Double Wall Ring Geometry for Measuring Interfacial Rheology in Food Emulsions

Accurate characterization of interfacial rheological properties is critical in understanding the stability and structure of food emulsions. Traditional geometries such as the Du Noüy ring and the bi-cone present limitations, including undefined sub-phase contributions and poor interface pinning, respectively. The Double Wall Ring (DWR) geometry offers significant improvements in sensitivity and data accuracy. This application note demonstrates the advantages of DWR in evaluating two common emulsifiers: monoglycerides and sodium caseinate, using interfacial rheological measurements. Frequency sweeps, time sweeps, and strain sweeps were conducted at various concentrations. The results highlight improved resolution, reduced sub-phase influence, and capability to study interfacial structure formation dynamics with the DWR setup.

Interfacial rheology provides insight into the behavior of emulsions, which are multiphase systems widely used in the food industry. Emulsifiers play a key role in stabilizing these systems by forming viscoelastic films at the interface between immiscible phases. The accurate measurement of these interfacial properties requires specialized geometries.

Traditional devices like the Du Noüy ring and bi-cone geometries suffer from limitations. The Du Noüy ring often fails to pin the interface reliably and has undefined contributions from the liquid within the ring. The bi-cone geometry, while more accurate in defining shear rate, suffers from significant bulk contributions. In contrast, the Double Wall Ring (DWR) geometry was developed to overcome these shortcomings [1]. It combines the benefits of both earlier designs, providing high torque, reliable interface pinning, and a well-defined shear field while minimizing bulk interference.

Two types of emulsifiers were analyzed:

1. Monoglycerides at concentrations of 0.54% and 1.1%
2. Sodium Caseinate at a concentration of 10%

A TA Instruments™ Discovery™ Hybrid Rheometer (HR) 20 rheometer was used for testing the samples. Measurements were conducted at 25 °C on water/air and water/oil interfaces. The following interfacial rheology tests were performed:

• Frequency sweeps
• Time sweeps
• Strain sweeps

The DWR was positioned at the fluid interface, with the denser phase (typically distilled water) placed first in the bottom cup. If not already in the water phase, the emulsifier was applied directly to the interface or mixed into the oil phase. The instrument setup allowed precise measurement of interfacial moduli and viscosity without sub-phase correction in most cases.

Monoglycerides (Low Molecular Weight Emulsifier)

At 0.54%, monoglyceride films showed a clear structure formation over time (Figure 1). A modulus crossover (G’ = G”) was observed after 2 minutes, indicating network formation.

Frequency sweeps revealed viscoelastic behavior, with a relaxation time of approximately 2.94 s (1/ω, where w is in rad/s), as shown in Figure 2. At 1.1%, the viscoelasticity increased, with a higher relaxation time (~5.88 s) and elevated interfacial viscosity.

In water/oil interfaces, 1.1% monoglyceride samples consistently exhibited G’ > G” across the frequency range, indicating a more elastic behavior. The 0.54% samples remained more viscous (G” > G’). These results highlight the concentration-dependent strengthening of interfacial films. At 1.1%, the crossover between G′ and G″ occurs at lower frequencies, indicating that over a broader frequency range, G′ exceeds G″, and the material exhibits more elastic-dominant behavior. In contrast, at 0.54%, the crossover shifts to higher frequencies, suggesting a wider viscous-dominant region where G″ is greater than G.

Sodium Caseinate (High Molecular Weight Emulsifier)

Sodium caseinate formed stronger interfacial films, as expected from its large molecular weight. However, the film formation was significantly slower due to the delayed migration of the protein to the interface. Time sweeps showed gelation occurring after approximately 30 hours, as shown in Figure 3.

Frequency sweep results of the sodium caseinate are shown in Figure 4. After 50 hours, frequency sweeps revealed high G’ and G” values with a relatively low phase angle (~20°), confirming the formation of a strong elastic interfacial network. This behavior is attributed to protein unfolding and network formation at the interface, providing superior elasticity compared to monoglycerides.

Advantages of DWR Geometry

The DWR geometry allowed detection of surface viscosities as low as 10-4 N·s/m without correction and down to 2 × 10-6 N/m using Boussinesq number adjustments. Key advantages include:

• Small gap design to help ensure the narrow gap assumption is valid
• Elimination of undefined sub-phase contributions in most cases
• Enhanced interface pinning due to square-edged ring
• Increased torque and sensitivity from larger ring diameter and dual-wall configuration

The Boussinesq number is critical for assessing the relative contribution of surface and bulk stresses. With a ring side length of only 0.7 mm, the DWR achieves a much higher Boussinesq number compared to bi-cone geometries, which helps ensure reliable surface-specific measurements.