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Particle Suspensions in Complex Formulations

Papadopoulou, Anastasia; (2021) Particle Suspensions in Complex Formulations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Particle suspensions are ubiquitous in nature and engineering/manufacturing applications. Suspensions exhibit complex rheological phenomena even in the simplest cases of monodisperse rigid spheres in Newtonian media. To complicate matters further, industrial formulations, toothpaste being a typical example, often involve particles that vary in size, surface morphology and porosity, suspended in non-Newtonian and viscoelastic solvent media. Such systems entail new particle and polymer dynamics which are poorly understood, introducing new challenges in manufacturing and product stability. Experimental evidence considering the combined effects of these parameters on suspension rheology is limited. The present study attempts to address this gap by examining two types of non- colloidal, commercial silica particles, commonly used in toothpaste formulations and differing in surface area, roughness and porosity, suspended in various non-aqueous Newtonian and non-Newtonian solvents. Their suspension rheology was investigated under steady state and oscillatory shear at different volume fractions and compared to that of relatively smooth glass sphere suspensions with a similar size distribution, used as a control system and investigated under the same experimental and solution conditions. Particle surface roughness and porosity increased suspension viscosity and induced non-Newtonian rheological phenomena at lower particle volume fractions in both the Newtonian and non-Newtonian suspending media. This was due to the increased particle specific surface area and effective volume fraction, as the solvent gets absorbed into the pores, leading to enhanced particle-particle or particle-solvent interactions. Glycerol and mineral oil were first used as the suspending medium to probe two mechanisms for suspension shear thinning: a friction driven and an adhesion driven respectively. Suspensions in glycerol exhibited shear thinning at __ ≥ 0.25 due to the elastic deformation of the surface asperities at increasing shear leading to a reduction in the friction coefficient. In contrast, suspensions in mineral oil showed pronounced shear thinning and elasticity at __ ≥ 0.02 due to particle agglomeration; this was due to the agglomerates breaking down upon increasing the shear rate. The use of an optical shearing technique enabled the monitoring of particle deagglomeration in situ which was quantified using image analysis and aggregation metrics. Based on these findings, tuning of suspension shear thinning was demonstrated by inhibiting particle agglomeration through particle surface chemistry modification. To explore the effects of solvent elasticity and shear thinning on suspension rheology and provide links with industrially relevant formulations, the same particles were studied in two non-Newtonian solvents: a viscoelastic Boger fluid and a shear thinning and weakly viscoelastic Xanthan gum solution. Elastic thickening was induced for certain suspensions in the Boger fluid. The shear thinning nature of the Xanthan gum solution seemed to suppress this phenomenon giving rise to strong shear thinning response, compared to the same suspensions in the corresponding Newtonian solvents, especially in the dilute concentration regime. The effect of temperature on suspension rheology was also investigated by heating the suspensions to 60oC, a typical temperature that solids are added in industrial manufacturing. Heating significantly decreased suspension viscosity and suppressed the shear thickening response. The suppression of the shear thickening response was either due to a decrease in the possibility of hydroclustering in the Newtonian solvents or the decrease in the flexibility of the polymer chains at elevated temperature, requiring higher stresses to show strain hardening. In contrast, suspension shear thinning was found to both increase and decrease with temperature. The thesis offers new insights in tuning suspension rheology through the particle surface morphology and chemistry as well as the physical and chemical properties of the suspending media. Suspension rheology is complex and thus, understanding the mechanisms governing it, will aid manufacturers in addressing challenges during industrial processing and developing guidelines for the design and optimization of formulations tailored to specific applications. This work forms part of an EPSRC funded research programme on addressing manufacturing challenges of Future Formulations and in particular, non-aqueous paste formulations. It also involves the collaboration with leading companies in the pharmaceutical and inkjet printing fields (GlaxoSmithKline, Xaar Plc).

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Particle Suspensions in Complex Formulations
Event: UCL (University College London)
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10120433
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