Fig. 5. Characterization of forces in flow-stretch experiments. (A) A cartoon depicting geometry on a flow-stretch experiment. Under laminar flow conditions, the velocity profile is parabolic. (B) Mean DNA extension as a function of flow rate in a flow stretch experiment using 1 μm beads and λ/2 DNA molecules. The solid line is the fit to eqn (8) with P = 40 nm as a fixed parameter, obtaining L = 8.6 μm. (C) Mean force as a function of flow rate for the same data set. The force was determined from a calibration force–extension curve obtained from the vertical configuration in the absence of flow. The force increases linearly up to a maximum value of ∼1.5 pN, in accordance with eqn (9). Error bars in B and C are the standard deviation of the mean from measurements of multiple beads.