Quick Assessment of Yield Stress in Soft Solids

Figure 1 demonstrates how rapidly yield behavior can be characterized using oscillatory strain amplitude sweeps on two soft solid formulations. A lotion was tested from 0.01 % to 100 % strain and an after‑sun gel from 0.1 % to 1000 % strain, both at 10 points per decade, giving a full test time of only ≈ 122 s per sample. From the modulus curves, the yield point (critical strain) is identified at the G′–G″ crossover. The lotion exhibits a crossover at 41.76 % strain, while the after‑sun gel shows a much higher 177.43 % strain, consistent with its stronger network structure.

To convert this yield point into a yield stress, the data can be replotted as versus oscillation stress. As shown in Figure 2, the crossover for the after‑sun gel corresponds to a yield stress of 132.65 Pa, representing the stress required to initiate flow and disrupt the material’s structure.

Although amplitude sweeps are often run with 10 points per decade to produce smooth curves, this density is not always required for accurate identification of the yield point or yield stress. Figure 3 shows that using only 3 points per decade preserves the same crossover behavior while reducing total test time to under 40 seconds. This significant acceleration is especially valuable in QA/QC environments where rapid and repeatable assessment of yield properties is critical.

These tests were conducted using 1‑second conditioning and sampling times, which are sufficient for fast‑responding materials such as lotions and after‑sun gels. Although the measurements are performed within the LVR, where the structure is not being damaged, materials with slower structural relaxation may still require longer equilibration. The conditioning time may be increased to 2 seconds, though this extends the total test duration from 122 s to approximately 189 s.

For these fast‑responding materials, the HR rheometer likewise exhibits rapid responsiveness, allowing conditioning times as short as half a second (although this is generally not recommended) while still producing results near equivalent to those obtained with a 2‑second conditioning time. In this example, all 41 data points were collected in approximately 100 seconds (Figure 4).

A way to further accelerate the workflow is to automate the crossover analysis. After performing the analysis, the crossover determination can be saved as an analysis template, as shown in Figure 5.

This saved template can then be loaded into the end of test analysis. With this setup, TRIOS Software automatically identifies the G′–G″ crossover as soon as the test completes, eliminating manual post‑processing (Figure 6).