Fingering instability of a suspension film spreading on a spinning disk

We have experimentally investigated the spreading of a suspension drop when rotated atop a spinning disk using flow visualization techniques. The suspension is made of $50 \pm 10$ micron glass beads suspended in a low viscosity, partially wetting Newtonian liquid having same density as the glass beads. The suspension drop is placed centrally on a horizontal disc and the disc is then rotated at a desired speed. The spreading behavior is captured using a high speed camera and the acquired images are analysed to find the edges of the spreading film. For all the particle volume fractions ($\phi_{p}$) studied, the suspension drops spread radially until they reach a critical radius, following which the contact line develops instabilities which further grow into fingers. The critical radius for the onset of instability shows an increase with increase in the particle fraction ($\phi_{p}$) before decreasing slightly at the highest value of $\phi_{p}$ studied, while the instability wavelength ($\lambda$) exhibits a non-monotonic dependence. The value of $\lambda$ is close to that for a partially wetting liquid at lower $\phi_{p}$, it decreases with increasing $\phi_{p}$ to a minimum before increasing again at the largest $\phi_{p}$.