481 research outputs found
Microtransformers: controlled microscale navigation with flexible robots
Artificial microswimmers are a new technology with promising microfluidics
and biomedical applications, such as directed cargo transport, microscale
assembly, and targeted drug delivery. A fundamental barrier to realising this
potential is the ability to control the trajectories of multiple individuals
within a large group. A promising navigation mechanism for "fuel-based"
microswimmers, for example autophoretic Janus particles, entails modulating the
local environment to guide the swimmer, for instance by etching grooves in
microchannels. However, such techniques are currently limited to bulk guidance.
This paper will argue that by manufacturing microswimmers from phoretic
filaments of flexible shape-memory polymer, elastic transformations can
modulate swimming behaviour, allowing precision navigation of selected
individuals within a group through complex environments
Thrifty swimming with shear-thinning
Microscale propulsion is integral to numerous biomedical systems, for example
biofilm formation and human reproduction, where the surrounding fluids comprise
suspensions of polymers. These polymers endow the fluid with non-Newtonian
rheological properties, such as shear-thinning and viscoelasticity. Thus, the
complex dynamics of non-Newtonian fluids presents numerous modelling
challenges, strongly motivating experimental study. Here, we demonstrate that
failing to account for "out-of-plane" effects when analysing experimental data
of undulatory swimming through a shear-thinning fluid results in a significant
overestimate of fluid viscosity around the model swimmer C. elegans. This
miscalculation of viscosity corresponds with an overestimate of the power the
swimmer expends, a key biophysical quantity important for understanding the
internal mechanics of the swimmer. As experimental flow tracking techniques
improve, accurate experimental estimates of power consumption using this
technique will arise in similar undulatory systems, such as the planar beating
of human sperm through cervical mucus, will be required to probe the
interaction between internal power generation, fluid rheology, and the
resulting waveform
Local drag of a slender rod parallel to a plane wall in a viscous fluid
The viscous drag on a slender rod by a wall is important to many biological and industrial systems. This drag critically depends on the separation between the rod and the wall and can be approximated asymptotically in specific regimes, namely far from, or very close to, the wall, but is typically determined numerically for general separations. In this article we determine an asymptotic representation of the local drag for a slender rod parallel to a wall which is valid for all separations. This is possible through matching the behavior of a rod close to the wall and a rod far from the wall. We show that the leading order drag in both these regimes has been known since 1981 and that they can be used to produce a composite representation of the drag which is valid for all separations. This is in contrast to a sphere above a wall, where no simple uniformly valid representation exists. We estimate the error on this composite representation as the separation increases, discuss how the results could be used as resistive-force theory, and demonstrate their use on a two-hinged swimmer above a wall
Modelling the Fluid Mechanics of Cilia and Flagella in Reproduction and Development
Cilia and flagella are actively bending slender organelles, performing
functions such as motility, feeding and embryonic symmetry breaking. We review
the mechanics of viscous-dominated microscale flow, including time-reversal
symmetry, drag anisotropy of slender bodies, and wall effects. We focus on the
fundamental force singularity, higher order multipoles, and the method of
images, providing physical insight and forming a basis for computational
approaches. Two biological problems are then considered in more detail: (1)
left-right symmetry breaking flow in the node, a microscopic structure in
developing vertebrate embryos, and (2) motility of microswimmers through
non-Newtonian fluids. Our model of the embryonic node reveals how particle
transport associated with morphogenesis is modulated by the gradual emergence
of cilium posterior tilt. Our model of swimming makes use of force
distributions within a body-conforming finite element framework, allowing the
solution of nonlinear inertialess Carreau flow. We find that a three-sphere
model swimmer and a model sperm are similarly affected by shear-thinning; in
both cases swimming due to a prescribed beat is enhanced by shear-thinning,
with optimal Deborah number around 0.8. The sperm exhibits an almost perfect
linear relationship between velocity and the logarithm of the ratio of zero to
infinite shear viscosity, with shear-thickening hindering cell progress.Comment: 20 pages, 24 figure
Efficient Implementation of Elastohydrodynamics via Integral Operators
The dynamics of geometrically non-linear flexible filaments play an important
role in a host of biological processes, from flagella-driven cell transport to
the polymeric structure of complex fluids. Such problems have historically been
computationally expensive due to numerical stiffness associated with the
inextensibility constraint, as well as the often non-trivial boundary
conditions on the governing high-order PDEs. Formulating the problem for the
evolving shape of a filament via an integral equation in the tangent angle has
recently been found to greatly alleviate this numerical stiffness. The
contribution of the present manuscript is to enable the simulation of non-local
interactions of multiple filaments in a computationally efficient manner using
the method of regularized stokeslets within this framework. The proposed method
is benchmarked against a non-local bead and link model, and recent code
utilizing a local drag velocity law. Systems of multiple filaments (1) in a
background fluid flow, (2) under a constant body force, and (3) undergoing
active self-motility are modeled efficiently. Buckling instabilities are
analyzed by examining the evolving filament curvature, as well as by
coarse-graining the body frame tangent angles using a Chebyshev approximation
for various choices of the relevant non-dimensional parameters. From these
experiments, insight is gained into how filament-filament interactions can
promote buckling, and further reveal the complex fluid dynamics resulting from
arrays of these interacting fibers. By examining active moment-driven
filaments, we investigate the speed of worm- and sperm-like swimmers for
different governing parameters. The MATLAB(R) implementation is made available
as an open-source library, enabling flexible extension for alternate
discretizations and different surrounding flows.Comment: 37 pages, 17 figure
Depressive Symptoms Affect Working Memory in Healthy Older Adult Hispanics
Objectives: Low and middle income nations will experience an unprecedented growth of the elderly population and subsequent increase in age-related neurological disorders. Worldwide prevalence and incidence of all-types of neurological disorders with serious mental health complications will increase with life expectancy across the globe. Onein- ten individuals over 75 has at least moderate cognitive impairment. Prevalence of cognitive impairment doubles every 5 years thereafter. Latin America’s population of older adult’s 65 years and older is growing rapidly, yet little is known about cognitive aging among healthy older Latinos. Clinically significant depressive symptomatology is common among community-dwelling older adults and is associated with deficits across multiple cognitive domains, however much of the literature has not modeled the unique effects of depression distinct from negative and low positive affect. Our objective was to understand how mental health affects cognitive health in healthy aging Latinos.
Methods: The present study used confirmatory factor analysis (CFA) and structural equation modeling (SEM) to examine the relative effects of Negative Affect, Positive Affect and Geriatric Depression on Verbal Memory, Verbal Reasoning, Processing Speed, and Working Memory in healthy aging Latinos. Data was collected from a sample of healthy community dwelling older adults living in San Jose, Costa Rica. Modeling of latent variables attenuated error and improved measurement reliability of cognition, affect, and depression variables.
Results: Costa Ricans enjoy a notoriety for being much happier than US citizens and are renowned as one of the happiest nations in the world in global surveys. This was born out in these data. Costa Rican affective profiles differed substantively from US profiles. Levels of negative affect and depression were similar to US samples, but their levels of positive affect were much higher. Cognitive performance of these Costa Rican older adults was similar to USage and education matched peers. CFA and SEM found that increased depressive symptomatology had deleterious effects on Working Memory made up of subtest scores sampling simple attention and vigilance for numbers. Verbal Memory, Verbal Reasoning, and Processing Speed were not affected by self-reported Positive Affect, Negative Affect or Depressive symptoms.
Conclusion: Costa Rican older adults were happy, as evidenced by the high ratio of positive affect to relatively low negative affect. Thus, we were somewhat surprised to find that depressive symptoms were selectively correlated to decrements in working memory and that negative and positive affect contributed negligible amounts of variance to any of the cognitive factors. Because of the methodological rigor of latent variable analysis, these results are very specific. The Working Memory factor is not contaminated with Speed of Processing or other measured cognitive factors. Likewise, the measured Geriatric Depression represents symptoms that are richly cognitive, not overtly affective
Three-dimensional flow in Kupffer's Vesicle.
Whilst many vertebrates appear externally left-right symmetric, the arrangement of internal organs is asymmetric. In zebrafish, the breaking of left-right symmetry is organised by Kupffer's Vesicle (KV): an approximately spherical, fluid-filled structure that begins to form in the embryo 10 hours post fertilisation. A crucial component of zebrafish symmetry breaking is the establishment of a cilia-driven fluid flow within KV. However, it is still unclear (a) how dorsal, ventral and equatorial cilia contribute to the global vortical flow, and (b) if this flow breaks left-right symmetry through mechanical transduction or morphogen transport. Fully answering these questions requires knowledge of the three-dimensional flow patterns within KV, which have not been quantified in previous work. In this study, we calculate and analyse the three-dimensional flow in KV. We consider flow from both individual and groups of cilia, and (a) find anticlockwise flow can arise purely from excess of cilia on the dorsal roof over the ventral floor, showing how this vortical flow is stabilised by dorsal tilt of equatorial cilia, and (b) show that anterior clustering of dorsal cilia leads to around 40Â % faster flow in the anterior over the posterior corner. We argue that these flow features are supportive of symmetry breaking through mechano-sensory cilia, and suggest a novel experiment to test this hypothesis. From our new understanding of the flow, we propose a further experiment to reverse the flow within KV to potentially induce situs inversus.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s00285-016-0967-
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