Observation of Nanoparticle-Enhanced Marangoni Transport near a Freezing Out Front

We report observation of an unusual nanoparticle (NP)-enhanced Marangoni flow near the melting point of a dodecanethiol ligated Au NP–octadecane solution. A thermal gradient (0.74 °C/cm) is applied to a thin film of this NP–octadecane mixture in an open Teflon trough parallel to the liquid–vapor surface. Half the mixture is frozen, and the other half is liquid. Over a period of many hours, thermal and solutal Marangoni flow transports the liquid in the direction of the solidified mixture where a “mountain” of this material builds up over time at the liquid–solid boundary due to the presence of this liquid flow. Small quantities of NPs in the bulk solution (NP volume fractions of ∼10–5) enhance this Marangoni flow by upward of factors of 4 relative to pure octadecane. We believe that this effect is due to the freezing out of NPs (relative to the liquid octadecane) within a thin liquid film that flows over the solidified NP–octadecane mixture; this freezing out of NPs within the thin film creates a large NP gradient at the liquid–vapor surface which generates an enhanced solutal Marangoni flow.