Dynamics and Scission of Rodlike Cationic Surfactant Micelles in Shear Flow

Flow-induced configuration dynamics and scission of rodlike micelles are studied for the first time using molecular dynamics simulations in the presence of an explicit solvent and salt. Predicted dependence of tumbling frequency and orientation distribution on shear rate $S$ agrees with mesoscopic theories. However, micelle stretching increases the distance between the cationic head groups and adsorbed counterions, which reduces electrostatic screening and increases the overall energy $\mathrm{\Phi }$ linearly with micelle length. Micelle scission occurs when $\mathrm{\Phi }$ exceeds a threshold value, independent of $S$.

Figure 1

(a) Schematic of NEMD simulations (high resolution image in SI). ${\mathrm{CTA}}^{+}$ surfactants (purple head and green tails), ${\mathrm{Sal}}^{-}$ ions (yellow), ${\mathrm{Na}}^{+}$ (red) and ${\mathrm{Cl}}^{-}$ (blue) ions and water (cyan). (b)–(e) Plots of $\theta$ versus $t$ at for various Wi. (f) (i) Flow-aligned, (ii) gradient-aligned, (iii) misaligned (in the $xy$ plane), and (iv) misaligned (in the $yz$ plane) states (not to scale). (g) Probability distribution function $p(\theta )$ at various Wi. The dotted line represents a uniform distribution and the vertical bars show the standard deviation of $\mathit{p}(\theta )$ at $\mathrm{Wi}=0$ in the simulations within each interval. Inset shows a plot of $\mathrm{\Delta }\theta$ as a function of Wi showing $\mathrm{\Delta }\theta \propto {\mathrm{Wi}}^{-0.31}$.

Figure 2

Normalized tumbling frequency $f$ versus Wi. Open circles indicate stochastic diffusion regime while filled circles lie in the flow-aligned regime. Power law fitting in the flow-aligned regime yields $f\sim {\mathrm{Wi}}^{0.68}$.

Figure 3

(a) Pair potential energy ${\mathrm{\Phi }}^{*}(l)$ (black dots) and probability distribution of length $p(l)$ (blue triangles) from the length relaxation simulations are plotted as a function of $l$. (b) ${\mathrm{\Phi }}^{*}(l)$ and $l$ versus $t$ showing micelle scission for $\mathrm{Wi}=76$. Snapshots at different locations show (1) stretched micelle, (2) micelle with most probable $l$, and (3) micelle after scission. (c) ${\mathrm{\Phi }}^{*}(l)$ versus $t$ for four different simulations. Dotted lines indicate the range for micelle scission for $\mathrm{Wi}>70$.