Inhomogeneous driving forces and the appearance of defects. Rotor chains possess a Volvox-inspired intrinsic driving force , where εi = εi(σ) is a random number chosen from a normal distribution with mean of zero and standard deviation σ. The frequency bias along the chain is governed by the parameter C. (a–d) Kymographs showing the phase, sin(Φi), and phase difference, sin(Φi+1 − Φi), for various values of C, with σ = 0 (no polydispersity). An example phase defect is circled in white. (e) Kymographs for C = 0.02 with σ = 0.0279. (f) Intrinsic frequency distributions (black) for 60 different realizations with C = 0.02 and σ = 0.0279, along with their average (yellow). The magenta curve corresponds to the frequency profile used in (e) and the inset (g) shows power spectra of sin(Φi) for oscillators 1 (red), 8–9 (green) and 30 (blue) in this chain. (h) Extracted metachronal wavenumber k. Simulations without polydispersity (σ = 0, black, 19 simulations) and with polydispersity (σ = 0.0279, red, 210 simulations) are shown. Other parameters are given by a/d = 0.01, r0/d = 0.5, l/d = 2 and Λi centred at Λ = 0.1.