44 research outputs found
Multipolar expansion of the electrostatic interaction between charged colloids at interfaces
The general form of the electrostatic potential around an arbitrarily charged
colloid at an interface between a dielectric and a screening phase (such as air
and water, respectively) is analyzed in terms of a multipole expansion. The
leading term is isotropic in the interfacial plane and varies with
where is the in--plane distance from the colloid. The electrostatic
interaction potential between two arbitrarily charged colloids is likewise
isotropic and , corresponding to the dipole--dipole interaction
first found for point charges at water interfaces. Anisotropic interaction
terms arise only for higher powers with .Comment: 6 pages, mathematical details adde
Charge renormalization for effective interactions of colloids at water interfaces
We analyze theoretically the electrostatic interaction of surface-charged
colloids at water interfaces with special attention to the experimentally
relevant case of large charge densities on the colloid-water interface. Whereas
linear theory predicts an effective dipole potential the strength of which is
proportional to the square of the product of charge density and screening
length, nonlinear charge renormalization effects change this dependence to a
weakly logarithmic one. These results appear to be particularly relevant for
structure formation at air-water interfaces with arbitrarily shaped colloids.Comment: 4 page
Transplantation of Encapsulated Dopamine-Secreting Cells as a Treatment for Parkinson's Disease
Warming Up Density Functional Theory
Density functional theory (DFT) has become the most popular approach to
electronic structure across disciplines, especially in material and chemical
sciences. Last year, at least 30,000 papers used DFT to make useful predictions
or give insight into an enormous diversity of scientific problems, ranging from
battery development to solar cell efficiency and far beyond. The success of
this field has been driven by usefully accurate approximations based on known
exact conditions and careful testing and validation. In the last decade,
applications of DFT in a new area, warm dense matter, have exploded. DFT is
revolutionizing simulations of warm dense matter including applications in
controlled fusion, planetary interiors, and other areas of high energy density
physics. Over the past decade or so, molecular dynamics calculations driven by
modern density functional theory have played a crucial role in bringing
chemical realism to these applications, often (but not always) with excellent
agreement with experiment. This chapter summarizes recent work from our group
on density functional theory at non-zero temperatures, which we call thermal
DFT. We explain the relevance of this work in the context of warm dense matter,
and the importance of quantum chemistry to this regime. We illustrate many
basic concepts on a simple model system, the asymmetric Hubbard dimer
Direct measurements of the effects of salt and surfactant on interaction forces between colloidal particles at water-oil interfaces
The forces between colloidal particles at a decane-water interface, in the
presence of low concentrations of a monovalent salt (NaCl) and of the
surfactant sodium dodecylsulfate (SDS) in the aqueous subphase, have been
studied using laser tweezers. In the absence of electrolyte and surfactant,
particle interactions exhibit a long-range repulsion, yet the variation of the
interaction for different particle pairs is found to be considerable. Averaging
over several particle pairs was hence found to be necessary to obtain reliable
assessment of the effects of salt and surfactant. It has previously been
suggested that the repulsion is consistent with electrostatic interactions
between a small number of dissociated charges in the oil phase, leading to a
decay with distance to the power -4 and an absence of any effect of electrolyte
concentration. However, the present work demonstrates that increasing the
electrolyte concentration does yield, on average, a reduction of the magnitude
of the interaction force with electrolyte concentration. This implies that
charges on the water side also contribute significantly to the electrostatic
interactions. An increase in the concentration of SDS leads to a similar
decrease of the interaction force. Moreover the repulsion at fixed SDS
concentrations decreases over longer times. Finally, measurements of three-body
interactions provide insight into the anisotropic nature of the interactions.
The unique time-dependent and anisotropic interactions between particles at the
oil-water interface allow tailoring of the aggregation kinetics and structure
of the suspension structure.Comment: Submitted to Langmui
Health information seeking on the Internet: a double divide? Results from a representative survey in the Paris metropolitan area, France, 2005–2006
<p>Abstract</p> <p>Background</p> <p>The Internet is a major source of information for professionals and the general public, especially in the field of health. However, despite ever-increasing connection rates, a digital divide persists in the industrialised countries. The objective of this study was to assess the determinants involved in: 1) having or not having Internet access; and 2) using or not using the Internet to obtain health information.</p> <p>Methods</p> <p>A cross-sectional survey of a representative random sample was conducted in the Paris metropolitan area, France, in the fall of 2005 (n = 3023).</p> <p>Results</p> <p>Close to 70% of the adult population had Internet access, and 49% of Internet users had previously searched for medical information. Economic and social disparities observed in online health information seeking are reinforced by the economic and social disparities in Internet access, hence a double divide. While individuals who reported having a recent health problem were less likely to have Internet access (odds ratio (OR): 0.72, 95% confidence interval (CI): 0.53–0.98), it is they who, when they have Internet access, are the most likely to search for health information (OR = 1.44, 95% CI = 1.11–1.87).</p> <p>Conclusion</p> <p>In the French context of universal health insurance, access to the Internet varies according to social and socioeconomic status and health status, and its use for health information seeking varies also with health beliefs, but not to health insurance coverage or health-care utilisation. Certain economic and social inequalities seem to impact cumulatively on Internet access and on the use of the Internet for health information seeking. It is not obvious that the Internet is a special information tool for primary prevention in people who are the furthest removed from health concerns. However, the Internet appears to be a useful complement for secondary prevention, especially for better understanding health problems or enhancing therapeutic compliance.</p