"You need two to tango . And as Mexico seeks to shut down the flow of drugs coming into the United States from Mexico and South America, we need the support of the United States to shut down the flow of weapons ,".

VOA: standard.2009.04.12

They move from regions of high concentration to regions of low concentration by this natural flow down a concentration gradient.

从高浓度区域,向低浓度区域流动,顺浓度梯度流动

耶鲁公开课 - 生物医学工程探索课程节选