Optical Control of Thermocapillary Effects in Complex Nanofluids

We study the strong coupling of light and nanoparticle suspensions and their surface tension effect in capillaries. We show experimentally and theoretically that increasing the intensity of a narrow laser beam passing through a capillary far away from the surface results in a significant decrease in the fluid level. The underlying mechanism relies on light-induced redistribution of nanoparticles in the bulk and the surface of the fluid, facilitating continuous optical control over the surface position. The experiments manifest optical control from afar over properties of fluid surfaces.

Figure 1

(a) Experimental setup. (b),(c) The capillary with nanoparticle fluid illuminated by a laser beam before (b) and after (c) the height of the surface changes. The bright yellow line is due to the fluorescence nature of the nanoparticles at the vicinity of the beam. (d),(e) A heating wire around a capillary filled with a CdTe solution at room temperature before (d) and after (e) heating. The solution turns clear where the temperature is higher, due to the thermophobic nanoparticles.

Figure 2

(a),(b) Steady-state height change $\Delta H$ in a solution with 3.5 nm CdTe particles, vs initial distance between the beam and the surface, $\Delta X_0$, for various beam powers. (c) Same as (a) with 2.5 nm CdSe particles. (d) Steady-state height change in a solution with 3.5 nm CdTe particles vs beam power, for $\Delta X_0=0.75$, 1.5 mm. Solid lines represent theoretical results, with no fitting parameters.

Figure 3

(a) Maximum refractive index change ($\Delta n$ at the beam vicinity) vs beam power, for CdTe and CdSe solutions. (b) Profile of the refractive index change in a CdTe particle solution, inside the capillary (beam position set as zero). (c) Measured (circles) and calculated (solid line) temperature profiles in the capillary for CdSe solution ($\Delta X_0=4.5\mathrm{mm}$, $I_{\mathrm{beam}}=120\mathrm{mW}$, surface position set at zero). The temperature at the surface is higher than room temperature.