Rheology of Dispersions

Table of Contents

Introduction
Types of Dispersions
Understanding the Stability of Dispersions
     Zero-Sheer Viscosity
     Elastic Strength and Cohesive Energy
     Particle-Particle Interactions
     Conclusion

Introduction

A dispersion is a non-homogenous system of two immiscible phases in which the dispersed phase is finely distributed in the continuous dispersion medium. Rheology is the study and control of the flow properties1,2. Understanding the principles that govern the flow characteristics of the dispersions is essential for formulating a dispersion and its long-term stability1,2.

Types of Dispersions

For example, emulsions occur as oil in water (o/w) emulsions or water in oil (w/o) emulsions. The relative amounts of the phases within the dispersion system determines their role as a dispersed phase or as a dispersed medium. In the o/w emulsions, the oil droplets are dispersed in the relatively larger volume of aqueous dispersion medium, and vice versa in the case of w/o emulsions. While emulsions are a type of liquid in liquid dispersions, suspensions are a solid in liquid dispersions wherein insoluble solid particles are suspended in a liquid medium. Other examples of dispersions include foams and geld. Rheological measurements obtained by instruments such as a rheometer can be used to predict the stability of dispersions without diluting or disturbing the structure.

Understanding the Stability of Dispersions

Zero-Sheer Viscosity

When the dispersions are at rest, gravitational force is the only factor that determines the settling of particles3. Increasing the viscosity of dispersions to a certain extent prevents the settling of particles over time. Zero-sheer viscosity is defined as the viscosity exhibited by the dispersion system when the sheer rate is approaching zero3. Therefore, increasing the zero-sheer viscosity increases the stability of the dispersions. It should be noted that the highest zero-sheer viscosity possible would be infinity. This yield stress type behavior makes the dispersion act like a solid at rest and prevents the sedimentation of the particles3.

Elastic Strength and Cohesive Energy

The elastic strength of the material’s internal structure also influences the stability of the dispersions. Cohesive energy is in correlation with the yield stress and describes the measurement of the elastic strength of the internal structure of the dispersion. An amplitude sweep test can be performed to assess the cohesive energy in the dispersion system3. The stability of the system is greater in dispersions with higher cohesive energy. A frequency sweep analysis can be utilized to determine the viscoelastic spectrum of the dispersion and assess its behavior over different time scales. Due to the liquid-like behavior exhibited with increase in phase angles under low frequencies, viscoelastic liquids tend to be less stable3. The liquid-like behavior allows for better flow, but the particles in the dispersion settle down at rest or when they are left undisturbed. Therefore, dispersion systems containing gel structures with a phase angle below 45° independent of the frequency show greater stability over prolonged period of time3.

Particle-Particle Interactions

Another important factor that influences the stability of the dispersions is the number of the particle-particle interactions3. These inter-particular interactions can be increased with increase in the number of particles in the dispersion. Smaller particles are subjected to lesser gravitational force3. Therefore, reducing the particle size of the dispersed phase or increasing the particle concentration in the non-homogenous dispersion systems would allow for decreased sedimentation of the particles3. Dispersion systems with a narrow particle size distribution tend to be more stable as compared to dispersions with wider particle size distribution because the wider particle size distribution allows for better packing and easier flow of the system3. Therefore, the stability of the dispersion can be improved by narrowing the particle size distribution. This is an important factor to be considered in conditions where the particle concentration in the dispersion cannot be altered.

Conclusion

Taken together, control of the rheological parameters is essential for the long-term stability of multi-phase dispersion systems1,2. Various rheological tests can be performed to assess the parameters that govern the flow as well as stability. Taking advantage of these tests and adjusting the rheological parameters accordingly would allow for better design and stability of the different types of dispersion systems.

References:

  1. “Rheology of Dispersions. Principles and Applications” – satPRnews
  2. “Rheology of Dispersions – Principles and Applications” – Tharwat F. Tadros
  3. “Multiple Ways to Optimize Rheology for Increased Dispersion, Colloidal and Emulsion Stability”

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