Meniscope: A Low-Cost Fluid Interface Visualizer

Abstract

In this work, we describe the development and application of a low-cost fluid interface visualizer referred to as the ``Meniscope.'' The device works using a color-based surface gradient detector method that maps the gradient of an air-water interface to a specific color on a target pattern below using a converging lens. Sample experiments are outlined that showcase the working principle and functional versatility of the device. The device and assembly instructions were piloted in a hands-on workshop, with pertinent feedback reviewed herein. The Meniscope is a low-cost device that is capable of producing striking visualizations of static and dynamic free-surface deformations while introducing users to free-surface measurement techniques in an accessible and hands-on manner.

Figures (3)

• Figure 1: Schematic highlighting the key optical properties underlying the surface gradient detector method (zhang1994measuring). A vertical ray first passes through a fluid interface with local slope $\frac{dh}{dx}$, thereby being reoriented by an angle $\varphi$. The ray then passes through a focusing lens with focal length $f$, focusing to a point on the focal plane a distance $D$ away from the central optical axis. As such, an interface slope $\frac{dh}{dx}$ is mapped to a unique position $D$ on the focal plane. In the absence of interfacial disturbances (i.e. $h(x)=H$), all incident rays map to the center of the focal plane, coincident with the optical axis of the lens.
• Figure 2: (a) Schematic of the working principle behind the Meniscope, known as the color-based surface gradient detector method (zhang1994measuring). When viewed from above, each surface gradient is mapped to a different point (encoded by color) on a screen placed at the focal plane of the lens. (b) Fully assembled device placed atop a cell phone displaying a target color pattern.
• Figure 3: (a) A small water droplet is deposited on the Meniscope Petri dish. (b) Typical target color pattern used for the visualization technique that maps hue and saturation to gradient direction and magnitude, respectively. (c) Discrete color "bulls-eye" pattern used for example visualizations shown herein that was found to be easier to work with for demonstrative and exploratory purposes. (d) Image through the viewing cup of the sessile droplet. Away from the droplet, the color on the lens appears uniformly white, representing successful alignment with the center of the target pattern shown in (c). Some aberrations are visible near the edge of the lens due to practical limitations of the inexpensive optical setup. (e) Two Cheerios floating at an air-water interface are surrounded by a meniscus and attract one another through capillary attraction. (f) Periodic excitations lead to capillary waves forming at the air-water interface. White stripes correspond to regions of nearly zero slope, indicative of peaks and troughs of the excited wavefield.