Automated system for measuring the surface dilational modulus of liquid–air interfaces

Abstract

The surface dilational modulus is a crucial parameter for describing the rheological properties of aqueous surfactant solutions. These properties are important for many technological processes. The present paper describes a fully automated instrument based on the oscillating bubble technique. It works in the frequency range from 1 Hz to 500 Hz, where surfactant exchange dynamics governs the relaxation process. The originality of instrument design is the consistent combination of modern measurement technologies with advanced imaging and signal processing algorithms. Key steps on the way to reliable and precise measurements are the excitation of harmonic oscillation of the bubble, phase sensitive evaluation of the pressure response, adjustment and maintenance of the bubble shape to half sphere geometry for compensation of thermal drifts, contour tracing of the bubbles video images, removal of noise and artefacts within the image for improving the reliability of the measurement, and, in particular, a complex trigger scheme for the measurement of the oscillation amplitude, which may vary with frequency as a result of resonances. The corresponding automation and programming tasks are described in detail. Various programming strategies, such as the use of MATLAB ® software and native C++ code are discussed. An advance in the measurement technique is demonstrated by a fully automated measurement. The instrument has the potential to mature into a standard technique in the fields of colloid and interface chemistry and provides a significant extension of the frequency range to established competing techniques and state-of-the-art devices based on the same measurement principle.

The Role of Surface Viscosity in the Escape Mechanism of theStenusBeetle

“…The system response is detected by a pressure sensor and evaluated by a lock-in scheme. A cross-sectional view of the apparatus is given in Figure . , …”

“…A cross-sectional view of the apparatus is given in Figure 2. 15,16 For each studied solution, three independent measurements were carried out; i.e., the sample chamber was opened and refilled with the solution to be studied. The frequencydependent results shown in Figures 4 and 5 represent the averaged values obtained from these measurements.…”

Interfacial Dynamics and Rheology of a Crude-Oil Droplet Oscillating in Water at a High Frequency

“…Most of the time, these devices are operated at low frequency (ω/2π ≲ 1 Hz), in order to facilitate the accounting of bulk hydrodynamic effects in the determination of interfacial properties. Oscillations of a hemispherical bubble were used to investigate rheological interface properties in the range of 1–500 Hz. − Despite the high-frequency range considered, because of the small size of the bubble ( d ≈ 0.5 mm), the forcing frequency was negligible compared to those of the shape eigenmode (ω 2 /2π ≈ 10 3 Hz). The shape thus remained hemispherical, the interface experienced a uniform dilation, and inertia was considered to be negligible.…”