The effects of surfactant monolayers on free surface natural convection

An experimental investigation of the effects of surfactant monolayers on free surface natural convection is presented in which the Rayleigh number, Ra, is used to parameterize the Nusselt number, Nu, with the power law scaling Nu = A · PrnRam. Experiments were conducted in water under the following free surface conditions: (1) clean surface, (2) oleyl alcohol covered surface, (3) stearic acid covered surface, and (4) stearyl alcohol covered surface. The use of an infrared (IR) camera permitted confirmation of these surface conditions and the measurement of the surface temperature. The results reveal a reduction of Nu by approximately one order of magnitude in the presence of any of the three surfactants investigated, compared to the clean surface at equivalent Ra. This reduction in convective ability is attributed to the shear-yielding hydrodynamic boundary condition imposed by the monolayers compared to the shear free condition of the clean surface. All four surface conditions yield an exponent m > 1/3, which is attributed to a relatively non-homogeneous temperature boundary condition at the free surface compared to the boundary condition of a Rayleigh–Bénard study. A dependence of Nu upon aspect ratio Γ was also discovered in contrast to earlier work which suggested that Nu is independent of Γ, at least for large Γ.