Abstract

It is estimated that approximately 20 million tons of plastics enter the ocean each year. But only about 300 thousand tons are observed to be floating on the surface or washed up on shore. That a tiny percent of plastic pollution is observed is surprising because most produced plastics are less dense than water. So where are the missing plastics? Answering this question poses several new challenges to the fluid dynamics community. In particular, it is known that large plastic detritus eventually breaks up into microplastics, which are more difficult to observe and collect. But how the break up occurs is not well understood, particularly for particles that are millimeter scale and smaller. It is also known that marine organisms can grow on floating plastics causing them to become more dense and sink. But the settling of such “biofouled” plastics is not well understood. In this talk, I will first survey some of these challenges, based on the outcomes of recent workshops in Banff, Canada and Lerici, Italy. I will then present experimental work examining how buoyant plastics can become more dense not due to biology, but due to the attraction of clay, glass and sand to microplastics.