Viscosity of Colloidal Suspensions

Simple expressions are given for the Newtonian viscosity $\eta_N(\phi)$ as well as the viscoelastic behavior of the viscosity $\eta(\phi,\omega)$ of neutral monodisperse hard sphere colloidal suspensions as a function of volume fraction $\phi$ and frequency $\omega$ over the entire fluid range, i.e., for volume fractions $0 < \phi < 0.55$. These expressions are based on an approximate theory which considers the viscosity as composed as the sum of two relevant physical processes: $\eta (\phi,\omega) = \eta_{\infty}(\phi) + \eta_{cd}(\phi,\omega)$, where $\eta_{\infty}(\phi) = \eta_0 \chi(\phi)$ is the infinite frequency (or very short time) viscosity, with $\eta_0$ the solvent viscosity, $\chi(\phi)$ the equilibrium hard sphere radial distribution function at contact, and $\eta_{cd}(\phi,\omega)$ the contribution due to the diffusion of the colloidal particles out of cages formed by their neighbors, on the P\'{e}clet time scale $\tau_P$, the dominant physical process in concentrated colloidal suspensions. The Newtonian viscosity $\eta_N(\phi) = \eta(\phi,\omega = 0)$ agrees very well with the extensive experiments of Van der Werff et al and others. Also, the asymptotic behavior for large $\omega$ is of the form $\eta_{\infty}(\phi) + A(\phi)(\omega \tau_P)^{-1/2}$, in agreement with these experiments, but the theoretical coefficient $A(\phi)$ differs by a constant factor $2/\chi(\phi)$ from the exact coefficient, computed from the Green-Kubo formula for $\eta(\phi,\omega)$. This still enables us to predict for practical purposes the visco-elastic behavior of monodisperse spherical colloidal suspensions for all volume fractions by a simple time rescaling.Comment: 51 page

Sound-propagation gap in fluid mixtures

We discuss the behavior of the extended sound modes of a dense binary hard-sphere mixture. In a dense simple hard-sphere fluid the Enskog theory predicts a gap in the sound propagation at large wave vectors. In a binary mixture the gap is only present for low concentrations of one of the two species. At intermediate concentrations sound modes are always propagating. This behavior is not affected by the mass difference of the two species, but it only depends on the packing fractions. The gap is absent when the packing fractions are comparable and the mixture structurally resembles a metallic glass.Comment: Published; withdrawn since ordering in archive gives misleading impression of new publicatio

Dynamic structure factors of a dense mixture

We compute the dynamic structure factors of a dense binary liquid mixture. These describe dynamics on molecular length scales, where structural relaxation is important. We find that the presence of a few large particles in a dense fluid of small particles slows down the dynamics considerably. We also observe a deep narrowing of the spectrum for a disordered mixture composed of a nearly equal packing of the two species. In contrast, a few small particles diffuse easily in the background of a dense fluid of large particles. We expect our results to describe neutron scattering from a dense mixture

The role of area level social deprivation on childhood and adolescent consultation rate in primary care:a population based, cohort study

BACKGROUND: Studies show that children and adolescents in the most socially deprived areas (SDA) consult their general practitioner (GP) more often than those in the least socially deprived areas (Non-SDA). Given that GPs see a wide range of diseases, it is important to know which clinical diagnoses are shaped by socioeconomic factors. The primary objective was to determine the association between area level social deprivation and consultation rates in a pediatric population. The secondary objective was to explore this association across a wide range of clinical diagnoses. METHODS: A cohort study using the Rijnmond Primary Care Database (RPCD) was conducted. Between 2013 and 2020, a total of 69,861 patients aged 0 to 17 years registered with a GP were analysed. A consultation was defined as patient contact and entry of a diagnosis using the International Classification of Primary Care (ICPC-1) code. Associations between consultation rates, ICPC-1 codes and area level social deprivation were explored using a Poisson regression model. The incidence risk ratio (IRR) and 95% confidence interval (CI) were reported. RESULTS: Over the 7-year study period the consultation rate of the study population was 3.8 per person-years. The top 5 reasons for children and adolescents to consult their GP was related to skin, respiratory, general unspecified, musculoskeletal and digestive symptoms or diagnoses. Consultation rate was higher in SDA group compared to Non-SDA group (IRR 1.20, 95% CI 1.19–1.20). Consultation rate for ICPC-1 code related to pregnancy and family planning was significantly lower in SDA group compared to Non-SDA group. Upon further exploration of this code, SDA group were less likely to consult for oral contraception and more likely to contact a GP for induced termination of pregnancy compared to Non-SDA group (IRR 0.36; 95% CI 0.33–0.44 and IRR 2.94; 95% CI 1.58–5.46 respectively). CONCLUSIONS: Overall, SDA group had higher GP consultation rates for the majority of clinical diagnoses except for pregnancy and family planning. In this latter category, adolescent females in SDA consulted less frequently for oral contraception. This study illustrates the need to understand the underlying health seeking behaviors of children and adolescents at different development phases of their lives. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12875-022-01873-x

Theorem on the Distribution of Short-Time Particle Displacements with Physical Applications

The distribution of the initial short-time displacements of particles is considered for a class of classical systems under rather general conditions on the dynamics and with Gaussian initial velocity distributions, while the positions could have an arbitrary distribution. This class of systems contains canonical equilibrium of a Hamiltonian system as a special case. We prove that for this class of systems the nth order cumulants of the initial short-time displacements behave as the 2n-th power of time for all n>2, rather than exhibiting an nth power scaling. This has direct applications to the initial short-time behavior of the Van Hove self-correlation function, to its non-equilibrium generalizations the Green's functions for mass transport, and to the non-Gaussian parameters used in supercooled liquids and glasses.Comment: A less ambiguous mathematical notation for cumulants was adopted and several passages were reformulated and clarified. 40 pages, 1 figure. Accepted by J. Stat. Phy

Short-wavelength collective modes in a binary hard-sphere mixture

We use hard-sphere generalized hydrodynamic equations to discuss the extended hydrodynamic modes of a binary mixture. The theory presented here is analytic and it provides us with a simple description of the collective excitations of a dense binary mixture at molecular length scales. The behavior we predict is in qualitative agreement with molecular-dynamics results for soft-sphere mixtures. This study provides some insight into the role of compositional disorder in forming glassy configurations.Comment: Published; withdrawn since already published. Ordering in the archive gives misleading impression of new publicatio