Adsorptive pressure-sensitive coatings for unsteady flow measurements

Time response of pressure-sensitive luminescent coatings has been investigated theoretically and experimentally. The present coating consists of a thin anodized aluminum layer and luminophores. The layer is formed onto the surface of pure aluminum by an electro-chemical process. The luminophores are absorbed onto the surface of the layer via chemical and physical adsorption. A method of making this coating is described in detail. The theoretical analysis shows that the effective diffusion coefficient for oxygen permeation in the anodized aluminum layer is up to 1×10^-5 m²/s. This implies that the present coating should have the response time of the order of microseconds. For three kinds of luminophores, two porphyrin compounds and a ruthenium (II) complex, the response to a step change in pressure was studied using a pressure jump apparatus and a shock tube. It has been found that the response time of coating with tris(4,7-diphenylphenanthroline)ruthenium(II) ([Ru(dpp)₃]²⁺) is longer than 20 μs, and depends on the thickness of anodized aluminum layer. On the other hand, tetrakis-(4-carboxyphenyl)porphyrin (TCPP) coating has the time response less than 10 μs, which is independent of the thickness of layer.