Measured drag coefficient and effective charge plotted as a function of salt concentration, expressed here as κ a, where κ = 1 / λ D . (a) Open symbols denote the values of measured γ E / γ S plotted as a function of κ a. Here γ _S denotes the calculated Stokes drag coefficient, and γ _E is obtained by using measured δ in Eq. (9). Error bars on the open circles arise from the errors in amplitude and phase measurements. (b) Measured effective charge magnitude | Q eff | is plotted as a function of κ a with open symbols. Filled symbols indicate the surface charge magnitude | Q S | obtained from simulations with measured mobility μ _E as the only input from the experiment. Error bars are from the mobility measurements. All results are for PS spheres with a = 0.75 μ m .