An experimental model of vitreous motion induced by eye rotations

Background:During eye rotations the vitreous humour moves with respect to the eye globe.This relative motion has been suggested to possibly have an important role in inducing degradation of the gel structure,which might lead to vitreous liquefaction and/or posterior vitreous detachment.Aim of the present work is to study the characteristics of vitreous motion induced by eye rotations.Methods:We use an experimental setup,consisting of a Perspex model of the vitreous chamber that,for simplicity,is taken to have a spherical shape.The model is filled with an artificial vitreous humour,prepared as a solution of agar powder and hyaluronic acid sodium salt in deionised water,which has viscoelastic mechanical properties similar to those of the real vitreous.The model rotates about an axis passing through the centre of the sphere and velocity measurements are taken on the equatorial plane orthogonal to the axis of rotation,using an optical technique.Results:The results show that fluid viscoelasticity has a strong influence on flow characteristics.In particular,at certain frequencies of oscillation of the eye model,fluid motion can be resonantly excited.This means that fluid velocity within the domain can be significantly larger than that of the wall.Conclusions:The frequencies for which resonant excitation occurs are within the range of possible eye rotations frequencies.Therefore,the present results suggest that resonant excitation of vitreous motion is likely to occur in practice.This,in turn,implies that eye rotations produce large stresses on the retina and within the vitreous that may contribute to the disruption of the vitreous gel structure.The present results also have implications for the choice of the ideal properties for vitreous substitute fluids.

A rheological method for selecting nano-kaolin powder as filler in SBR rubber

This paper shows that the stiffness ofstyrene-butadiene solid rubber with added kaolin powder is related to the yield stress of kaolin dispersion in liquid polybutadiene rubber up to the percolation threshold. For five kinds of kaolin powder, the value of τ° spans the range of 100-430 Pa, while the corresponding compressive elastic constant of SBR varies from 12 to 21 MPa. A relationship between τ° and △E^i^＊/ER^＊ is proposed. Critical examination of these data infers that kaolin powder dispersed in solid rubber matrix acts as an additive which decreases the random movement of the polybutadiene chains. Consequently, dispersions of nano-particles in liquid and in solid SBR are considered to be related, thus leading to a theological method for selecting nano-particles as fillers in solid SBR.

Yield stress measurements and microstructure of colloidal kaolin powders clusterized and dispersed in different liquids

Rheologicat measurements of four colloidal kaolin powders dispersed in water, paraffinic oil and liquid rubber have been done at solid concentration （9 （VS/VL） in the range 8-33%. In quasi-static conditions the yield stress τ° values were derived. An S-shaped relationship between τ° and （9 exists, that allows to evaluate the percolation threshold θc values. In water θc ranges between 29% and 33%, while it is between 17% and 21% in oil and in liquid rubber. In water the microstructure units, i.e. the primary clusters among kaolin crystallites, are denser and bigger than the ones in the two other liquids. Furthermore, in quasistatic condition, the aqueous microstructure units have less tendency to cling together to form a network that can span all over the liquid phase. The different kaolin dispersions at θ equal to 22% are characterized by yield stress ranging between 7.S Pa and 59 Pa in water, 66-250 Pa in oil and 230-770 Pa in liquid rubber. It has been found that for any kaolin types the log τ°/ θ evaluated near to the percolation threshold is a conservative parameter of the three different liquids. These results are useful to design colloidal dispersions with selected microstructure.