Clogging is one of those phenomenons that we encounter constantly, from overflowing storm drains to the traffic jam at the door when a lecture ends. It happens at all scales, too; ink-jet cartridges and microfluidic circuits can jam up just as thoroughly as a grain silo. Although there are many complexities to clogging, the basic mechanisms fall into three categories: sieving, bridging, and aggregation.
Of these, sieving is the most familiar; it occurs when a particle too large for the constriction gets stuck. That includes both a rock too large to fit down a storm drain and a leaf that gets caught in the wrong orientation.
Bridging, on the other hand, occurs when too many small particles reach a constriction at the same time. Although each one is small enough to fit on its own, their simultaneous arrival means that they jam together into a bridge that blocks the constriction. Given time, all flow comes to a stand still, as seen in the images below.
The last mechanism, aggregation, is a more gradual blockage, formed as individual particles begin sticking to a surface, making the constriction progressively smaller. Think of those hard-water buildups that eventually block your shower head.
Some of these mechanisms are easier to prevent or clear than others, but researchers are making progress. For an overview of the field’s current standing, check out this Physics Today article. (Image credit: drain – R. Rampsch, bridging – D. Jeong et al.; see also B. Dincau et al. at Physics Today)