Sentence:  <WSD>Essentially</WSD> these birefringence studies show that at low rates of shear a tension is present at 45 ` to the direction of shear , and as the rate of shear increases , the direction of the maximum tension moves asymptotically toward the direction of shear . 
Meaning of the word marked with <WSD> tags:  in essence; at bottom or by one's (or its) very nature