title: Control of force through feedback in small driven systems.
abstract: Controlling a time-dependent force applied to single molecules or colloidal particles is crucial for many types of experiments. Since in optical tweezers the primary controlled variable is the position of the trap, imposing a target force requires an active feedback process. We analyze this feedback process for the paradigmatic case of a nonequilibrium steady state generated by a dichotomous force protocol, first theoretically for a colloidal particle in a harmonic trap and then with both simulations and experiments for a long DNA hairpin. For the first setup, we find there is an optimal feedback gain separating monotonic from oscillatory response, whereas a too strong feedback leads to an instability. For the DNA molecule, reaching the target force requires substantial feedback gain since weak feedback cannot overcome the tendency to relax towards the equilibrium force.
authors: II. Institut für Theoretische Physik, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors: E
authors: E
authors: Dieterich
authors: Departament de Física Fonamental, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.|CIBER-BBN de Bioingeniería, Biomateriales y Nanomedicina, Instituto de Salud Carlos III, 28029 Madrid, Spain.
authors: J
authors: J
authors: Camunas-Soler
authors: Departament de Física Fonamental, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.|CIBER-BBN de Bioingeniería, Biomateriales y Nanomedicina, Instituto de Salud Carlos III, 28029 Madrid, Spain.
authors: M
authors: M
authors: Ribezzi-Crivellari
authors: II. Institut für Theoretische Physik, Universität Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany.
authors: U
authors: U
authors: Seifert
authors: Departament de Física Fonamental, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain.|CIBER-BBN de Bioingeniería, Biomateriales y Nanomedicina, Instituto de Salud Carlos III, 28029 Madrid, Spain.
authors: F
authors: F
authors: Ritort