Microscale Marangoni Surfers

We apply laser light to induce the asymmetric heating of Janus colloids adsorbed at water-oil interfaces and realize active micrometric “Marangoni surfers.” The coupling of temperature and surfactant concentration gradients generates Marangoni stresses leading to self-propulsion. Particle velocities span 4 orders of magnitude, from microns/s to cm/s, depending on laser power and surfactant concentration. Experiments are rationalized by finite elements simulations, defining different propulsion regimes relative to the magnitude of the thermal and solutal Marangoni stress components.

Figure 1

(a) Diagram of the experimental setup. (b) Schematic of a gold-coated Janus particle at the water-oil interface. Laser illumination induces a temperature increase $\mathrm{\Delta }T$ in the fluid in contact with the cap, leading to asymmetric Marangoni stresses and particle propulsion with velocity $V$. (c) Overlay of different trajectories of Marangoni surfers propelling from the center of the illuminated region. The color code indicates normalized velocity ranging between zero (black) and a power-density-dependent $V_{\max }$ (white). $\text{Scale bar}=30\mu \mathrm{m}$.

Figure 2

Simulated particle speed versus temperature increase $\mathrm{\Delta }T$ in the fluid next to the cap of a Janus particle at a pristine water-dodecane interface (purple squares) and with excess concentrations ${\mathrm{\Gamma }}_0={10}^{-7}$ (green circles), ${10}^{-6}$ (blue triangles), and ${10}^{-5}\mathrm{mole}/{\mathrm{m}}^2$ (orange diamonds). The symbols are simulation results, and the connecting lines are to guide the eye. Solid black line: theoretical prediction by Würger [24]. The insets show isosurfaces of the dimensionless temperature $T^{*}=(T-T_0)/\mathrm{\Delta }T$ (top) and excess concentration ${\mathrm{\Gamma }}^{*}=\mathrm{\Gamma }/{\mathrm{\Gamma }}_0$ (bottom) fields for selected simulations corresponding to different regimes of thermal and surface Péclet numbers $P{e}_T$ and $P{e}_s$, respectively.

Figure 3

Experimental particle speed $V$ as a function of $\mathrm{\Delta }T$. Allegedly pristine interface (purple squares) and increasing concentrations of SDS in the water phase: $C={10}^{-7}$ (green pentagons), ${10}^{-5}$ (orange triangles), and ${10}^{-3}\mathrm{mole}/\mathrm{L}$ (blue diamonds). Black, open symbols are simulated data with the symbol shape corresponding to Fig. 2. The error bars represent the standard deviations of particle velocities.

Figure 4

Rescaled experimental and simulation data. For the experiments the excess concentrations used are ${\mathrm{\Gamma }}_0=2\times {10}^{-7}$, $4\times {10}^{-7}$, $1\times {10}^{-6}$, and $2\times {10}^{-6}\mathrm{mole}/{\mathrm{m}}^2$ for SDS concentrations of $C=0$, ${10}^{-7}$, ${10}^{-5}$, and ${10}^{-3}\mathrm{mole}/\mathrm{L}$, respectively. The symbols and error bars correspond to the ones in Fig. 3. The solid black line is a guide to the eye. Inset: Image overlay of a particle propelling with $V\simeq 10\mathrm{mm}/\mathrm{s}$ ($\text{scale bar}=10\mu \mathrm{m}$).