Effects of moving contact line on filament pinch-off dynamics of viscoelastic surfactant fluids

Many practical applications (e.g., coating, painting, printing, or agrochemical spraying) involve depositing complex fluids onto a solid substrate using a predominantly uniaxial extensional flow. In this work, we conduct experiments by gradually depositing non-Newtonian surfactant fluids onto a horizontal solid substrate via a vertical needle. We investigate the extent to which, the spreading dynamics of the fluid contact line on the solid substrate can affect the thinning dynamics of the fluid filament formed between the needle and the substrate. Our work considers two model viscoelastic surfactant fluids based on cetylpyridinium chloride and sodium salicylate (CPyCl/NaSal) and octadecyltrimethylammonium bromide and sodium oleate $\left({\mathrm{C}}_8\mathrm{TAB}\text{/}\mathrm{NaOA}\right)$ in deionized water. Experiments are performed using two flat substrates: a big substrate, where fluid contact line is free to move, and a finite-size substrate, where fluid contact line is pinned. The fluid wetting on the substrate is characterized by measuring the contact-line velocity and dynamic contact angle, while the extensional flow is evaluated by measuring the fluid midfilament diameter. Two novel regimes are identified: In regime I, fluid wetting and filament pinch-off dynamics are independent, while in regime II, the fluid wetting significantly affects the extensional flows by lowering the material extensional relaxation times and Trouton ratios. Our analysis shows that spreading of these viscoelastic surfactant fluids are surprisingly well captured by Tanner's law suggested for spreading of a Newtonian fluid on solid substrates. Finally, we propose a scaling analysis based on a combination of the wetting forces and viscous dissipation that can successfully explain the effects of wetting on filament pinch-off dynamics.

Figure 1

A schematic of the custom-made dripping onto substrate device that is used to perform experiments in this study. In this figure $U$, $R$, $\theta$, and $D$ denote the contact-line velocity, radius of the contact line, contact angle, and the midfilament diameter.

Figure 2

Zero-shear-rate viscosity of the viscoelastic micellar solutions.

Figure 3

Filament thinning and contact-line dynamics in a viscoelastic micellar fluid based on CPyCl/NaSal (100 mM/60 mM) when the contact line is (a) free and (b) pinned to the lower plate. The filament pinch-off time is denoted as $\mathrm{t}{}_f$. The length of the scale bar in panels (a) and (b) is 1.5 mm.

Figure 4

Time-resolved variation of the dimensionless filament diameter for the two viscolastic surfactant fluids based on (a) ${\mathrm{C}}_8\mathrm{TAB}$/NaOA and (b) CPyCl/NaSal system. The filled symbols denote the results with pinned contact line, while empty symbols correspond to the freely moving contact line. The solid curves correspond to the best fit of Eq. (6) to the elastic regime. The midfilament diameter at the beginning of the experiments is denoted as $\mathrm{D}{}_0$.

Figure 5

Filament lifetime as a function of (a) ${\mathrm{C}}_8\mathrm{TAB}$ weight percentage for ${\mathrm{C}}_8\mathrm{TAB}$/NaOA viscoelastic solutions and (b) NaSal concentration in the CPyCl/NaSal systems.

Figure 6

Extensional relaxation time of the viscoelastic surfactant fluids as determined by the best fit to Eq. (6) for (a) ${\mathrm{C}}_8\mathrm{TAB}$/NaOA and (b) CPyCl/NaSal fluids. Empty symbols correspond to the experiments where contact line is pinned, while, filled symbols correspond to the freely moving contact lines.

Figure 7

Maximum Trouton ratio $\left(\mathrm{Tr}{}_{\text{max}}\right)$ as a function of salt to surfactant ratios for (a) ${\mathrm{C}}_8\mathrm{TAB}$/NaOA and (b) CPyCl/NaSal viscoelastic fluids. Filled symbols indicate a freely moving contact line, while empty symbols correspond to pinned contact lines.

Figure 8

(a) Dimensionless radius of the contact-line as a function of time for sample viscoelastic surfactant fluids. The dashed curve corresponds to $R\left(t\right)\propto t^{\alpha }$ when $\alpha =0.1$. (b) The power exponent ($\alpha$) as a function of salt concentrations (NaSal) and surfactant concentration (${\mathrm{C}}_8\mathrm{TAB}$) in viscoelatic fluids under consideration in this study.

Figure 9

Dimensionless contact-line velocity as a function of contact angle for viscoelastic surfactant fluids. Empty symbols correspond to the rapid decay of the contact angle after deposition of the fluid onto the solid substrate and filled symbols indicate the contact-line dynamics during extensional flow measurements.

Figure 10

Capillary number as a function of salt (NaSal) or surfactant (${\mathrm{C}}_8\mathrm{TAB}$) concentrations for the two viscoelastic surfactant fluids studied in this work. Filled symbols indicate experiments where the contact-line wetting does not influence the extensional flow measurements, while empty symbols are those cases that contact-line dynamics significantly affect the extensional flow properties.