19 research outputs found
On the nature of long-range contributions to pair interactions between charged colloids in two dimensions
We perform a detailed analysis of solutions of the inverse problem applied to
experimentally measured two-dimensional radial distribution functions for
highly charged latex dispersions. The experiments are carried out at high
colloidal densities and under low-salt conditions. At the highest studied
densities, the extracted effective pair potentials contain long-range
attractive part. At the same time, we find that for the best distribution
functions available the range of stability of the solutions is limited by the
nearest neighbour distance between the colloidal particles. Moreover, the
measured pair distribution functions can be explained by purely repulsive pair
potentials contained in the stable part of the solution.Comment: 6 pages, 5 figure
Direct measurement of three-body interactions
Three-body interactions have been measured among three charged colloidal
particles in deionized solvent. Two of the particles have been confined to an
optical line-trap while the third one was approached by means of a focused
laser beam. The experimentally determined three-body interactions are
attractive and roughly of the same magnitude and range as the
pair-interactions. In addition, numerical calculations have been performed,
which show good agreement with the experimental results
Three-body interactions in colloidal systems
We present the first direct measurement of three-body interactions in a
colloidal system comprised of three charged colloidal particles. Two of the
particles have been confined by means of a scanned laser tweezers to a
line-shaped optical trap where they diffused due to thermal fluctuations. Upon
the approach of a third particle, attractive three-body interactions have been
observed. The results are in qualitative agreement with additionally performed
nonlinear Poissson-Boltzmann calculations, which also allow us to investigate
the microionic density distributions in the neighborhood of the interacting
colloidal particles
The osmotic pressure of charged colloidal suspensions: A unified approach to linearized Poisson-Boltzmann theory
We study theoretically the osmotic pressure of a suspension of charged
objects (e.g., colloids, polyelectrolytes, clay platelets, etc.) dialyzed
against an electrolyte solution using the cell model and linear
Poisson-Boltzmann (PB) theory. From the volume derivative of the grand
potential functional of linear theory we obtain two novel expressions for the
osmotic pressure in terms of the potential- or ion-profiles, neither of which
coincides with the expression known from nonlinear PB theory, namely, the
density of microions at the cell boundary. We show that the range of validity
of linearization depends strongly on the linearization point and proof that
expansion about the selfconsistently determined average potential is optimal in
several respects. For instance, screening inside the suspension is
automatically described by the actual ionic strength, resulting in the correct
asymptotics at high colloid concentration. Together with the analytical
solution of the linear PB equation for cell models of arbitrary dimension and
electrolyte composition explicit and very general formulas for the osmotic
pressure ensue. A comparison with nonlinear PB theory is provided. Our analysis
also shows that whether or not linear theory predicts a phase separation
depends crucially on the precise definition of the pressure, showing that an
improper choice could predict an artificial phase separation in systems as
important as DNA in physiological salt solution.Comment: 16 pages, 5 figures, REVTeX4 styl
Triplet correlations in two-dimensional colloidal model liquids
Three-body distribution functions in classical fluids have been theoretically
investigated many times, but have never been measured directly. We present
experimental three-point correlation functions that are computed from particle
configurations measured by means of video-microscopy in two types of
quasi-two-dimensional colloidal model fluids: a system of charged colloidal
particles and a system of paramagnetic colloids. In the first system the
particles interact via a Yukawa potential, in the second via a potential
. We find for both systems very similar results: on increasing
the coupling between the particles one observes the gradual formation of a
crystal-like local order due to triplet correlations, even though the system is
still deep inside the fluid phase. These are mainly packing effects as is
evident from the close resemblance between the results for the two systems
having completely different pair-interaction potentials.Comment: many pages, 8 figures, contribution to the special issue in J.Phys.
Cond. Mat. of the CECAM meeting in LYON ''Many-body....'
Wenn drei Körper mehr sind als drei Paare
Das Superpositionsprinzip ist ein zentrales Konzept der Physik. So lassen sich mit seiner Hilfe mehrere gleichzeitig an ein Teilchen angreifende Kräfte zu einer einzigen effektiven Kraft zusammenfassen, was Probleme oft wesentlich vereinfacht. Doch nicht immer genügt es, die Gesamtwechselwirkung eines Systems als Summe von Paarwechselwirkungen aufzufassen. Es gibt durchaus Fälle, wo die Wechselwirkung eines Paares durch die räumliche Nähe weiterer Teilchen beeinflusst wird, sodass Kräfte eben nicht mehr paarweise superponierbar sind. Solche Mehrkörper-Wechselwirkungen haben oft ganz konkrete physikalische Effekte zur Folge. Dreikörper-Wechselwirkungen sind nun in Kolloidsuspensionen das erste Mal direkt vermessen worden.publishe
Entropische Kräfte : warum sich repulsiv wechselwirkende Teilchen anziehen können
Entropie, eine anschaulich schwer faßbare Größe, kann in Vielteilchensystemen zu direkt beobachtbaren Kräften führen. Taucht man zwei große harte Kugeln in ein Bad aus kleinen harten Kugeln, so läßt sich eine effektive Kraft zwischen beiden großen Kugeln feststellen, die anziehend ist, und dies, obwohl keine attraktiven Paarwechselwirkungen existieren. Schon 1954 vorhergesagt, lassen sich nun solche entropischen Kräfte direkt nachweisen und vermessen. Sie sind in der statistischen Physik und der Biophysik von grundlegender Bedeutung, etwa für die Entmischung binärer Hartkugelmischungen oder die Koagulation roter Blutkörperchen
Phase transitions in two-dimensional colloidal systems
This chapter is an introduction to phase transitions in two-dimensional (2D) systems. In contrast to three dimensions (3D), microscopic theories of melting exist in 2D. The most well known of them was developed more than 30 years ago by Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY theory). This theory predicts the unbinding of topological defects to break the symmetry in two steps at two distinct temperatures. Dissociation of dislocation pairs first melts the crystal into a still orientationally ordered (hexatic) phase and, in the second step, dissociation of free dislocations causes the system to go over to an isotropic fluid. Colloidal systems are used to verify experimentally the predictions of KTHNY theory in detail as colloids provide the possibility to visualize the change in symmetry on an "atomic" level by simple video-microscopy. Elastic moduli like Young's modulus and Frank's constant are deduced from microscopic trajectories of colloids in order to quanify the softening of the 2D ensemble in the vicinity of the phase transitions