{HEAD ~ A fresh supply} of solution to {TAIL ~ the face} of the membrane is provided by a stirrer 
{ HEAD ~ A fresh supply } of solution to { TAIL ~ the face } of the membrane is provided by a stirrer
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH