Dynamics of falling droplet and elongational properties of dilute nonionic surfactant solutions with drag-reducing ability

Abstract

The dynamics of the falling droplet through a nozzle for dilute nonionic surfactant (oleyl-dimethylamine oxide, ODMAO) aqueous solutions with viscoelastic and drag-reducing properties were investigated at different concentrations of ODMAO solutions Cs = 500, 1000, and 1500 ppm by weight. The effects of the flow rate and tube outer diameter on the length of the filament, which was the distance between the tube exit and the lower end of a droplet at the instant when the droplet almost detached from the tube, were clarified by flow visualization measurements by a high-speed video camera. Two types of breaking-off processes near the base of the droplet and within the filament were classified by the Ohnesorge number Oh and the Weber number We. In the regime of the higher Oh and We, the length of the filament became drastically larger at Cs = 1000 and 1500 ppm, whose high spinnability represented the strong viscoelasticity of ODMAO solutions. In the case where the filament was broken up near the lower end of the neck and thinning in time, the thinning of the diameter of the filament was measured by a light-emitting diode micrometer. As for the elasto-capillary thinning of dilute nonionic surfactant solutions, the initial necking process was similar to that of Newtonian fluids and then followed the exponential thinning like polymer solutions. The apparent elongational viscosity of the dilute nonionic surfactant solution was evaluated in the elasto-capillary thinning regime, in which the elongation rate was almost constant. At Cs = 1000 and 1500 ppm, the Trouton ratio, which was the ratio of the apparent elongational viscosity to the shear viscosity, was found to be several orders of magnitude larger than that of Newtonian fluids, while the shear viscosity measured by the capillary viscometer was almost the same order of the Newtonian fluids. The higher elongational property would be closely related to the higher drag-reducing ability of dilute nonionic surfactant solutions.

On the consistent modeling of shear-thickening polymer solutions

“…We have showcased that the model can predict the shear viscosity of dilute nonionic surfactant (ODMAO) solutions. 55 Furthermore, we have shown that, in addition to the shear viscosity, the first and second normal stress coefficients also shear thicken, in line with previous works. 24 However, we have been unable to judge whether this behavior is a physical one since no such rheological data are available.…”

“…Overall, the predictions concerning finite values of the finite extensibility parameter and a 6 ¼ 0 are in line with available rheological measurements of STFs. 2,16,55 In Fig. 3, we depict the predictions of the steady-state dimensionless first [panel (a)] and second [panel (b)] normal stress coefficient as a function of Wi for various values of the model parameters.…”

“…Next, we validate the capacity of the derived model to compare with available experimental measurements. Figure 5 presents a comparison between model predictions and experimental data 55 on the shear viscosity of a dilute aqueous (thus g s ¼ 1 mPa s) nonionic surfactant, mainly consisted of oleyl-dimethylamine oxide [ODMAO, C 18 H 37 N(CH 3 ) 2 O], solutions for different ODMAO concentrations C s ¼ 500; 1000; and 1500 ppm at T ¼ 20 C. Since no data for ÀW 2 are available, we consider a ¼ 0, and for simplicity, we consider k 1 ¼ 1. As no information pertinent to the calculation of the extensibility parameter of ODMAO can be found, we will consider it here as an adjustable parameter; given the small size of ODMAO chains, we consider b ¼ 5.…”

“…. Comparison of the model predictions for the shear viscosity with the experimental rheological data of three ODMAO solutions 55.…”

Material assembly by droplet drying: From mechanics theories to applications

“…76 Furthermore, surfactants can reduce the surface tension and alter the contact angle of the droplet, leading to more uniform spreading and a more homogeneous deposition pattern, with less pronounced coffee ring formation. 107,108 Surfactants can also have a destabilizing effect, leading to nonuniform deposition patterns of so-called "fingers" or "channels" 109,110 particularly for certain types of surfactants, such as ionic surfactants, which can induce electrostatic interactions between the droplet and the substrate.…”