Measurements

The dynamic interfacial tension and dilational visco-elasticity (IFT and elasticity, hereafter) were measured using a drop profile analysis tensiometer (PAT1, Sinterface Technology, Germany). To this end, a pendant drop of the aqueous phase was created at the tip of a stainless steel capillary in a glass cuvette filled with pure heptane. The profile coordinates were extracted from the drop images and the IFT was measured by fitting the Gauss–Laplace equation to the experimental drop profile coordinates. The fully computer-controlled dosing system kept the drop surface/volume constant in the course of reaching the interface to its equilibrium adsorption.

The elasticity of interfacial layer was also evaluated by applying harmonic perturbations on the drop surface. In this way, the drop surface was subjected to sinusoidal oscillations and the response of the pre-equilibrated interface layer to its perturbations was analyzed using a Fourier transformation program. To remain in the range of a linear perturbation response, the frequency range and the amplitude of these perturbations were between 0.01 and 0.1 Hz and 8% of the initial drop surface area, respectively. The calculation procedure was described elsewhere in detail⁵⁶. Moreover, the adsorption process of nanoparticle at the interface was evaluated by conducting high amplitudes interface compression. In this case, after the system reached the equilibrium IFT, the drop volume was undergone to a linear compression. Then, the variations of IFT and the deviation of the drop profile from its Laplacian shape were evaluated over the course of this process. The rate of compression and the amplitude of perturbation were 2.0 × 10⁻⁵ ml/s and about 70% of the initial interface area, respectively.

Zeta potentials of nanoparticles in aqueous nanofluid dispersions with varying surfactant/nanoparticle concentration and ionic strength were obtained using a Zetasizer ZEN3600 apparatus (Malvern Instruments, UK). All the experiments were conducted at 25 °C. All glassware and Teflon equipment in contact with the liquid phases during sample preparation or measurements were cleaned by concentrated sulfuric acid and washed carefully afterward with distilled water. The purity of the aqueous and oil phases and the absence of any contaminants in the systems were ensured by measuring the water/heptane dynamic IFT and elasticity before each measurement. All experiments were conducted at least three times to assure the repeatability of the results.