Differential diffusion between species often substantially effects the
results of laboratory-scale experiments in turbulent combustion. The
present work documents these effects for a simple jet flow by simultan
eous 2D imaging of LIF from biacetyl and Lorenz-Mie scattering from su
bmicron TiO2 particles, at low to moderate Reynolds numbers. This nove
l technique, involving a frequency-tripled Nd:YAG laser and two slow-s
can CCD cameras, captures directly the structural differences that ari
se when two species with widely different Schmidt numbers mix in a cla
ssical jet flow where molecular diffusion makes a substantial contribu
tion to overall mixing. Matched instantaneous image pairs of biacetyl
and particle concentration show differences, with the particle images
exhibiting sharper contours and a more convoluted structure with finer
detail. The fluorescence images show a ''superlayer'' similar to that
found in Rayleigh images in flows of this kind. Subtraction of one (n
ormalized) image from the other in each pair reveals greater diffusive
spreading of the biacetyl gas, a direct indication of its molecular c
ontribution to total mixing. These findings support the argument that
molecular diffusivity in general, and differential diffusion between s
pecies in particular, must be accounted for in the interpretation of l
aboratory flame results that are used to validate and test combustion
models.