1,788 research outputs found
How to Track Protists in Three Dimensions
We present an apparatus optimized for tracking swimming microorganisms in the
size range 10-1000 microns, in three dimensions (3D), far from surfaces, and
with negligible background convective fluid motion. CCD cameras attached to two
long working distance microscopes synchronously image the sample from two
perpendicular directions, with narrowband dark-field or bright-field
illumination chosen to avoid triggering a phototactic response. The images from
the two cameras can be combined to yield 3D tracks of the organism. Using
additional, highly directional broad-spectrum illumination with millisecond
timing control the phototactic trajectories in 3D of organisms ranging from
Chlamydomonas to Volvox can be studied in detail. Surface-mediated hydrodynamic
interactions can also be investigated without convective interference. Minimal
modifications to the apparatus allow for studies of chemotaxis and other taxes.Comment: 8 pages, 7 figure
Fluid dynamics of bacterial turbulence
Self-sustained turbulent structures have been observed in a wide range of
living fluids, yet no quantitative theory exists to explain their properties.
We report experiments on active turbulence in highly concentrated 3D
suspensions of Bacillus subtilis and compare them with a minimal fourth-order
vector-field theory for incompressible bacterial dynamics. Velocimetry of
bacteria and surrounding fluid, determined by imaging cells and tracking
colloidal tracers, yields consistent results for velocity statistics and
correlations over two orders of magnitude in kinetic energy, revealing a
decrease of fluid memory with increasing swimming activity and linear scaling
between energy and enstrophy. The best-fit model parameters allow for
quantitative agreement with experimental data.Comment: 5 pages, 4 figure
Oscillatory Flows Induced by Microorganisms Swimming in Two-dimensions
We present the first time-resolved measurements of the oscillatory velocity
field induced by swimming unicellular microorganisms. Confinement of the green
alga C. reinhardtii in stabilized thin liquid films allows simultaneous
tracking of cells and tracer particles. The measured velocity field reveals
complex time-dependent flow structures, and scales inversely with distance. The
instantaneous mechanical power generated by the cells is measured from the
velocity fields and peaks at 15 fW. The dissipation per cycle is more than four
times what steady swimming would require.Comment: 4 pages, 4 figure
Meso-scale turbulence in living fluids
Turbulence is ubiquitous, from oceanic currents to small-scale biological and
quantum systems. Self-sustained turbulent motion in microbial suspensions
presents an intriguing example of collective dynamical behavior amongst the
simplest forms of life, and is important for fluid mixing and molecular
transport on the microscale. The mathematical characterization of turbulence
phenomena in active non-equilibrium fluids proves even more difficult than for
conventional liquids or gases. It is not known which features of turbulent
phases in living matter are universal or system-specific, or which
generalizations of the Navier-Stokes equations are able to describe them
adequately. Here, we combine experiments, particle simulations, and continuum
theory to identify the statistical properties of self-sustained meso-scale
turbulence in active systems. To study how dimensionality and boundary
conditions affect collective bacterial dynamics, we measured energy spectra and
structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D
geometries. Our experimental results for the bacterial flow statistics agree
well with predictions from a minimal model for self-propelled rods, suggesting
that at high concentrations the collective motion of the bacteria is dominated
by short-range interactions. To provide a basis for future theoretical studies,
we propose a minimal continuum model for incompressible bacterial flow. A
detailed numerical analysis of the 2D case shows that this theory can reproduce
many of the experimentally observed features of self-sustained active
turbulence.Comment: accepted PNAS version, 6 pages, click doi for Supplementary
Informatio
Direct measurement of the flow field around swimming microorganisms
Swimming microorganisms create flows that influence their mutual interactions
and modify the rheology of their suspensions. While extensively studied
theoretically, these flows have not been measured in detail around any
freely-swimming microorganism. We report such measurements for the microphytes
Volvox carteri and Chlamydomonas reinhardtii. The minute ~0.3% density excess
of V. carteri over water leads to a strongly dominant Stokeslet contribution,
with the widely-assumed stresslet flow only a correction to the subleading
source dipole term. This implies that suspensions of V. carteri have features
similar to suspensions of sedimenting particles. The flow in the region around
C. reinhardtii where significant hydrodynamic interaction is likely to occur
differs qualitatively from a "puller" stresslet, and can be described by a
simple three-Stokeslet model.Comment: 4 pages, 4 figures, accepted for publication in PR
Stability of dancing Volvox
Biflagellate algal cells of the genus Volvox form spherical colonies that
propel themselves, vertically upwards in still fluid, by the coordinated
beating of thousands of flagella, that also cause the colonies to rotate about
their vertical axes. When they are swimming in a chamber of finite depth, pairs
(or more) of Volvox carteri colonies were observed by Drescher et al. [Phys.
Rev. Lett. 102, 168101 (2009)] to exhibit hydrodynamic bound states when they
are close to a rigid horizontal boundary. When the boundary is above, the
colonies are attracted to each other and orbit around each other in a `waltz';
when the boundary is below they perform more complex `minuet' motions. These
dances are simulated in the present paper, using a novel `spherical squirmer'
model of a colony in which, instead of a time-independent but
-dependent tangential velocity being imposed on the spherical surface
(radius ; is the polar angle), a time-independent and uniform
tangential shear stress is applied to the fluid on a sphere of radius
, where represents the length of
the flagella. The fluid must satisfy the no-slip condition on the sphere at
radius . In addition to the shear stress, the motions depend on two
dimensionless parameters that describe the effect of gravity on a colony:
, proportional to the ratio of the sedimentation speed of a non-swimming
colony to its swimming speed, and , that represents the fact that
colonies are bottom-heavy..
Guided Interface Waves
Many of tomorrowâs technologies are dependent upon the emergence of new advanced materials with superior stiffness and strength but reduced density. Metal matrix composites (MMCâs) consisting of light metal matrices (e. g., aluminum, titanium or magnesium) reinforced with very stiff ceramic fibers or particles (e. g. SiC, AI2O3 or graphite) show considerable promise for satisfying this need. However, the satisfactory performance of these materials has been found to be critically dependent upon the attainment of optimal properties at the metal-ceramic interface; a problem that is compounded by the possibility of chemical reactions between the reactive metal matrix and ceramic reinforcement. Of particular import are the interface adhesion and local elastic properties. Unfortunately no methods exist for the measurement of these quantities even for macroscopic interfaces let alone for the microscopic interfaces occurring within MMCâs
Gender Microaggression and Macroaggression Experiences of Women Athletic Trainers
Purpose: Although gender disparities and workplace vitality for women has been studied in athletic training, gender discrimination has not been widely studied. The purpose of this study was to describe gender-based aggressions experienced by women athletic trainers (ATs) within their educational and workplace environments. Methods: We used a cross-sectional design, and the web-based survey was comprised of demographic questions, the Schedule of Sexism Events (SSE), and questions on incident reporting. The SSE asks participant to rank items on a Likert Scale (1 = the event never happened to 6 = the event happens almost all the time). Items within the SSE are contextualized to either events in the past year or events in their entire life. The survey was distributed to 5,667 women ATs through the National Athletic Training Association (NATA). Five hundred thirty-nine (539) participants accessed the survey (9.5%). Four hundred seventy-eight (478) participants (age=34.2±8.6y [range=23â66y], experience=11.0±8.2y [range=0-40y]) completed the entire instrument (88.6%). We used descriptive statistics to analyze demographic variables, gender-based education- and work- related items of the SSE and incident reporting. Results: When asked about unfair treatment while interacting with teachers, professors, or engaging in academics in their lifetime, 89% (n=425) of participants indicated they had experienced unfair treatment. Comparatively, 53% (n=252) of participants experienced unfair treatment from teachers, or professors, or while engaging in academics in the last year. When asked about unfair treatment by an employer, boss, or supervisor, 88% (n=421) of participants experienced unfair treatment in their lifetime, where 55% (n=267) have experienced unfair treatment in the past year. Ninety percent (90%, n=430) indicated they experienced unfair treatment by co-workers, fellow students, or colleagues, in their lifetime compared to 61% (n=292) in the past year. When asked if they had experienced gender-based macroaggressions and microaggressions in the workplace, 41% (n=198) experienced both types of aggressions; 5% (n=22) experienced macroaggressions and 29% (n=137) experienced microaggressions. Only 25% (n=119) of participants have reported aggressions in the workplace. Conclusion: Women ATs experience gender-based aggressions in the workplace but they do not typically report these aggressions. All ATs have the responsibility to work towards an inclusive, equitable, and welcoming workplace that directly addresses aggressions
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