Free vs surface-tethered VWF extension under flowshows inconsistent behavior. (A) Diagram illustrating²¹ free-in-flow (blue) vs surface-tethered (red) VWF with applied shear flow. (B) Data from the study by Schneider et al²² (blue) showing normalized extension vs shear stress for free VWF and data from the study by Fu et al⁶ (red) showing normalized extension vs shear for tethered VWF. Required shear stress for free-VWF extension is expected to be higher than required shear stress for surface-tethered extension, but experimentally, the opposite was observed. (C) Predictions of mean extension in response to shear stress based on a Brownian dynamics model with a strong Lennard-Jones (LJ) interaction potential proposed by Schneider et al²² for both a free-in-flow (blue) and tethered (red) polymer. Lengths are normalized based on the maximum observed length in the direction of flow.