Trawling For Microplastics

My business mentor in the Venn Garage program, Alicia had suggested that it might be interesting to see if AMOS could be used to look for microplastics in water. Microplastics are generally defined as any fragment of plastic that is less than 5 mm in length. There are a number of ways that researchers can sample water to check for microplastics. One of the most common ways is to drag a net with very small pore holes behind a slowly moving boat. Usually the net has a pore hole diameter between 100 and 500 micrometers, depending on the minimum particle size desired. As a quick experiment with AMOS, I 3D-printed an opening for a reusable produce bag, made of a woven fabric material with a pore size somewhere in the 300 to 500 micrometer range.

I took AMOS back to the Kelly Creek Basin area of Woolastook Park, and set it for its usual course. The day before we had had our first major snowstorm of the year, about 10 cm of snow, and there was a bit of a chill in the air. Although the water temperature was around 9 degrees C, the air temperature was only -2 deg C at the conclusion of the test.

After traveling for about 4 km, I noticed that AMOS seemed to be stalled in the middle of the river. I paddled over to investigate, and found that the propeller motor was making a strange, high pitched whining noise. Thinking perhaps it was just a strange combination of the low temperature and low battery, I tried to change the battery for a spare that I had on hand. No difference. So I hitched up AMOS to my tow line and towed it 4 km back to the launch point.

The post-test analysis showed that the motor was completely seized up. Researching a bit about drone performance in cold weather: apparently you should avoid flying a drone in cold, damp weather, such as directly above water that is considerably warmer than freezing air. Humidity from the warm water can condense and build up on the propeller blades, in some cases causing excessive vibration that can damage the motor. That's my best guess as to what must have happened. I didn't think to look for ice buildup at the time, but it probably would not have taken very much to cause a significant vibration. The propeller is very thin and light, and it spins extremely fast.

Back on shore, I had a look at the material collected within the produce bag:

Mostly it was a species of river grass common to the area, but there was also a tiny snail shell, a strange 2 mm long shrimp-like creature, many small black dots (< 1 mm diameter) that appeared to be organic, but I couldn't be sure, and a 2 mm long piece of some white substance which was probably plastic, but might also have been a small bone fragment. Based on the diameter of the collection hole and the distance traveled, approximately 22000 L of water was sampled. There was no noticeable reduction in the speed of AMOS, which was encouraging to see. I guess it was also a good thing to learn that the water around Woolastook Park appears to be mostly free of microplastics (at least visible microplastics).