The drag force on a sphere in tangential and normal motion to a plane wall is evaluated in the limit of large Knudsen number and small Mach (and Strouhal) number assuming isothermal conditions and ...diffuse reflection of gas molecules on walls. In the limit of free molecular flow, the molecular distribution function of the gas is evaluated using a set of coupled Fredholm integral equations. The results are compared with direct simulation Monte Carlo calculations and extended for finite Knudsen numbers. In all cases stronger dependence of the force on the width of the gap is found for normal compared to tangential motion. When the flow within the gap can be considered as essentially collisionless in nature, a similar dependence of the force on the gap width is observed at finite Knudsen numbers as in the free molecular case.
This report shares practices from a successful Latinx Studies learning community model at an urban research-intensive university in the Midwestern United States. The learning community model offers a ...tiered developmental curriculum to support the three different stages of a student's journey, from enrollment to graduation. The three distinct phases of the learning community model are: a pre-college summer enrichment program, a first- and second-year college scholars program and a third and fourth-year college-to-career component. The model also integrates 10 out of 11 of Kuh's (2008) high impact practices demonstrated to support student engagement and success for students making a direct transition from high school to university. The authors share gains in student retention and five-year graduation rates as evidence of effectiveness.
This practice report shares practices from a successful Latinx Studies learning community model at an urban research-intensive university in the Midwestern United States. The learning community model ...offers a tiered developmental curriculum to support the three different stages of a student’s journey, from enrollment to graduation. The three distinct phases of the learning community model are: a pre-college summer enrichment program, a first- and second-year college scholars program and a third and fourth-year college-to-career component. The model also integrates 10 out of 11 of Kuh’s (2008) high impact practices demonstrated to support student engagement and success for students making a direct transition from high school to university. The authors share gains in student retention and five-year graduation rates as evidence of effectiveness.
Calanus marshallae is the dominant mesozooplankton copepod species over the south-eastern Bering Sea middle shelf. Climate-induced changes in the magnitude and timing of production by C. marshallae ...may affect the living marine resources of the Bering Sea shelf ecosystem. We examined springtime abundance, gonadal maturity and stage distributions of C. marshallae copepodites during five consecutive years (1995–1999) that spanned the range of variability observed over the past 34 years in terms of water temperature and ice cover. We compared our results with previous work conducted during cool (1980) and warm (1981) years Smith, S. L. and Vidal, J. (1986) Cont. Shelf Res., 5, 215–239. The spring phytoplankton bloom began relatively early in association with ice (1995, 1997, 1999), but began late when ice was absent or retreated early (1996, 1998). Egg production began well before the bloom and continued over a long duration. Copepodites, however, were recruited during a relatively short period, coincident with the spring phytoplankton bloom. The relationship between brood stock and spring-generation copepodite abundances was weak. Copepodite concentrations during May were greatest in years of most southerly ice extent. Copepodite populations were highly variable among years, reflecting interannual variability in the atmosphere–ice–ocean system.
Isotachophoresis with emulsions Goet, G.; Baier, T.; Hardt, S. ...
Biomicrofluidics,
07/2013, Letnik:
7, Številka:
4
Journal Article
Recenzirano
Odprti dostop
An experimental study on isotachophoresis (ITP) in which an emulsion is used as leading electrolyte (LE) is reported. The study aims at giving an overview about the transport and flow phenomena ...occurring in that context. Generally, it is observed that the oil droplets initially dispersed in the LE are collected at the ITP transition zone and advected along with it. The detailed behavior at the transition zone depends on whether or not surfactants (polyvinylpyrrolidon, PVP) are added to the electrolytes. In a system without surfactants, coalescence is observed between the droplets collected at the ITP transition zone. After having achieved a certain size, the droplets merge with the channel walls, leaving an oil film behind. In systems with PVP, coalescence is largely suppressed and no merging of droplets with the channel walls is observed. Instead, at the ITP transition zone, a droplet agglomerate of increasing size is formed. In the initial stages of the ITP experiments, two counter rotating vortices are formed inside the terminating electrolyte. The vortex formation is qualitatively explained based on a hydrodynamic instability triggered by fluctuations of the number density of oil droplets.
The southeastern Bering Sea shelf ecosystem is an important fishing ground for fin- and shellfish, and is the summer foraging grounds for many planktivorous seabirds and marine mammals. In 1997 and ...1998, Northern Hemisphere climate anomalies affected the physical and biological environment of the southeastern Bering Sea shelf. The resulting anomalous conditions provided a valuable opportunity to examine how longer-term climate change might affect this productive ecosystem. We compared historical and recent zooplankton biomass and species composition data for the southeastern Bering Sea shelf to examine whether or not there was a response to the atmosphere–ocean–ice anomalies of 1997 and 1998. Summer zooplankton biomass (1954–1994) over the southeastern shelf did not exhibit a decline as previously reported for oceanic stations. In addition, zooplankton biomass in 1997 and 1998 was not appreciably different from other years in the time series. Spring concentrations of numerically abundant copepods (Acartia spp., Calanus marshallae, and Pseudocalanus spp.), however, were significantly higher during 1994–1998 than 1980–1981; spring concentrations of Metridia pacifica and Neocalanus spp. were not consistently different between the two time periods. Neocalanus spp. was the only taxon to have consistent differences in stage composition between the two time periods—CV copepodites were much more prevalent in May of the 1990s than early 1980s. Since relatively high zooplankton concentrations were observed prior to 1997, we do not attribute the high concentrations observed in the summers of 1997 and 1998 directly to the acute climate anomalies. With the present data it is not possible to distinguish between increased production (control from below) and decreased predation (control from above) to explain the recent increase in concentrations of the species examined.
We surveyed 237 first-time college students to examine the extent to which social-cognitive factors--self-efficacy, perceptions of mentorship, high school GPA, ACT scores, first-semester college GPA, ...and demographic characteristics-- influence freshmen's intent to persist. Standard multiple regression and MANOVA were conducted to determine the influence of the selected characteristics on intended persistence. The findings show that college self-efficacy and perceptions of mentorship were the strongest predictors for intentions to persist past the first college semester, whereas ACT, GPA and socioeconomic status did not predict intent to persist. Implications for freshmen retention at 4-year institutions and directions for future research are discussed.
The interaction of the chaetognath Sagitta elegans with the copepod community of the southeast Bering Sea middle shelf was examined in relation to environmental conditions during 1995–1999. Predation ...impact was estimated for 2 years, 1995 and 1997, using gut content analysis, experimentally derived digestion time (DT) and abundances of chaetognaths and prey. Pseudocalanus concentrations correlated with water temperature and Calanus marshallae with sea ice extent. Sagitta elegans were less abundant but individuals were larger in 1995, when C. marshallae predominated, compared to 1997, when Pseudocalanus and Acartia were the primary prey. Predation by S. elegans removed <1% standing stock day−1 of Pseudocalanus or C. marshallae in 1995 and 1.7 to 2.3% of Pseudocalanus in 1997. The percent of the copepod community biomass required by chaetognaths was estimated to be <1% in 1995 compared with 8–12% in 1997. Calanus marshallae may be more vulnerable than Pseudocalanus to cumulative predation effects because of its reproductive strategy. The effect of chaetognath predation on the copepod community depends on which copepod species is predominant and its susceptibility to cumulative predation effects, as well as on daily predation impact, both of which varied between years with different climatic conditions.
We present a microfluidic centrifuge with no moving parts, relying on a vortex formed between two counterflowing liquid streams. The centrifuge is driven by streams with a speed of 0.6–2.6 m/s, ...resulting in accelerations applied to samples between 50 and 2,000
g
. The liquid flow in the centrifugation chamber and the transport of microparticles are visualized using epi-fluorescence microscopy and bright-field imaging with a high-speed camera. It is found that small particles follow the streamlines of the flow, whereas larger particles show a cross-stream migration. The size separation of different particles is demonstrated, and the experiments clearly indicate that as the flow speed increases, the particles in the vortex are increasingly driven outwards. Per construction, the centrifuge is ideally suited for handling small sample amounts and can be integrated with lab-on-a-chip systems.
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