Propulsion of catalytic Janus spheres in viscosified Newtonian solutions

The propulsion of active colloids made from silica microspheres half coated with platinum is experimentally probed in solutions of hydrogen peroxide as a function of solution viscosity. The velocity is shown to decay approximately inversely with viscosity. Further, the type of viscosifier used affects the interaction between fuel molecules and motor, which affects propulsion. The quantification of how a viscosifier alters both viscosity and interactions is interpreted using existing diffusion-reaction models of self-diffusiophoresis.

Figure 1

Velocity $V$ of the silica-Pt Janus spheres in solutions of varying viscosity obtained from quadratic fits using Eq. (2). Data are shown as (a) velocity directly and (b) velocity times viscosity. The concentration of hydrogen peroxide is $3M$ (circles) and $4M$ (squares). Blue markers show data points in aqueous solutions, black data points with sucrose as the viscosifier, and red data points with glycerol as the viscosifier.

Figure 2

Data of velocity time viscosity shown separately for (a) $3M {\text{H}}_2{\text{O}}_2$ and (b) $4M {\text{H}}_2{\text{O}}_2$. The horizontal blue solid lines are simply provided as guides for the eye. The dashed lines are the lines of best fit using the low-$\text{Da}$ equation (5) and the dotted lines are the lines of best fit using Eq. (6). Blue markers show data points in aqueous solutions, black data points with sucrose as the viscosifier, and red data points with glycerol as the viscosifier.

Figure 3

(a) Plot of $D_S/D_{\text{Br}}$ vs viscosity where $D_S$ has been determined from quadratic fits to the MSD for $\mathrm{\Delta }t\ll {\tau }_r$ [Eq. (2)]. The solid line is shown at a value of 1. (b) Plot of $D_L/D_{\text{Br}}$ vs viscosity where $D_L$ has been determined from linear fits to the MSD for $\mathrm{\Delta }t\gg {\tau }_r$ [Eq. (3)]. The solid line is shown at a value of 1. Blue markers show data points in aqueous solutions, black data points with sucrose as the viscosifier, and red data points with glycerol as the viscosifier. The hydrogen peroxide concentrations are $0M$ (triangles), $3M$ (circles), and $4M$ (squares).

Figure 4

Rotational time ${\tau }_r$ of the silica-Pt Janus spheres in $3M {\text{H}}_2{\text{O}}_2$ solutions of varying viscosity. Two different methods have been used: one using the short and long-time diffusivities (open symbols and another using the intercepts from quadratic and linear fits (closed symbols). The solid line is the theoretical prediction of ${\tau }_r$ as a function of viscosity using the Stokes-Einstein-Debye equation. Blue markers show data points in aqueous solutions, black data points with sucrose as the viscosifier, and red data points with glycerol as the viscosifier.