Experimental investigation of surface characterization of thin liquid layers on a solid substrate under different heating processes
Habeeb Mohamed Mohamed Khalifa
Date of Issue2009
School of Mechanical and Aerospace Engineering
Heating a thin layer of silicone oil gives rise to convective cell pattern formation on the surface. These convective cells are commonly termed as Benard cells which are gaining interests in both academic research and in industrial applications. Presently, extensive research has been done in single-thin-liquid-layers (STLL) but not in two-immiscible-liquid-layer (TILL) systems. Therefore, the author investigates the feasibility of convective cell pattern formation on the silicone oil surface in both STLL (Phase 1 to 3) and TILL (Phase 4 to 6) systems. A simple shadowgraph technique comprising a mirror, a torch and a “filter” medium was used. The viscosity (centistokes, cSt) is varied as 50, 5 and 100 respectively in Phase 1, 2 and 3. In Phase 4 (50cSt) and 5 (5cSt), water was used as a base medium for silicone oil. In Phase 6, varying concentrations of Sodium Dodecyl Sulphate (SDS) surfactant was used as a base medium for Si oil and this was to study the effect of reducing surface tension in pattern formation. Experimental results from STLL from Phase 1 to 3, reveal that increasing the depth, d, of liquid layer, decreases the number of cells but increase the relative size of the cells. This is related to lower aspect ratio (AR) and higher Biot number (Bi), Marangoni number (Ma) and Rayleigh number ( Ra). Moreover, the cell pattern is only initialized when the ∆T term in Ma and Ra is positive. However, the author postulates that the surface temperature, Ts, is more important in this pattern formation. The cells appear and fade at a particular range of Ts which is critical to a particular aspect ratio. Decreasing the viscosity lowers the Ts for pattern formation for a particular Ar . The pattern formation is more dynamic for lower Ar with lower viscosity. The author discovers that the TILL systems in Phase 4 to 6 closely follow the same principle as STLL systems. New types of cells which include square-like and “3D pentagonal-like” shapes are observed. In Phase 6, the Si oil only dissolves in the micelles and hence fails to give pattern when the SDS concentration is more than 5 times higher than CMC when small quantity of SDS solution is used. Detailed experimental procedures, results, discussions and conclusion are documented in this report. Recommendations for further investigations in this topic are also being highlighted.
DRNTU::Engineering::Mechanical engineering::Fluid mechanics
Final Year Project (FYP)
Nanyang Technological University