Direct Strain Oscillation: a new oscillatory method enabling measurements at very small shear stresses and strains

Abstract.

As shown previously, a rotational rheometer equipped with an electronically commutated motor (EC-motor) allows one to conduct stress and strain experiments with the same rheometer in rotational mode. A new method has now been developed to improve further strain controlled oscillatory measurements by adjusting the strain directly within a single oscillation cycle.

Generally, a strain controlled oscillatory test in a stress controlled rheometer consists of the following steps: applying one full oscillation cycle with an arbitrary stress amplitude, measuring the strain amplitude, adjusting the stress in the next oscillation cycle, and repeating this routine until the desired strain amplitude is reached. The newly developed direct strain oscillation mode employs a different approach. It does not require a full oscillation cycle but uses a real-time position control and adjusts to the desired strain directly on the sine wave. Therefore, the actual movement of the measuring system follows directly the required change in strain during each individual oscillation cycle.

This new oscillatory mode has several major advantages: (1) the possibility of conducting real strain controlled tests in oscillation, (2) the exact strain setting right from the first oscillation cycle, i.e., no or only very slight overshoot in strain, (3) faster data acquisition even within an oscillation cycle, (4) it allows the measurement at extremely low angular resolution and low torques.

Due to the absence of strain overshoots and the ability of testing at small deflection angles and low torques this new method is especially helpful for measurements on samples with low viscosities and weak structures such as gels, emulsions, suspensions, colloids, and foams.