approaching = moving like?

Context:

This is an artist's representation of epithelial cells (black) approaching the glass transition (blue). Increasingly large groups of cells (green, purple, red) are able to move together more rapidly than the surrounding cells. (Credit: Image courtesy of Thomas E. Angelini, University of Florida)

More (below the pic):

http://www.sciencedaily.com/releases/2011/02/110222092658.htm