Background

Traditional shadowgraphy is based on the slight shift of rays refracted from a transparent sample. Passing through a transparent medium with a variation in refractive index, such as a stream of gas or compressed air, the light ray is refracted, causing a slight change in direction by an angle [1]. Following the invention of German physicist August Toepler in 1864, a variety of shadowgraph imaging techniques, collectively known as Schlieren optics, have been developed to observe fluid flow by detecting light deflection caused by density gradients that vary the refractive index [2]. Thus, Schlieren optics are widely applied to capture the contours of transparent specimens, including liquids, gases, and air flows [3,4].

Modern Schlieren setups utilize optical components available from global vendors, such as Edmund Optics (https://www.edmundoptics.com). A typical setup involves an optical table, a laser, beam splitters, compound lenses, and a knife-edge strategically and precisely positioned on the optical path. The technique has also been applied in combination with interferometry [5]. A more recent technique, known as the BOS (background-oriented Schlieren) [6], detects fluid non-uniformities by observing the distortion to the image of background grids and is digitally processed to construct the Schlieren image with improved sensitivity.

Although Schlieren systems can effectively identify fluid flow patterns with high sensitivity, they can be affected by disturbances from the transparent medium, such as ambient air, surrounding the test samples [7]. The caveat of large, commercial Schlieren systems is that the light-path distance is rather long [8], especially compared to thin samples of interest. This might explain why we found few previously reported high-quality images on flow patterns of transparent fluids in small liquid volumes (on the order of microliters) or thin layers of fluid, particularly those of biological origin.

Here, we present the design of a compact Schlieren optics device (CSOD). Using a thick concave mirror, we show through optical alignment and calibration that CSOD capitalizes on light’s diffraction and interference properties to improve image contrast and sensitivity. We illustrate the usefulness of a simple CSOD in imaging bacterial swarm fronts and clusters of human cells on a culture dish. The simple device described in this protocol may lead to numerous applications, particularly in research involving aqueous, transparent, and biological materials.