Diffusion-Slip Boundary Conditions for Isothermal Flows in Micro- and Nano-Channels.

Continuum description of flows in micro- and nano-systems requires ad hoc addition of effects such as slip at walls, surface diffusion, Knudsen diffusion and others. While all these effects are derived from various phenomenological formulations, a sound theoretical ground unifying these effects and observations is still lacking. In this paper, adopting the definition and existence of various type of flow velocities beyond that of the standard mass velocity, we suggest derivation of model boundary conditions that may systematically justify various diffusion process occurring in micro- and nano-flows where the classical continuum model breaks down. Using these boundary conditions in conjunction with the classical continuum flow equations we present a unified derivation of various expressions of mass flow rates and flow profiles in micro- and nano-channels that fit experimental data and provide new insights into these flow profiles. The methodology is consistent with recasting the Navier-Stokes equations and appears justified for both gas and liquid flows. We conclude that these diffusion type of boundary conditions may be more appropriate to use in simulating flows in micro- and nano-systems and may also be adapted as boundary condition models in other interfacial flow modelling.

Effects of molecular diffusivity on shock-wave structures in monatomic gases.

We present a full investigation into shock-wave profile description using hydrodynamics models. We identified constitutive equations that provide better agreement for all parameters involved in testing hydrodynamic equations for the prediction of shock structure in a monatomic gas in the Mach number range 1.0-11.0. The constitutive equations are extracted from a previously derived thermomechanically consistent Burnett regime continuum flow model. The numerical computations of the resulting hydrodynamic equations along with classical ones are performed using a finite difference global solution (FDGS) scheme. Compared to previous studies that focused mainly on the density profile across the shock, here we also include temperature profiles as well as non-negativity of entropy production throughout the shock. The results obtained show an improvement upon those obtained previously in the bivelocity (or volume and mass diffusion) hydrodynamics and are more accurate than in the hydrodynamic models from expansions method solutions to the Boltzmann equation.

Cometary Comae-Surface Links: The Physics of Gas and Dust from the Surface to a Spacecraft.

A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading.

Predicting enhanced mass flow rates in gas microchannels using nonkinetic models.

Different nonkinetic approaches are adopted in this paper towards theoretically predicting the experimentally observed phenomenon of enhanced mass flow rates accompanying pressure-driven rarefied gas flows through microchannels. Our analysis utilizes a full set of mechanically consistent volume-diffusion hydrodynamic equations, allowing complete, closed-form, analytical solutions to this class of problems. As an integral part of the analysis, existing experimental data pertaining to the subatmospheric pressure dependence of viscosity were analyzed. The several nonkinetic approaches investigated were (1) pressure-dependent viscosity exponent model, (2) slip-velocity models, and (3) volume diffusion model. We explored the ability to predict the gas's mass flow rate over the full range of Knudsen numbers, including furnishing a physically sound interpretation of the well-known Knudsen minimum observed in the mass flow rate. Matching of a pressure-dependent viscosity model, one that follows the standard temperature-viscosity power law and its supporting single momentum diffusion mechanism, did not allow an accurate interpretation of the data. Rather, matching of this model with the flow rate was found to mismatch the experimental pressure dependence of the viscosity. An additional transport mechanism model, one based on volume diffusion, offered a comprehensive understanding of the Knudsen minimum, while also resulting in excellent agreement with experimental data well into the transition regime (up to a Knudsen number of 5).

Analysis of the thermomechanical inconsistency of some extended hydrodynamic models at high Knudsen number.

There are some hydrodynamic equations that, while their parent kinetic equation satisfies fundamental mechanical properties, appear themselves to violate mechanical or thermodynamic properties. This paper aims to shed some light on the source of this problem. Starting with diffusive volume hydrodynamic models, the microscopic temporal and spatial scales are first separated at the kinetic level from the macroscopic scales at the hydrodynamic level. Then, we consider Klimontovich's spatial stochastic version of the Boltzmann kinetic equation and show that, for small local Knudsen numbers, the stochastic term vanishes and the kinetic equation becomes the Boltzmann equation. The collision integral dominates in the small local Knudsen number regime, which is associated with the exact traditional continuum limit. We find a subdomain of the continuum range, which the conventional Knudsen number classification does not account for appropriately. In this subdomain, it is possible to obtain a fully mechanically consistent volume (or mass) diffusion model that satisfies the second law of thermodynamics on the grounds of extended non-local-equilibrium thermodynamics.

Time-delayed autosynchronous swarm control.

In this paper a general Morse potential model of self-propelling particles is considered in the presence of a time-delayed term and a spring potential. It is shown that the emergent swarm behavior is dependent on the delay term and weights of the time-delayed function, which can be set to induce a stationary swarm, a rotating swarm with uniform translation, and a rotating swarm with a stationary center of mass. An analysis of the mean field equations shows that without a spring potential the motion of the center of mass is determined explicitly by a multivalued function. For a nonzero spring potential the swarm converges to a vortex formation about a stationary center of mass, except at discrete bifurcation points where the center of mass will periodically trace an ellipse. The analytical results defining the behavior of the center of mass are shown to correspond with the numerical swarm simulations.

Molecular genetic studies of Anopheles stephensi in Pakistan.

Anopheles stephensi Liston s.l. (Diptera: Culicidae) is one of the major vectors of malaria in Pakistan, India, Iran and Afghanistan. In parts of its range this species has shown increases in both relative and absolute abundance in what is hypothesized to be a response to human-mediated environmental change resulting from extensive irrigation. We attempted to detect the molecular genetic signatures of this population instability based on three samples obtained from two villages (149/6R and 111/6R) within an irrigation zone in Punjab Province and from one village (Azakhel) outside the irrigation scheme in Northwest Frontier Province (NWFP), Pakistan, using seven microsatellite loci and 682 basepairs of the mitochondrial CO1 gene. For microsatellite loci, high levels of genetic diversity were observed within populations (mean alleles per locus 10.71-11.57; mean heterozygosity 0.703-0.733). Deviation from Hardy-Weinberg expectations was observed for only two microsatellite loci in 21 tests. No genetic differentiation was observed between populations and average pairwise F(ST) values did not differ significantly from zero for any population pair or either marker system. Tests of population expansion for both mitochondrial and microsatellite loci were inconclusive.

Surveillance of viral haemorrhagic fevers in ghana: entomological assessment of the risk of transmission in the northern regions.

SUMMARY OBJECTIVES: To assess the risk of transmission of viral haemorrhagic fevers in northern Ghana. DESIGN: A two-year cross-sectional entomological study was carried out in four communities in the northern part of Ghana. Standard WHO methods were used to collect adult and larvae of Aedes mosquitoes to estimate man-vector contact rates and larval indices. RESULTS: A total of 2804 households were surveyed to estimate larval indices and man-vector contacts of potential vectors of viral haemorrhagic fevers such as Yellow fever and Dengue. Over 56% households in each study site were positive for Aedes larvae. Relatively higher Breteaux index (BI) and Container index (CI) were estimated in Damongo (BI: 180 and CI: 44.8) and Jirapa (BI: 149.7 and CI: 41.5) compared to Tumu (BI: 76.1 and CI: 19.5) and Bolgatanga (BI: 72.4 and CI: 20.6). Man-biting rates of 9.8 and 18.5 bites /man/hour were estimated for Damongo and Jirapa respectively whilst Bolgatanga recorded 10 B/M/H. Generally, man-vector contact rates in all the study sites were higher during the dry season than the wet season. Larval indices showed seasonal variations and the dry season was identified as the high-risk period for transmission of viral haemorrhagic fevers and possible disease outbreaks. No flavivirus was detected in the 2034 Aedes mosquitoes from the study sites by RT-PCR. CONCLUSIONS: Aedes mosquito larval densities and adult biting rates, in all the study areas were sufficient to promote outbreaks of viral haemorrhagic fevers.

Lymphatic filariasis in Ghana: entomological investigation of transmission dynamics and intensity in communities served by irrigation systems in the Upper East Region of Ghana.

We conducted an entomological study to document the effect of irrigation on the vectors and transmission dynamics of lymphatic filariasis in the Upper East Region of Ghana. Mosquitoes were collected by indoor spraying of houses in a cluster of communities located around irrigation projects (Tono and Vea) and others without reservoirs (Azoka). Anopheles gambiae s.s. was the dominant species and major vector, followed by An. funestus. Anopheles arabiensis constituted 9--14% of the An. gambiae complex but none were infective. Culex quinquefasciatus was also not infective in these communities. Chromosomal examinations showed that >60% (n=280--386) of the An. gambiae s.s. in irrigated communities were Mopti forms whilst 73% (n=224) in the non-irrigated area were Savannah forms. Infectivity rates (2.3--2.8 vs. 0.25), worm load (1.62--2.04 vs. 1.0), annual bites per person (6.50--8.83 vs. 0.47) and annual transmission potential (13.26--14.30 vs. 0.47) were significantly higher in irrigated communities.

Delayed recovery of adrenocortical and testicular function after chemotherapy of human trypanosomiasis.

The following indicators of pituitary, adreno-cortical and testicular function were measured in 58 male African trypanosomiasis patients from Western Kenya; plasma cortisol, luteinizing hormone (LH) and testosterone levels. The measurements were carried out by specific radioimmunoassay methods in early and late stage infected patients on admission to hospital and in both groups of patients after one month of chemotherapy. Packed cell volume (PCV) and hemoglobin levels were also measured in all the patients to determine the extent of their anaemia and success of recovery. High parasitaemia, anaemia and clinical symptoms of human trypanosomiasis were found in the infected patients and were eliminated with chemotherapy in the infected/treated patients. Increased levels of cortisol and decreased concentrations of testosterone without significant changes in LH levels were evident in the infected patients; this condition remained unchanged even after one month of chemotherapy. Evidence is thus provided of persistent hormonal perturbations which probably indicate residual endocrine organ damage.