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Abstract Structural coloration in biological systems arises from the interaction of light with micro‐ and nanoscale structures, producing vivid, pigment‐free optical effects. While this phenomenon is well‐documented in butterflies and birds, recent reports have revealed that certain microorganisms, particularly those in theBacteroidetesphylum, also exhibit striking structural coloration when formed into biofilms. In the marine bacteriumCellulophaga lytica(C. lytica), iridescence emerges dynamically during biofilm development and is tightly coupled to gliding motility, a surface‐associated mechanism of locomotion. However, the influence of environmental mechanics on this self‐organizing photonic behavior remains poorly understood. This investegation demonstrates how substrate properties, specifically agar stiffness and salt‐modulated stress relaxation, regulate the gliding motility and emergent iridescence ofC. lyticabiofilms. Time‐lapse imaging, quantitative optical analysis, and bulk rheological measurements demonstrate that increasing agar stiffness enhances early‐stage collective motility and promotes the formation of green‐iridescent biofilms. Furthermore, salt concentration modulates the viscoelastic properties of the substrate, impacting both motility dynamics and the spatial evolution of structural color. Correlating substrate stiffness and development time with observed dominant iridescent hue enables the construction of a phase map revealing distinct regimes of photonic behavior, thus providing a framework for designing biologically‐inspired living optical systems with customizable structural colour. 

Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water. 

Using a single rotating magnetic field, RBC biohybrid micromotors can be controlled to achieve propulsionviaswimming and rolling modes. The propulsion mechanism, directional control, and behavior in different fluids is investigated.