Probing collective dynamics of active particles using modulation force spectroscopy

In this letter, we report a method of measuring the dynamic viscosity of self-propelled active particles using an intensity-modulated optical tweezer. We have used a 6 mum trapped polystyrene bead suspended in a bath of motile bacterial cells as a probe. The response function amplitude of the oscillatory bead directly measures the dynamics of the spatiotemporal structure of the motile particles. We find that unlike passive systems, the viscosity is defined by distributions of response function amplitudes that represent the long-range active temporal structures. Appropriate Langevin equations are set up that capture all these essential features. (C) 2004 American Institute of Physics.