Tracer diffusion in active suspensions

We study the diffusion of a Brownian probe particle of size $R$ in a dilute dispersion of active Brownian particles (ABPs) of size $a$, characteristic swim speed $U_0$, reorientation time $\tau_R$, and mechanical energy $k_s T_s = \zeta_a U_0^2 \tau_R /6$, where $\zeta_a$ is the Stokes drag coefficient of a swimmer. The probe has a thermal diffusivity $D_P = k_B T/\zeta_P$, where $k_B T$ is the thermal energy of the solvent and $\zeta_P$ is the Stokes drag coefficient for the probe. When the swimmers are inactive, collisions between the probe and the swimmers sterically hinder the probe's diffusive motion. In competition with this steric hindrance is an enhancement driven by the activity of the swimmers. The strength of swimming relative to thermal diffusion is set by $Pe_s = U_0 a /D_P$. The active contribution to the diffusivity scales as $Pe_s^2$ for weak swimming and $Pe_s$ for strong swimming, but the transition between these two regimes is nonmonotonic. When fluctuations in the probe motion decay on the time scale $\tau_R$, the active diffusivity scales as $k_s T_s /\zeta_P$: the probe moves as if it were immersed in a solvent with energy $k_s T_s$ rather than $k_B T$.Comment: 5 pages, 3 figures, submitted for publication. Please contact authors regarding supplemental informatio

Inferring muscle functional roles of the ostrich pelvic limb during walking and running using computer optimization

Owing to their cursorial background, ostriches (Struthio camelus) walk and run with high metabolic economy, can reach very fast running speeds and quickly execute cutting manoeuvres. These capabilities are believed to be a result of their ability to coordinate muscles to take advantage of specialized passive limb structures. This study aimed to infer the functional roles of ostrich pelvic limb muscles during gait. Existing gait data were combined with a newly developed musculoskeletal model to generate simulations of ostrich walking and running that predict muscle excitations, force and mechanical work. Consistent with previous avian electromyography studies, predicted excitation patterns showed that individual muscles tended to be excited primarily during only stance or swing. Work and force estimates show that ostrich gaits are partially hip-driven with the bi-articular hip–knee muscles driving stance mechanics. Conversely, the knee extensors acted as brakes, absorbing energy. The digital extensors generated large amounts of both negative and positive mechanical work, with increased magnitudes during running, providing further evidence that ostriches make extensive use of tendinous elastic energy storage to improve economy. The simulations also highlight the need to carefully consider non-muscular soft tissues that may play a role in ostrich gait

Fluctuation-dissipation in active matter

In a colloidal suspension at equilibrium, the diffusive motion of a tracer particle due to random thermal fluctuations from the solvent is related to the particle’s response to an applied external force, provided this force is weak compared to the thermal restoring forces in the solvent. This is known as the fluctuation-dissipation theorem (FDT) and is expressed via the Stokes-Einstein-Sutherland (SES) relation D = k_BT/ζ, where D is the particle’s self-diffusivity (fluctuation), ζ is the drag on the particle (dissipation), and k_BT is the thermal Boltzmann energy. Active suspensions are widely studied precisely because they are far from equilibrium—they can generate significant nonthermal internal stresses, which can break the detailed balance and time-reversal symmetry—and thus cannot be assumed to obey the FDT a priori. We derive a general relationship between diffusivity and mobility in generic colloidal suspensions (not restricted to near equilibrium) using generalized Taylor dispersion theory and derive specific conditions on particle motion required for the FDT to hold. Even in the simplest system of active Brownian particles (ABPs), these conditions may not be satisfied. Nevertheless, it is still possible to quantify deviations from the FDT and express them in terms of an effective SES relation that accounts for the ABPs conversion of chemical into kinetic energy

Fluctuation-dissipation in active matter

In a colloidal suspension at equilibrium, the diffusive motion of a tracer particle due to random thermal fluctuations from the solvent is related to the particle’s response to an applied external force, provided this force is weak compared to the thermal restoring forces in the solvent. This is known as the fluctuation-dissipation theorem (FDT) and is expressed via the Stokes-Einstein-Sutherland (SES) relation D = k_BT/ζ, where D is the particle’s self-diffusivity (fluctuation), ζ is the drag on the particle (dissipation), and k_BT is the thermal Boltzmann energy. Active suspensions are widely studied precisely because they are far from equilibrium—they can generate significant nonthermal internal stresses, which can break the detailed balance and time-reversal symmetry—and thus cannot be assumed to obey the FDT a priori. We derive a general relationship between diffusivity and mobility in generic colloidal suspensions (not restricted to near equilibrium) using generalized Taylor dispersion theory and derive specific conditions on particle motion required for the FDT to hold. Even in the simplest system of active Brownian particles (ABPs), these conditions may not be satisfied. Nevertheless, it is still possible to quantify deviations from the FDT and express them in terms of an effective SES relation that accounts for the ABPs conversion of chemical into kinetic energy

Housing First: Documenting the Need for Permanent Supportive Housing

Five years ago, very few people would have believed that it was possible to end homelessness for the most marginalized Americans. Today, the idea that we can end the cycle of homelessness and institutionalization for vulnerable Americans is so mainstream that the Bush administration declared in its 2003 budget proposal that it considers “ending chronic homelessness in the next decade a top objective.” The key to this turnaround in thinking is supportive housing, an approach that is both smart and compassionate” according to a recent editorial in the New York Times. It is a concept that is proving to be cost-effective in cities across the country

Housing First: Documenting the Need for Permanent Supportive Housing (Executive Summary)

Five years ago, very few people would have believed that it was possible to end homelessness for the most marginalized Americans. Today, the idea that we can end the cycle of homelessness and institutionalization for vulnerable Americans is so mainstream that the Bush administration declared in its 2003 budget proposal that it considers “ending chronic homelessness in the next decade a top objective.” The key to this turnaround in thinking is supportive housing, an approach that is both smart and compassionate” according to a recent editorial in the New York Times. It is a concept that is proving to be cost-effective in cities across the country

Housing First: Documenting the Need for Permanent Supportive Housing (Executive Summary)

Five years ago, very few people would have believed that it was possible to end homelessness for the most marginalized Americans. Today, the idea that we can end the cycle of homelessness and institutionalization for vulnerable Americans is so mainstream that the Bush administration declared in its 2003 budget proposal that it considers “ending chronic homelessness in the next decade a top objective.” The key to this turnaround in thinking is supportive housing, an approach that is both smart and compassionate” according to a recent editorial in the New York Times. It is a concept that is proving to be cost-effective in cities across the country

Evaluation of Neighborhood Progress, Inc.\u27s Community Organizing Support Program

In 1998, the Sisters of Charity Foundation of Cleveland began a program-related initiative to increase the availability, affordability and quality of permanent, affordable housing units for low-income and underserved families and persons in Cuyahoga and Lorain Counties. The Affordable Housing Initiative will award approximately six million dollars to community organizations over a five-year period. As part of this initiative, the Foundation funded more than 25 grantees to undertake a variety of non-capital projects. The Maxine Goodman Levin College of Urban Affairs, Cleveland State University was asked to evaluate the overall initiative and the projects of each of the grantees

Potential Phytoplankton Productivity of Three Iowa Streams

A series of experiments were conducted to test the hypothesis that the concentrations of suspended algal populations in central Iowa streams are not limited by nutrient concentrations. River water samples with their natural plankton populations were collected from the Des Moines River, Skunk River, and Squaw Creek and were cultured under controlled conditions without the addition of nutrients. In 23 of 24 experiments significant increases in algal chlorophyll a were found with an average replication factor of 14 times. The data indicate that nutrients are not limiting suspended algal densities in the streams. A second series of experiments indicated that nitrate concentrations of up to 20 mg/I nitrate-nitrogen did not inhibit algal growth in these river waters

Effects of the COVID-19 pandemic on academic preparation and performance: a complex picture of equity

IntroductionMany experts have predicted a drop in students’ academic performance due to an extended period of remote instruction and other harmful effects of the pandemic.MethodsAs university instructors and education researchers, we sought to investigate the effects of the pandemic on students’ preparation for college-level coursework and their performance in early college using mixed effects regression models. Data were collected from STEM students at a public research university in the southeastern United States.ResultsWe found that demographic gaps in high school preparation (as measured by ACT scores) between men and women, as well as underrepresented minority and majority students, remained relatively consistent after the start of the pandemic. These gaps were approximately 1 point (out of 36) and 3 points, respectively. However, the gap between first generation and continuing generation students increased from prior to 2020, to after 2020, going from approximately 1 point to 2 points. This gap in preparation was not accompanied by a corresponding shift in the demographics of the student population and there was no corresponding increase in the demographic gaps in students’ first term grades.DiscussionThe data seem to suggest that first-generation students in STEM suffered more from the changes to secondary instruction during the pandemic, but that college instructors were able to mitigate some of these effects on first-semester grades. However, these effects were only mitigated to the extent that they preserved the status quo of pre-pandemic inequities in undergraduate STEM education