Experimental observation of synchronization in a biomechanical rotational motors system

Summary form only given. Spatio-temporal dynamics of complex collective systems are of general interest in nonlinear physics due to the general underlying principles of complex systems' dynamics that can be found in many disciplines. Among these scenarios, synchronization demonstrates impressively how order can emerge from chaos by the coordination of single entities to operate a collective system in unison. Synchronization is known to emerge in mechanical, optical or biological systems [1,2] for example in coupled oscillators as they are represented by metronomes, coupled lasers or nonlinear optical systems [3,4]. Despite of vast theoretical and numerical modeling [2] and impressive demonstrations in nature (fire flies) [5], sociology (clapping hands), or technical systems, experimental verification is still limited for small scales. Especially biomechanical systems on the micro and nano scale, as flagellated bacteria, feature one of the smallest known rotational motors, which drive the flagella and allow the cell to propel through its environment, seem to be interesting candidates to observe synchronization. However, the biophysical properties of this kind of motors are not yet fully understood and even less is known about the role of hydrodynamic interactions of multiple motors, including possible cooperative effects and the resulting spatio-temporal dynamics. Therefore, nonlinear dynamical effects observed with bacterial molecular motors are typically limited to collective effects of thousands of bacteria [6]. We investigate interactions of multiple bio-rotational motors in optically induced arrangements of surfaceadhered bacteria. For the induction of defined configurations, we employ holographic optical tweezers (HOT), as a precise and at the same time flexible positioning and orientation scheme which has been optimized for rodshaped bacteria [7]. By means of structured, optically induced adherence at homogeneous surfaces, monitoring without possible influences of the trapping laser is achieved [8]. Figure 1 shows a schematic representation of the configuration of a pair of bacteria adhered to the surface. The distance between the two bacteria is varied from 2.5μm to 32μm (Fig. 1a). Digital image processing of video data with high temporal resolution allows accessing information on rotation frequencies and of the instantaneous bacterial body orientation (see Fig.1b) of single and multiple bacterial cells [9]. This provides the prerequisites to study their interaction and effects on their rotation behavior directly. We present our experimental platform for the investigation of bacterial coupling, discuss conditions for successful measurements of bacterial synchronization and show first experimental demonstration of phase synchronization and effects of (harmonic) frequency coupling.