diffusiophoresis I...mentioning
Shear-thinning viscosity is a non-Newtonian behaviour that active particles often encounter in biological fluids such as blood and mucus. The fundamental question of how this ubiquitous non-Newtonian rheology affects the propulsion of active particles has attracted substantial interest. In particular, spherical Janus particles driven by self-diffusiophoresis, a major physico-chemical propulsion mechanism of synthetic active particles, were shown to always swim slower in a shear-thinning fluid than in a Newtonian fluid. In this work, we move beyond the spherical limit to examine the effect of particle eccentricity on self-diffusiophoretic propulsion in a shear-thinning fluid. We use a combination of asymptotic analysis and numerical simulations to show that shear-thinning rheology can enhance self-diffusiophoretic propulsion of a spheroidal particle, in stark contrast to previous findings for the spherical case. A systematic characterization of the dependence of the propulsion speed on the particle's active surface coverage has also uncovered an intriguing feature associated with the propulsion speeds of a pair of complementarily coated particles not previously reported. Symmetry arguments are presented to elucidate how this new feature emerges as a combined effect of anisotropy of the spheroidal geometry and nonlinearity in fluid rheology.
“…2021; Guo et al. 2021). Here, we adopt the widely used definition of swimming efficiency introduced by Lighthill (1975) for low-Reynolds-number swimmers, , to characterize the efficiency of swimming in a shear-thinning fluid.…”
Section: Resultsmentioning
confidence: 99%
“…In addition to propulsion speed, efficiency is another relevant performance measure of the swimming motion. Recent studies have investigated how the geometrical shapes of active particles influence their efficiency of swimming in a Newtonian fluid (Daddi-Moussa-Ider et al 2021;Guo et al 2021). Here, we adopt the widely used definition of swimming efficiency introduced by Lighthill (1975) for low-Reynolds-number swimmers, η = F • U/P, to characterize the efficiency of swimming in a shear-thinning fluid.…”
Section: Effect Of Active Surface Coverage On Self-diffusiophoresis I...mentioning
Shear-thinning viscosity is a non-Newtonian behaviour that active particles often encounter in biological fluids such as blood and mucus. The fundamental question of how this ubiquitous non-Newtonian rheology affects the propulsion of active particles has attracted substantial interest. In particular, spherical Janus particles driven by self-diffusiophoresis, a major physico-chemical propulsion mechanism of synthetic active particles, were shown to always swim slower in a shear-thinning fluid than in a Newtonian fluid. In this work, we move beyond the spherical limit to examine the effect of particle eccentricity on self-diffusiophoretic propulsion in a shear-thinning fluid. We use a combination of asymptotic analysis and numerical simulations to show that shear-thinning rheology can enhance self-diffusiophoretic propulsion of a spheroidal particle, in stark contrast to previous findings for the spherical case. A systematic characterization of the dependence of the propulsion speed on the particle's active surface coverage has also uncovered an intriguing feature associated with the propulsion speeds of a pair of complementarily coated particles not previously reported. Symmetry arguments are presented to elucidate how this new feature emerges as a combined effect of anisotropy of the spheroidal geometry and nonlinearity in fluid rheology.
“…2021), and the increase in efficiency with increased eccentricity (Guo et al. 2021). Figure 11.( a ) Swimming efficiency as a function of the fluid resistance for pullers/pushers.…”
Biological microorganisms and artificial micro-swimmers often locomote in heterogeneous viscous environments consisting of networks of obstacles embedded into viscous fluid media. In this work, we use the squirmer model and present a numerical investigation of the effects of shape on swimming in a heterogeneous medium. Specifically, we analyse the microorganism's propulsion speed as well as its energetic cost and swimming efficiency. The analysis allows us to probe the general characteristics of swimming in a heterogeneous viscous environment in comparison with the case of a purely viscous fluid. We found that a spheroidal microorganism always propels faster, expends less energy and is more efficient than a spherical microorganism in either a homogeneous fluid or a heterogeneous medium. Moreover, we determined that above a critical eccentricity, a spheroidal microorganism in a heterogeneous medium can swim faster than a spherical microorganism in a homogeneous fluid. Based on an analysis of the forces acting on the squirmer, we offer an explanation for the decrease in the squirmer's speed observed in heterogeneous media compared with homogeneous fluids.
“…The minimum amount of external dissipation needed by a swimmer of a given shape with a given swimming velocity therefore represents a fundamental problem of low-Reynolds-number hydrodynamics that has been solved analytically for spherical 30 – 33 and spheroidal swimmers 25 , 34 , 35 , as well as numerically for arbitrary axisymmetric shapes 36 , 37 . The propulsive motion can either be stationary 25 , 34 , 36 or periodic in time, representing a squirming motion or the motion of the ciliary envelope 30 , 33 , 37 . More lately, the swimming efficiency in non-Newtonian fluids has also been investigated 32 , 35 .…”
The energy dissipation and entropy production by self-propelled microswimmers differ profoundly from passive particles pulled by external forces. The difference extends both to the shape of the flow around the swimmer, as well as to the internal dissipation of the propulsion mechanism. Here we derive a general theorem that provides an exact lower bound on the total, external and internal, dissipation by a microswimmer. The problems that can be solved include an active surface-propelled droplet, swimmers with an extended propulsive layer and swimmers with an effective internal dissipation. We apply the theorem to determine the swimmer shapes that minimize the total dissipation while keeping the volume constant. Our results show that the entropy production by active microswimmers is subject to different fundamental limits than the entropy production by externally driven particles.
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