Under the microscope, the synthetic fibers were fibrillating rapidly when exposed to electric current.

fibrillating 是动词 fibrillate 的现在分词形式。fibrillate 由词根 fibril（"小纤维"）和后缀 -ate（表示"使成为..."）组成，意思是"使纤维化"或"呈现纤维状颤动"。在这里指纤维在电流作用下产生的快速颤动现象。