Time-resolved sedimentation of dense potato-starch suspensions measured by optical coherence tomography

Abstract

We demonstrate optical coherence tomography (OCT) as a measurement technique for dense, optically opaque suspensions. Conventional optical methods cannot access the interior of such suspensions. OCT resolves individual potato-starch particles ($\sim 20\mu \mathrm{m}$) as distinct scatterers, even though the suspension appears opaque to the eye. By tracking the vertical centroid position of the particle-laden layer $⟨Z⟩(t)$ and the supernatant boundary $Z_{\sup }(t)$ in the same OCT image sequence, we obtain the instantaneous settling velocity $V(t)$ and the time-evolving effective volume fraction ${\varphi }_{\mathrm{eff}}(t)$ simultaneously and continuously in time. To our knowledge, this is the first measurement to combine settling velocity and particle concentration into a single continuous trajectory within one sedimentation run. Conventional batch measurements yield only one velocity value per run. We applied this method to dense potato-starch suspensions, varying the initial volume fraction ${\varphi }_0$ from 0.30 to 0.50 and the solvent density ${\rho }_{\mathrm{L}}$ from 1.0 to $1.3\times 10^3\mathrm{kg}{\mathrm{m}}^{-3}$ using aqueous sodium polytungstate solutions. The normalized velocity $V/V_{\mathrm{Stokes}}$ plotted against ${\varphi }_{\mathrm{eff}}$ collapses onto a common trend consistent with both the Krieger--Dougherty model and the Richardson--Zaki law over ${\varphi }_{\mathrm{eff}}\simeq 0.30$--$0.52$, confirming that the method captures physically reasonable hindered-settling behavior. These results establish OCT as a viable tool for probing internal dynamics in dense suspensions that were previously inaccessible to optical measurement.

8 pages, 5 figures