Long-range repulsion of colloids driven by ion exchange and diffusiophoresis.

Interactions between surfaces and particles in aqueous suspension are usually limited to distances smaller than 1 µm. However, in a range of studies from different disciplines, repulsion of particles has been observed over distances of up to hundreds of micrometers, in the absence of any additional external fields. Although a range of hypotheses have been suggested to account for such behavior, the physical mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed experiments on a well-defined experimental system, using a setup that minimizes the effects of gravity and convection. Our experiments clearly indicate that the observed long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. We develop a simple model that accounts for our data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.

Convection associated with exclusion zone formation in colloidal suspensions.

The long-range repulsion of colloids from various interfaces has been observed in a wide range of studies from different research disciplines. This so-called exclusion zone (EZ) formation occurs near surfaces such as hydrogels, polymers, or biological tissues. It was recently shown that the underlying physical mechanism leading to this long-range repulsion is a combination of ion-exchange at the interface, diffusion of ions, and diffusiophoresis of colloids in the resulting ion concentration gradients. In this paper, we show that the same ion concentration gradients that lead to exclusion zone formation also imply that diffusioosmosis near the walls of the sample cell must occur. This should lead to convective flow patterns that are directly associated with exclusion zone formation. We use multi-particle tracking to study the dynamics of particles during exclusion zone formation in detail, confirming that indeed two pronounced vortex-like convection rolls occur near the cell walls. These dramatic flow patterns persist for more than 4 hours, with the typical velocity decreasing as a function of time. We find that the flow velocity depends strongly on the surface properties of the sample cell walls, consistent with diffusioosmosis being the main physical mechanism that governs these convective flows.

Case Report: Right atrial organized thrombus three years after tricuspid annuloplasty.

Background: Occurrence of right atrial masses, especially in patients with history of cardiac surgery, is rare. Differential diagnosis between malignant and non-malignant aetiologies might be cumbersome, and surgery is often required to prevent complications or disease evolution. Case: We report the case of a 16-year-old girl from a rural area of Sudan, who underwent surgery for a modified De Vega's tricuspid annuloplasty, and mitral and aortic valve replacement with mechanical prostheses. The patient was on regular follow-up but demonstrated a poor compliance to anticoagulation therapy with a time in therapeutic range between 52% and 20%. She remained asymptomatic, but a right atrial mass was diagnosed by transthoracic echocardiography during a follow-up visit 41 months after the first operation. Surgical removal of the mass revealed an organized thrombus arising from the point where the Prolene stitches for the tricuspid annuloplasty were previously passed. The patient recovered from surgery, was discharged home on post-operative day 10 and the first follow-up visit at 30 days after discharge confirmed a good clinical status and a normal transthoracic echocardiography (TTE). Conclusions: This case report describes the diagnostic and therapeutic work-out of a thrombus formation on the suture lines of a tricuspid annuloplasty. Moreover, it highlights the importance of a strict and long follow-up after valvular surgery and of the adherence to anticoagulation therapy, especially for patients living in rural areas of developing countries.

Experimental study of bent multimode optical waveguides.

Bend losses of bent multimode waveguides have been experimentally studied. The results experimentally verify a simple analytical expression based on ray optics that can be used to determine bend losses in bent multimode waveguides. We have also shown that bend-induced losses in such multimode optical waveguides are wavelength independent.