The determination of local mass-transfer coefficients by holographic interferometry—I

Abstract Methods are described of applying the powerful techniques of holographic interferometry to the profilometric measurement of mass-transfer rates at solid-fluid interfaces, with great advantages of speed, precision and comprehensiveness of data acquisition and display. Maintenance of virtually constant optical quality at the transferring surface, vital for the production of satisfactory interferograms, is effected by the use of a mass-transferring coating consisting of a smooth layer of elastomer initially charged with a volatile (or soluble) swelling agent. Shrinkage of this coating due to transfer of the swelling agent to the fluid stream can then be recorded as an array of interference fringes when reflection or transmission holograms of the initial and final states of the surface layer are superimposed. Such interferograms provide a synoptic record of the spatial variation of transfer coefficient over the entire surface viewed at the holographic plate. The precision and completeness of the data provided by these methods is demonstrated here by their application to the mapping of the transfer coefficient variation over the surface of a flat plate exposed to normal or tangential laminar air jets of known velocity. For the former case the results agree closely with an available analytical solution.