Silicone Rubber Floaters With Oomoo 30

To make floatable circular disks to investigate the Cheerios effect (illustrated in the video below), we use Oomoo 30, a silicone rubber compound made up of two separate liquids. There is a Part A and Part B Oomoo that, according to the box, are to be used in roughly equal parts when preparing a batch. However, we found a ratio that seems to work better for our purposes which will be discussed in step 1 below. Oomoo is a simple and easy-to-use product due to its quick mixing process and hydrophobic properties which allow for the finalized disks to sit easily atop the fluid surface. It is important to ensure that Oomoo 30 is used. There is a very similar product, Oomoo 25, which begins hardening at a quicker rate, making it more difficult to use.

Shown above is the initial set-up and the materials used.

List of Supplies:

• 60mL of Oomoo 30 part A (pink)
• 40mL of Oomoo 30 part B (blue)
• 2 popsicle sticks
• 1 straight-edge razor blade
• 1 graduated plastic cup
• 3D printed mold
• vacuum chamber
• compressed air gun
• paper towels

To begin with, create and 3D print a design for the molds. Shown above are the molds we designed using the software Autodesk Fusion 360 and then 3D printed to create the floating disks. Provided below is the STL file for our molds. Once the molds are ready to be used, remove any dirt or dust with compressed air. For this example, the molds were printed on an SLA resin 3D printer.

Shake both bottles vigorously to make sure the substances have an even composition throughout. To begin the molding and mixing process, first, open the Part A bottle. Using a Popsicle stick, stir Part A for 10 seconds. Then, add the desired amount of Part A of the Oomoo 30 to the plastic cup. We used disposable, graduated cups for our mixing container. Open and stir Part B with a new Popsicle stick (to avoid contamination) for 10 seconds, and pour into the plastic cup. Both substances should be poured carefully into the center of the cup so as to not hide the volume markers.

Once both Parts A and B are in the mixing vessel, mix vigorously with either of the Popsicle sticks for 10 seconds or until the color becomes uniform (the final target color is shown in the image on the right). At the same time, try to avoid taking the stir stick in and out of the mixture to reduce the potential for air bubble formation. When we used equal amounts of both liquids, as instructed on the Oomoo box, the mixture was bluer than desired so we added more of Part A. In the end, we found that placing the floaters on the water's surface was easiest when we used a ratio of Part A to Part B around 3:2.

Due to the rather short 20-30 minute pot-life of Oomoo 30, it is important to act fast once the two Oomoo liquids are mixed. Once well-combined, begin to pour the Oomoo mixture into the pre-made molds from a high distance. For this, the cup is held at least an arms-length above the molds so that a very thin stream of the Oomoo pours down. The high fall height, along with the thinned stream, allows for the Oomoo to make its way into some of the smaller crevices in the molds. After the mixture is poured into each of the molds, take a straight-edge razor blade and loosely scrape the top of the mold to remove significant amounts of excess Oomoo. I still try to leave a little film of Oomoo though for the moment.

Although the Oomoo box states no vacuuming is necessary, bubbles may start to form. Unfortunately, bubble production is essentially inevitable. It can come as a result of a non-ideal ratio of Part A to Part B, not pouring the mixture into the mold quickly enough, overvacumming, or not stirring the Part A and Part B mixtures separately at the start. Conduct several vacuuming sessions to attempt to remove any air bubbles which may be inside the viscous mixture. Vacuuming is done for 15 seconds, released, and repeated. I vacuumed 4 times, however, this number depends on the age of the Oomoo since the substance degrades over time. The Oomoo Part A and Part B both seem to harden and thus, as the bottles of Oomoo age, one may be more prone to bubbles getting trapped in the final floaters. As a result, it may be necessary to vacuum up to 4-5 times. After vacuuming, continue scraping with the razor blade, leaving behind little to no excess Oomoo. Any excess left on the mold's surface will make it more difficult to remove the floaters cleanly the following day. Finally, after every few uses of the vacuum, clean the cast polymer inside of the chamber from the previous sessions.

The molds should sit for at least 6 hours, preferably overnight. To remove the floaters from the mold efficiently, the following morning, rub the mold to scrape off any excess from its surface. One could also use a paper towel for some added friction. Given the challenge of plucking the shapes out of the molds, it is easiest to use a compressed air gun to pop them out. I tend to aim the compressed air gun at an angle to a side of the mold while pressing my thumb down on the opposite side to try and flick the disk out. This avoids some earlier problems occurring where the tweezers used for removal were damaging/pinching the shapes too much. Warning: If the compressed air comes out too fast, the shape will go flying and may be hard to find! Once they are out of the molds the disks can be placed easily on the surface of the water.