Mechanical Properties of Colloidal Gels

A colloidal suspension can be either dispersed or flocculated depending on the interaction between the colloidal particles. If the interaction is repulsive, particles can relax to the minimum of the potential due to their neighboring particles, and the system can reach an equilibrium dispersed state. In the case of attractive interaction, particles form aggregates that settle to the bottom of the container. As the concentration of particles is increased, the overcrowding of the aggregates produces a continuous network throughout the suspension before they settle and a colloidal gel is formed. A major difference between a colloidal gel and a colloidal suspension is that the gel can sustain finite stress and is therefore viscoelastic. Previously we studied the storage modulus and the yield strain of boehmite gels and found that they are related to the particle concentration in a power-law fashion [1]. Similar scaling behavior of the shear modulus was found for other colloidal particulate networks by Buscall et al. [2]. We developed a scaling theory [1] which successfully explains the experimental results on boehmite gels. The theory further predicts that there can be two types of power-law behavior depending on the relative elastic strength of the clusters to that of the links between clusters within the gel network. Furthermore, there can be a crossover from one type of behavior to the other as the particle concentration is varied.

Hepatitis A virus infection in Taipei in 1999

BACKGROUND AND PURPOSE

Hepatitis A is a disease that is heavily affected by sanitation status. Hepatitis A is much less prevalent compared with decades ago in Taiwan, as in many rapidly developing regions. Hepatitis A vaccine is still self-paid under the National Health Insurance program and is still not widely utilized by the general public in Taiwan. This seroepidemiologic study evaluated the prevalence of antihepatitis A virus (anti-HAV) seropositivity in Taipei in 1999.

METHODS

A total of 1017 serum samples from healthy inhabitants in Taipei were examined for anti-HAV antibody by qualitative enzyme immunoassay.

RESULTS

The overall seroprevalence rate was 25.2% (255/1013) in the nonvaccinated population. The seropositivity rate for anti-HAV antibody among children younger than 12 months old was 23.3%. The rates dropped to between 1% and 4.8% among subjects between 1 and 20 years of age. A markedly higher rate of 40% was observed in subjects aged between 20 and 30 years. The seropositivity rate in subjects aged 31 to 50 was 80%. More than 90% of subjects older than 50 years were seropositive. The vaccination rate was low (0.5%).

CONCLUSION

Our findings indicate that Taipei is an area of intermediate endemicity for hepatitis A virus. To achieve better herd immunity, a more active approach to the adoption of hepatitis A vaccine is warranted.

Monte Carlo simulations of the nucleation and growth process of colloidal particles

We have examined the effect of the total initial monomer concentration and that of the monomer-monomer attraction energy on the nucleation and growth process of colloidal particles using a reversible aggregation model (Shih-Aksay-Kikuchi model) with the Monte Carlo method. We showed that the equilibrium monomer concentration c(e) exhibited a peak with respect to the total initial monomer concentration c(t). Furthermore, the solution may be divided into three regimes with respect to c(t). In the first regime where the initial monomer concentration was low, all monomers remained as individual monomers in the solution and c(e) increased linearly with c(t). In the second regime where small clusters of monomers began to form, c(e) underwent a peak with respect to c(t). In the third regime where large particles form, c(e) slowly decreased with c(t). Moreover, with increasing monomer-monomer attraction energy, the peak in c(e) moved to a lower c(t) and became sharper. The equilibrium monomer concentration surrounding a particle with respect to particle size was shown to agree with the Kelvin equation, indicating that the model can indeed capture the equilibrium solution physics involving colloidal particles. The peak exhibited in c(e) versus c(t) was manifested as a peak in the monomer concentration versus time under conditions where monomers were gradually fed to the solution. The present simulation is a simulation model for illustrating a peaked solute concentration with respect to time first proposed by LaMer and Dinegar. We further showed that the supersaturation peak in the monomer concentration versus time depended on the feeding rate. The peak height increased with an increasing feeding rate.

Pneumoconiosis and pulmonary function defects in silica-exposed fire brick workers

This study was conducted to investigate the health effects of exposure to silica dust in firebrick manufacturing plants. A cross-sectional study was designed, and the health effects in 526 exposed workers were compared with 164 nonexposed control workers. The medical examination included chest x-rays, pulmonary function tests, and a respiratory symptom questionnaire. Radiological evidence of pneumoconiosis was evident in 6.9% of the exposed firebrick workers, and the prevalence of pneumoconiosis increased with duration of employment. No differences in forced vital capacity were noted between the exposed and control groups; however, in both smokers and nonsmokers of the exposed group, forced expiratory flow in 1 s/forced vital capacity, mean maximal expiratory flow, forced expiratory flow at 50% vital capacity, and forced expiratory flow at 75% vital capacity were significantly lower than in the control group. Pulmonary function decreased as the duration of employment increased. The exposed group experienced significantly more wheezing, compared with the control group. There was evidence of a dose-response relationship between prevalence of pneumoconiosis/pulmonary function defects and estimated dust exposure levels. Pneumoconiosis and pulmonary function defects occurred frequently in workers who participated in the crushing and burning operations, both of which cause exposure to high levels of silica dust. We concluded that exposure to silica dust in workers at firebrick factories leads to radiological changes, pneumoconiosis, and pulmonary function damage.