Abstract
Understanding the sedimentation behaviour of colloidal suspensions is crucial in determining their stability. Since sedimentation rates are often very slow, centrifugation is used to expedite sedimentation experiments. The effect of centrifugal acceleration on sedimentation behaviour is not fully understood. Furthermore, in sedimentation models, interparticle interactions are usually omitted by using the hard-sphere assumption. This work proposes a one-dimensional model for sedimentation using an effective maximum volume fraction, with an extension for sedimentation under centrifugal force. A numerical implementation of the model using an adaptive finite difference solver is described. Experiments with silica suspensions are carried out using an analytical centrifuge. The model is shown to be a good fit with experimental data for 480 nm spherical silica, with the effects of centrifugation at 705 rpm studied. A conversion of data to Earth gravity conditions is proposed, which is shown to recover Earth gravity sedimentation rates well. This work suggests that the effective maximum volume fraction accurately captures interparticle interactions and provides insights into the effect of centrifugation on sedimentation.
Highlights
Suspensions of colloidal particles can be found in many industrial applications, such as consumer products, surface coatings, and printer inks.1,2 Examples of such consumer products include fabric enhancers, cosmetics, and shampoos.3 In many of these applications, the dispersed phase has a higher density than the continuous phase
Phase separation can occur when the dispersed phase is less dense than the continuous phase, with the dispersed phase rising to the top in a process known as creaming
Metin23 has demonstrated the use of an effective particle size for describing colloidal suspensions, but taking into account the interparticle interactions using an effective maximum volume fraction, which has the advantage of remaining constant for a given suspension
Summary
Suspensions of colloidal particles can be found in many industrial applications, such as consumer products, surface coatings, and printer inks. Examples of such consumer products include fabric enhancers, cosmetics, and shampoos. In many of these applications, the dispersed phase has a higher density than the continuous phase. A common assumption made is the hard-sphere assumption, an idealisation which assumes no interparticle interactions other than infinite repulsion at contact.19,20 This means that there is no restriction on the minimum separation distance between two colloidal particles, so the maximum volume fraction is determined by random sphere packing theory in this case. Metin has demonstrated the use of an effective particle size for describing colloidal suspensions, but taking into account the interparticle interactions using an effective maximum volume fraction, which has the advantage of remaining constant for a given suspension. Hindered settling effects are taken into account in a one-dimensional sedimentation model using the suspension viscosity, rather than a hindrance function like the Richardson-Zaki relationship. By introducing an effective maximum volume fraction, a method which has been successfully validated for describing the shear rheology of colloidal suspensions in the past, interparticle interactions are taken into account in the model. An investigation into the effects of centrifugation on sedimenting suspensions is described, which leads to the validation of a proposed method for successfully converting centrifugal force sedimentation data to Earth gravity conditions and an improved understanding of the behaviour of colloidal suspensions under centrifugation
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