Figure A.1.
Diagram of tethered bead with tether stretched by flow to the right, for calculation of tether length × as a function of drag force Fdrag. Bead of radius r is attached to flow cell surface via a single DNA tether. Tension in DNA FT increases until it balances the drag force on bead. When the bead is stationary in flow, the height h of the tether attachment point is such that torque from shear flow (clockwise arrow) balances that from off-center tether attachment (counter-clockwise arrow) and FT cos(ϕ) = Fdrag. Angles ϕ and κ are defined in the figure. A relation between force and extension from the worm-like chain model of DNA and fluid dynamics predictions of force and torque on a sphere as a function of flow rate for a sphere resting on a surface allow all variables to be determined as a function of lateral flow rate (see appendix).