2,944 research outputs found
Polyelectrolyte-colloid complexes: polarizability and effective interaction
We theoretically study the polarizability and the interactions of neutral
complexes consisting of a semi-flexible polyelectrolyte adsorbed onto an
oppositely charged spherical colloid. In the systems we studied, the bending
energy of the chain is small compared to the Coulomb energy and the chains are
always adsorbed on the colloid. We observe that the polarizability is large for
short chains and small electrical fields and shows a non-monotonic behavior
with the chain length at fixed charge density. The polarizability has a maximum
for a chain length equal to half of the circumference of the colloid. For long
chains we recover the polarizability of a classical conducting sphere. For
short chains, the existence of a permanent dipole moment of the complexes leads
to a van der Waal's-type long-range attraction between them. This attractive
interaction vanishes for long chains (i.e., larger than the colloidal size),
where the permanent dipole moment is negligible. For short distances the
complexes interact with a deep short-ranged attraction which is due to
energetic bridging for short chains and entropic bridging for long chains.
Exceeding a critical chain length eventually leads to a pure repulsion. This
shows that the stabilization of colloidal suspensions by polyelectrolyte
adsorption is strongly dependent on the chain size relative to the colloidal
size: for long chains the suspensions are always stable (only repulsive forces
between the particles), while for mid-sized and short chains there is
attraction between the complexes and a salting-out can occur.Comment: 13 pages, 14 figure
In memoriam: Celso-Ramon Garcia, M.D. (1922–2004), reproductive medicine visionary
This article traces the career of Celso-Ramon Garcia (1922–2004), noted physician, educator, and internationally renowned pioneer in the field of reproductive endocrinology. His work helped to formulate oral contraceptives used by millions of women throughout the world. Garcia's research collaborators included Gregory Pincus and John Rock, who together finalized the landmark clinical data needed to secure initial FDA approval for "the pill" in 1960. In addition to Garcia's monumental work in contraceptive endocrinology, his scholarly interests encompassed physiology of the menopause, minimally invasive reproductive surgery, as well as psychological aspects of infertility. Closely identified with the University of Pennsylvania, Garcia was instrumental in establishing the first formal clinical program in reproductive biology and influenced countless young scientists whose training he supervised and mentored. His distinguished career was emblematic of the best of the medical profession, characterized by compassion, intellect, and a sincere desire to help others. Our manuscript outlines Garcia's wide range of interests, acknowledges his superior fund of knowledge, and honors his humanitarian spirit – all of which contributed to an impressive legacy of medical discoveries. The impact of Prof. Garcia's work will continue to be felt for many years
Counterion density profiles at charged flexible membranes
Counterion distributions at charged soft membranes are studied using
perturbative analytical and simulation methods in both weak coupling
(mean-field or Poisson-Boltzmann) and strong coupling limits. The softer the
membrane, the more smeared out the counterion density profile becomes and
counterions pentrate through the mean-membrane surface location, in agreement
with anomalous scattering results. Membrane-charge repulsion leads to a
short-scale roughening of the membrane.Comment: 4 pages, 4 figure
Singularity results for functional equations driven by linear fractional transformations
We consider functional equations driven by linear fractional transformations,
which are special cases of de Rham's functional equations. We consider
Hausdorff dimension of the measure whose distribution function is the solution.
We give a necessary and sufficient condition for singularity. We also show that
they have a relationship with stationary measures.Comment: 14 pages, Title changed, to appear in Journal of Theoretical
Probabilit
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