Keywords: Rheo-Impedance Spectroscopy, emulsions, Pickering
RH161
Abstract
Rheo-impedance spectroscopy is a powerful analytical technique that can be used to explore instabilities in complex fluids, such as emulsions. Many emulsions can suffer from phase separation under processing conditions, a complex instability that can be difficult to characterize. While traditional rotational rheology can detect flow anomalies under shear, it cannot definitively confirm whether shear-induced phase separation is occurring or accurately determine the critical shear rates where these instabilities begin. In an oil-in-water emulsion, there is a distinct contrast between the conductive aqueous continuous phase and the insulating dispersed oil droplets. By exploiting this difference, rheo-impedance spectroscopy provides a new level of insight into shear-induced phase separation.
This study investigates the shear stability of different oil-in-water emulsions through rheology and rheo-impedance spectroscopy. The formulations are characterized as oil-in-water emulsions comprising a dispersed oil phase greater than 30% oil and several percent hydrophobically modified silica by weight, stabilized by a hybrid system of traditional emulsifiers and yield polymers. The sample set within this study encompassed slight variations within this general formulation space. It was hypothesized that under varying conditions of temperature and shear, the hydrophobically modified silica particles contribute to the destabilization of the emulsions.
Emulsions are colloidal systems consisting of two immiscible liquids, typically oil and water. They are ubiquitous in industrial applications, ranging from pharmaceuticals and food science to agrochemicals and personal care products. As these systems are thermodynamically unstable, stabilizing agents are used to maintain homogeneity and mitigate phase separation. Stabilizing agents can include amphiphilic surfactant molecules that adsorb at an interface to reduce interfacial tension, thus facilitating mixing and emulsification while providing electrostatic or steric repulsion to inhibit coalescence. Another distinct class of stabilization is achieved using solid colloidal particles, resulting in what are known as Pickering emulsions.
The wettability of these solid colloidal particles dictates which emulsion configuration is more thermodynamically stable. It determines whether the system favors oil-in-water emulsions, where oil droplets are dispersed in a continuous aqueous phase, or water-in-oil emulsions, where water droplets are dispersed in oil. When particles are hydrophilic (θ < 90°), they are preferentially wetted by the aqueous phase and naturally favor the formation of oil-in-water emulsions. Conversely, when particles are hydrophobic (θ > 90°), they are preferentially wetted by the oil phase and therefore favor water-in-oil emulsions.
For the emulsions in this study, hydrophobically modified silica particles are employed within an oil-in-water emulsion to impart specific sensory properties. However, this combination creates an inherent instability, as the hydrophobic particles possess a thermodynamic drive to destabilise the interface. Consequently, the system depends on emulsifiers and yield polymers for stability.
The Hypothesized Mechanism of Instability
The emulsion formulations in this study undergo semi-phase separation under shear. The hypothesized failure mode is best understood by examining the breakdown of the formulation’s two primary stabilization mechanisms:
- The Bulk Polymer Network – The continuous aqueous phase contains a yield polymer that effectively immobilizes the dispersed oil droplets.
- Amphiphilic Emulsifiers – At the oil-water interface, a composite layer of amphiphilic emulsifiers provides steric repulsion. This layer creates a thermodynamic barrier that helps to prevent the droplets from making close contact.
The transition from stable state to phase separation is triggered when the system is subjected to sufficient shear stress. It is hypothesized that the applied shear disrupts the yield polymer network, removing the physical immobilization of the droplets. Simultaneously, these shear forces can overcome the short-range steric barrier provided by the emulsifiers, forcing droplets into close proximity.
Once the steric barrier is breached, the partially wetted oleophilic silica particles play a detrimental role. Silica particles adsorbed on the surface of one droplet can now interact strongly with the surface of a neighboring droplet. Effectively, the silica particles act as a localized magnet, initiating a solid-particle bridge between the interfaces and creating a pathway for droplet fusion. As the silica particles are hydrophobic in this study, their contact angle is greater than 90°, meaning most of the particle resides in the oil phase, as seen in Figure 3C. When another oil droplet comes into contact with this particle, the oil from that droplet attempts to satisfy the same contact angle. This interaction effectively pulls the new interface to the interior of the original droplet, rapidly accelerating coalescence. Furthermore, by merging multiple droplets into a larger volume, the system minimizes the high surface energy associated with the exposed, partially wetted particle-fluid interface. This drive to satisfy the contact angle and reduce the total surface area leads to coalescence, ultimately manifesting as macroscopic semi-phase separation.
A TA Instruments™ Discovery™ HR 10 Rheometer equipped with a 40 mm crosshatch geometry was used to perform controlled rate viscosity profiles, to serve as a fundamental first step in understanding the flow behavior of the emulsion formulations under shear.
Testing revealed an anomaly for all samples at specific shear rates where the measured shear stress deviates from the expected monotonic increase. Instead of a smooth curve, the data exhibits erratic fluctuations in the stress response. This erratic stress response remains as an indirect indication of something being wrong, with rheological techniques alone not being able to definitively confirm any microstructural changes as a result of the applied shear.
To gain insights into microstructural changes beyond what rheology alone could provide, we utilized rheo-impedance spectroscopy. This technique essentially uses electricity to further probe the microstructure of a material. While impedance spectroscopy in isolation is typically limited to static measurements, its integration with Discovery Hybrid Rheometers by TA Instruments allows for characterization under dynamic shear and temperature conditions.
An alternating current is applied across the sample geometry, and the instrument measures impedance, which is a measure of the system’s opposition to the flow of alternating current. In the context of complex fluids, a sample exhibiting high impedance indicates a more insulating microstructure, whereas lower impedance suggests a microstructure that facilitates current flow.
Traditional rheo-impedance configurations often require physical contacts connected to the geometry. These physical connections generate friction, which limits the instrument’s measurement range. To mitigate this, the Rheo-Impedance setup by TA Instruments utilizes both electrodes located on the stationary bottom plate, separated by a thin insulator. The upper geometry then serves as a bridge to close the circuit. As the upper tool requires no physical wiring, the measurement remains friction-free, preserving the inherent torque sensitivity of the rheometer.
The motivation for using rheo-impedance spectroscopy in this study is derived from the electrical properties of the components of the emulsion. The continuous water phase conducts electricity well, while the dispersed oil droplets act as an electrical insulator. Therefore, the measured impedance is directly related to the continuity of the water phase. As previously discussed, as shearinduced phase separation occurs, the insulating oil droplets grow in size. This forms a continuous insulating barrier within the sample, which would manifest as a large increase in the impedance of the sample.
To confirm this hypothesis, a stable control sample, known to remain homogenous under the applied shear rates, was also tested under wider shear conditions. This comparison allows us to confidently attribute any impedance increases in the sample set to shearinduced destabilization, rather than any experimental artefacts.
In this study, samples were subjected to a series of shear rate peak holds ranging from 0.1 to 10 s-1. To evaluate temperature dependency, this protocol was replicated at both 25 and 60 °C; however, testing at 60 °C was extended to include higher shear rates up to 100 s-1 to capture a broader range of instability onset. During each peak hold, voltage was set to 0.1V and frequency was swept logarithmically from 100 Hz to 8 MHz.
Samples A, B, and C exhibited a sharp step change in impedance at different shear rates, further highlighted in Table 1. This point marks the precise onset of instability, interpreted as the moment the steric barrier is breached and silica-particle bridging initiates. Following this initial onset, impedance continued to increase with increasing shear. This behavior suggests continuous droplet growth within the microstructure, where localized bridging events propagate into droplet coalescence and semi-phase separation. In contrast, the positive control maintained a stable impedance profile across a wider shear rate range, up to 100 s-1.
When testing was replicated at 60 °C, the onset of instability shifted to higher shear conditions for samples A and B. This indicates that at elevated temperatures, the emulsion network requires high shear forces to induce the bridging-dewetting event. Interestingly, Sample C showed no large increase in impedance up to 100 s-1, signifying no observable instability at this elevated temperature.
Table 1. Onset point of instability metrics
| Sample | Shear Rate of Instability Onset (s-1) | |
|---|---|---|
| 25°C | 60°C | |
| Sample A | 1 | 5 |
| Sample B | 5 | 7.5 |
| Sample C | 7.5 | – |
| Positive Control | >– | >– |
The investigation into the shear stability of Pickering emulsions in this study revealed that while rotational rheology alone indicated flow instability, it could not provide direct insights into whether phase separation was occurring. However, through the application of rheo-impedance spectroscopy, the distinct increase in impedance under shear offered compelling evidence of the formation of an insulating, coalesced oil phase. This data supported the hypothesis of the failure mode and validated rheoimpedance spectroscopy as a power characterization technique, capable of uncovering microstructural insights under shear and temperature changes that rheology alone is not sensitive enough to identify. Furthermore, the technique successfully mapped the onset of instability across different thermal conditions, highlighting the temperature-dependency of the shear-induced instability.
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Acknowledgement
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This paper was written by Wasif Altaf, Applications Scientist and Technical Marketer at Centre for Industrial Rheology, Jeffrey Martin, Principal Scientist at Kenvue, and Neil Cunningham, Founder and CEO of Centre for Industrial Rheology.
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