Monitoring heterogeneity and diffusive processes in therapeutic samples using schlieren

Heterogeniety in therapeutic samples can result in an overall reduction in quality. We present a simple and portable all-lens Schlieren setup to detect, visualize, and quantify heterogeneities in therapeutic solutions during and after thawing in real-time. The local concentration gradients in a thawing sample lead to light intensity variations which are captured using the Schlieren technique. The sample heterogeneity can be quantified by relating these light intensity variations to concentration gradients. To this end, we first measure the refractive index of the sample solutions, which varies linearly with the sample concentration. This linear relation is then used to extract the concentration gradient field from the light intensity data. We illustrate the capabilities of the proposed method by visualizing and quantifying heterogeneities during the thawing of different protein solutions. We establish the validity of the proposed approach by demonstrating its accuracy in measuring the diffusion coefficient of a diffusing interface.