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12 September 2003

How to Build a Hele-Shaw Cell

by Jearl Walker
Excerpted from "Scientific American's The Amateur Scientist", first published October, 1989.

Air bubbles rising through a viscous fluid sometimes take on curious shapes, combining and interacting in perplexing ways. Such behavior often goes unnoticed because it is hard to observe the bubbles in an ordinary container, particularly when the fluid is opaque. One way to enhance the visibility and at the same time somewhat reduce the complexity of the various goings-on is to place the fluid in a Hele-Shaw cell, a device named for Henry S. Hele-Shaw, the English engineer who devised it around the turn of the century.

Figure 1: The experimental setup with the Hele-Shaw cell. Click image to enlarge.

The cell consists primarily of two transparent plates separated by a narrow gap. A thin spacer runs along the internal edges of the plates to maintain their separation and keep the fluid from leaking out. Air bubbles are introduced into the cell through a port along one of the edges. The fluid can be pushed or pulled through the cell by a pump connected to other ports. Alternatively, the cell can simply be propped up at a slant or mounted vertically so that gravity and buoyancy move the fluid and the bubbles.

The most obvious advantage of a Hele-Shaw cell is that the bubbles are always visible. Even if the fluid in bulk is opaque, a thin layer of it is transparent because it hardly absorbs any light. Moreover, the flow is effectively two-dimensional and so is easier to analyze than when it is three-dimensional in a wider container.

From a local store specializing in plastic supplies I purchased two square acrylic plates that measured 15 inches on a side and were 3/4 inch thick. To separate the plates I used narrow, thin strips of Styrofoam that are sold in hardware stores for weatherproofing windows. The strips come with a sticky side that adheres well to the acrylic. I applied strips along three edges on the face of one plate. Along the fourth edge I applied two shorter strips so that a wide central hole was left between them to serve as an air vent.

I next laid the plate on paper towels and poured or squirted a fluid onto the surface inside the strips to cover about half of the area. Then I placed the second plate on the first one, taking care to align their edges. I squeezed the plates by tightening four C-clamps along their left and right sides, one near each corner. The plates were then separated by less than a millimeter, but the width varied across the cell because of the pressure of the fluid. The fluid filled most of the cell.

Using a Hele-Shaw cell many other fascinating phenomena can be observed involving gas bubbles rising through liquids, or interactions between liquids that don't mix.