A device to manufacture spherical alginate beads of suitable diameter (~ 2 mm diameter) for experiments was constructed from a thin aluminum sheet (0.079 cm).
The device is essentially an open box with an array of 64 specially-shaped apertures formed in the surface of the bottom plate. Each aperture consists of three parts: a dimple, a shoulder, and an orifice (see diagram). Liquid alginate is poured into the box and beads will form under gravity flow. The liquid drips into a receptacle containing a hardening solution. As the liquid level in the device decreases, the flow rate slows. Liquid alginate has high surface tension; hence, during their fall and quick hardening, the beads are spherical. The showerhead satisfies the following set of conditions: no pressurized air is needed; beads form under the pull of gravity; beads are a suitably small diameter; bead diameters are consistent; rate of formation is rapid; apparatus can be autoclaved for sterilization, and adequate amounts of alginate solution can be processed at one time.
The main design problems were to establish the appropriate diameter of the holes to control flow rate and to prevent alginate from bridging the space between the adjacent orifices, which would create beads of inconsistent diameter. The diameter of the apertures (holes) (0.137 cm) is small enough that, when the surface of the liquid is 15 cm above the surface of the showerhead, the flow of liquid alginate forms individual beads, but is large enough for the container to completely drain in a few minutes.
To prevent liquid alginate from bridging, dimples were punched into the surface of the aluminum sheet metal. The dimples are deep enough to prevent alginate from climbing up the dimple surface to an adjacent dimple. Dimples were formed by deforming 0.079-cm aluminum sheet metal between a hemispherical 0.635-cm-diameter punch and a steel plate with a concentric 0.673-cm hole through a distance of 0.216 cm. Dimples were then perforated in the center with a 0.137-cm diameter needle from the inside. The perforation operation formed a drip edge shoulder of torn aluminum approximately 0.064 cm deep. The low area drip edge is necessary to combat the high adhesion of the alginate and to form
beads that separate from the aperture under gravity, when suitably small. The device produces a half-liter of alginate beads in about 5 min. Larger devices, with the same nozzle design, will produce larger quantities of beads.