Oscillatory solutothermal convection-driven evaporation kinetics in colloidal nanoparticle-surfactant complex fluid pendant droplets

To elucidate the pure physics of evaporation which is free from surface effects, the pendant mode of evaporation is employed in the present study. The present study brings out the evaporation kinetics of a combined surfactant and nanoparticle colloidal system. We also segregate the contributing effects of surfactants alone, particle alone, and the combined effect of surfactant and particles in modulating the evaporation kinetics. It is observed that the rate of evaporation is a strong function of the particle concentration for nanocolloidal suspensions of particle alone and concentration of surfactant molecules up to the micellar concentration and thereafter insensitive to concentration for an aqueous surfactant solution. The combined colloidal system of nanoparticles and surfactant exhibited the maximum evaporation rate, and the rate is a strong function of the concentration of both the particle and surfactant. The theoretical classical diffusion-driven evaporation falls short of the experimentally observed evaporation rate in aqueous surfactant and colloidal solutions. Evidence of convective currents was observed in flow visualization studies in aqueous surfactant solutions, nanocolloidal solution of particle alone, and an oscillatory convective circulation in a combined surfactant-impregnated nanocolloidal solution. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce strong circulation currents in such systems from a stability map. Scaling analysis of solutal Marangoni is observed to be capable of inducing circulation from a stability map in all the systems and the enhanced thermophoretic drift and Brownian dynamics, and enhancement in the diffusion coefficient of the nanoparticles is also contributing to the enhanced evaporation rate for only nanocolloidal solutions. The oscillatory convective current arising out of two opposing driving potential enhances the evaporation rate of surfactant-impregnated nanocolloids. The present findings could reveal the effect of surfactants in tuning the evaporation rate of colloidal solutions.

Figure 1

Illustration of the experimental setup: (1) camera with a lens system, (2) syringe pump, (3) backlight with a diffuser, (4) temperature and humidity sensor, (5) syringe and needle system to generate pendant drop, (6) controlled enclosure chamber, (7) laser with a plano-convex lens, (8) laser power control unit, (9) backlight control unit, and (10) computer system.

Figure 2

(a) Plot of nondimensional square of diameter with the time factor τ for CTAB; (b) variation of the experimental evaporation rate constant with SDS and CTAB concentration. The scale bar represents 0.406 mm.

Figure 3

(a) Evaporation characteristics of only nanocolloidal solutions of CuO, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, and $\mathrm{Si}{\mathrm{O}}_2$ at concentrations of 0.04, 0.065, and 0.11 vol%, respectively. (b) The instantaneous variation of the evaporation rate constant $\left(K_{\mathrm{inst}}\right)$ with the time factor τ for CuO nanocolloidal solutions at a concentration of 0.04 vol%. (c) Variation of the evaporation rate constant with the particle concentration for various nanocolloidal solutions.

Figure 4

(a) Nature of variation of the square of the normalized diameter of the droplet with the time factor τ for surfactant solutions at a concentration of $C_s=0.25$ and the $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ nanocolloid at a particle concentration of 0.25 vol% and for surfactant-infused nanocolloids. (b) Experimentally observed $\left(K_{\exp }\right)$ and calculated diffusion-driven evaporation rate $\left(K_{\mathrm{th}}\right)$ for different complex fluids.

Figure 5

Surface plot illustrating the variation of the experimentally observed evaporation rate constant with the particle and surfactant concentration for (a) CTAB-infused $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, (b) SDS-infused MWCNT, and (c) SDS-infused graphene nanocolloids. (d) Evaporation rate of $\mathrm{Si}{\mathrm{O}}_2$ nanocolloid infused with SDS and CTAB surfactant.

Figure 6

Postprocess PIV analysis of internal circulation velocity vector plot for aqueous CTAB solution at normalized concentrations of (a) $C_s=0.25$, (b) $C_s=1.0$, and (c) $C_s=0.5$ with reversal dynamics of internal circulation pattern. (d) Variation of the measured circulation velocity with time in the case of water and aqueous CTAB solution at concentrations of $C_s=0.25$ and $C_s=0.5$ and for the $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ nanocolloidal solution at a particle concentration of 0.0125 vol%.

Figure 7

Velocity contours showing the velocity vectors for (a) nanocolloids without surfactants, (b) surfactant-infused nanocolloids exhibiting an oscillatory convection current, and (c) variation of the circulation velocity with time for CTAB-infused $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ nanocolloid at a particle concentration of 0.0125 vol% and surfactant concentration of $C_s=1.0$.

Figure 8

Thermal Marangoni number versus Rayleigh number stability plot with the stability boundary lines (stability curve 1 indicates that boundary proposed by Nield [32] and stability curve 2 indicates that proposed by Davis [34]) with water and (a) aqueous surfactant solutions at different concentrations, (b) nanocolloids of only particles at different particle concentration, and (c) surfactant-impregnated nanocolloids at a particle concentration of 0.025 vol% and at different surfactant concentrations.

Figure 9

Thermal Marangoni number versus the solutal Marangoni number plot with the stability curves at different Lewis numbers proposed by Joo [35] indicating the loci of points for different complex fluids considered.

Figure 10

Theoretical mean velocity from the calculations $\left(u_{\mathrm{theo}}\right)$ and the experimentally observed velocity $\left(u_{\exp }\right)$ for (a) aqueous SDS solutions at different concentration of the surfactants and (b) CTAB-infused $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ nanocolloids at various concentration of the surfactant at a given particle concentration of 0.025 vol%.