the point of straight line 1／4   
at the quarter of the line

At the quarter point of the straight line.

at a quarter of the straight line 四分之一是不是用泛指的a好一些

at the quarter of the straight line

plane = 平面   
surface = (或平或弯) 表面
  ---
沿某一方向 = along a certain direction
方向由所谓 outward normal 向外为正之矢量确定
boundary layer 表层为零, nnn beyond ... = ... (点或面) 之外 nnn 距离
  ---
(大约) 四分之一 = (about) a quarter

四分之一 距离之外 = about a quarter of the distance beyond (the frontal boundary)
  about / beyond  =  见上
  distance = 自内部某起点 (例: nucleus / nucleation site) 到表层之间的距离，

at the 1/4 point of the line