Abstract
Thermal convection is investigated experimentally in a microgel suspension that consists of core-shell colloids, which change their size with temperature. The swelling and shrinking of the particles strongly modify their volume fraction in the carrier fluid and therefore the viscosity of the suspension. In this experiment, thermal convection in a Hele–Shaw-like apparatus is monitored using the shadowgraph technique. When compared to a normal fluid, the threshold temperature difference is reduced dramatically, which is interpreted as a manifestation of the Soret effect, i.e. the temperature gradient applied to the suspension induces an unstable gradient of the colloid concentration. The wavelength in the nonlinear regime is very different from the one observed in water. Furthermore, transient oscillations of the patterns are detected in the nonlinear regime and are investigated as a function of the applied temperature difference.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. When a horizontal fluid layer is heated from below, motion in the form of spatially periodic convection rolls appears when the temperature difference between the top and bottom of the fluid layer exceeds a certain critical value. This phenomenon is known as Rayleigh–Bénard convection. In our experiment, we use a microgel suspension that consists of special colloids in water. These colloids change their size with temperature and thus also the viscosity of the fluid.
Main results. The critical temperature difference for the convection onset is reduced dramatically when compared to a normal fluid. Also, the wavelength of the convection rolls is very different. The most striking feature, however, is the occurrence of transient oscillations of the convection patterns in the nonlinear regime (see figure). When plotted against an appropriate control parameter (the Rayleigh number), the oscillaton period follows a power law behaviour with an exponent of approximately −1/2.
Wider implications. The decrease of the threshold value by a factor of about 400 000, together with the very large time scales involved, limits the experimentally accessible temperature range to very large values of the Rayleigh number. This provides an opportunity to investigate a remarkably overcritical regime in a very small apparatus. To explain the transient oscillations, a complete nonlinear model of the colloidal suspension needs to be established.
Figure. Space–time plot of temperature images averaged over the vertical dimension. The temperature difference is held constant, except that after 3 hours (see arrow) it has been switched off for 10 minutes. The bright spots correspond to the upstreaming, and the dark spots to the downstreaming, fluid.