Testing a Capillary Tube Feeder

Last week, we returned to the organic farm at Pomona College to test a new feeder design. The last time we went to the farm, we found that while the bees seemed to like our feeder, it had a few flaws. Nectar tended to leak out around the edges of the jar, and didn't flow all the way into the grooves in the plate. We decided to look into ways to improve that design while also considering some other designs.

One alternative design that seemed promising was a feeder based on the principle of capillary action. Capillary action describes the tendency of liquids to flow upward in thin tubes. There are three forces involved in capillary action: 
1. The cohesive forces between the liquid's molecules, which determine the strength of its surface tension. 
2. The adhesive forces between the liquid's molecules, which determine how strongly the liquid is attracted to the walls of the capillary. 
3. The force of gravity pulling the liquid down. 
Adhesion pulls the water molecules next to the wall upward along the walls surface, and surface tension holds the upper surface, or meniscus, intact, so instead of just the edges of the liquid rising, the whole meniscus rises. 
 

Where y is the surface tension, r is the radius of the tube, θ is the contact angle (a way of measuring adhesive force), ρ is the density of the liquid, g is gravity, and h is the height of the surface of the liquid.

As the height of the liquid increases, the force pulling its surface downward increases too. The liquid will keep rising until the downward force is as big as the upward force, and then the surface will stop rising and stay at that height. The smaller the radius of the tube is, the higher the liquid will rise.

We got the idea for this feeder from a paper¹ by James Hagler and Stephen Buchmann, who used a capillary tube feeder in their research. The basic design is a jar filled with sugar water, with capillary tubes pulling the sugar water up through holes in the jar lid. Hagler and Buchmann used pipettes for capillary tubes, so we decided to try it. We ordered 5, 10, and 50 µL pipettes and tested them in the lab to see how high each one could transport sugar water of varying concentrations. The sugar water rose 32-49 mm above the surface in the 5 µL pipettes, 22-33 mm in the 10 µL pipettes, and 8-12 mm in the 50 µL pipettes. We decided to make a prototype using the 5 µL pipettes and find out whether the bees would use it.

On Tuesday morning, we used an aspirator to pull the sugar water to the top of each of four 5 µL pipettes, which we then put in a jar of sugar water. We cut out a blue circle of foam to go at the top of each pipette, in the hope that the bright color would attract the bees to the openings of the pipettes. We used 2.5 mol/L sugar water, and anise-scented essential oil.

When we got to the farm, we put the jar close to the hive and made a trail of sugar water droplets from the hive entrance to the jar, and put a droplet of sugar water on each circle of foam by the pipette opening. Just as before, it took a long time for the first bee to find the feeder.

The first bee drinks the droplet of sugar water on the foam.

Once the first bee found the feeder, other bees slowly began to arrive. However, they never seemed to drink directly from the pipettes, preferring instead to drink the droplets on the foam and trying to drink from the holes in the lid. While five or six bees checked out the feeder over the course of an hour or so, the crowd we saw last time never materialized. 

Two bees exploring the "flower".

 

A bee attempts to drink from the hole in the jar.

We suspect that because the bees didn't drink from the pipettes, they may have viewed the feeder as a less plentiful resource and therefore not advertised it very strongly to their hivemates. Maybe they couldn't figure out how to drink from the pipettes, maybe their proboscises couldn't fit inside the pipettes, or maybe there was some other flaw in the prototype. The problem was not immediately clear, but the capillary tube feeder currently seems more problematic than our original design. 

Prof. Donaldson spoke with a fluid dynamics professor, and got some advice on things we could try in order to improve our original design. If the original design starts looking like a dead end, we may come back to the capillary tube design and try to make it better. But for now, we're going to focus on improving the original design.

References:
1. Hagler, James R., and Stephen L. Buchmann. "Honey Bee (Hymenoptera: Apidae) Foraging Responses to Phenolic-Rich Nectars." Journal of the Kansas Entomological Society 66.2 (1993): 223-30. JSTOR. Allen Press. Web. 5 Aug. 2014. <http://www.jstor.org/stable/25085437?origin=JSTOR-pdf>.