Measuring VWF elongation in shear flow. In shear flow, the fluid farther from the wall moves faster, as illustrated by the gray arrows. Free VWF moves during image acquisition, so the image forms a blur that is longer than the length of the protein, and depends on the distance of VWF from the wall. Surface-attached VWF does not move with the flow, so any elongation in the image reflects actual protein stretching. Professional illustration by Somersault18:24.