I will first show how to engineer spontaneously flowing colloidal liquids: flocks. Simply put our strategy consists in letting self-propelled colloids with velocity-alignment interactions to collide. After a short transient they self-assemble into liquids with emergent long-range orientational order which translates into spontaneous unidirectional flows.
I will then devote most of my talk to discussing how these colloidal flocks behave in challenging environments. (i) I will first discuss the robustness of their spontaneous flows to external pressure gradients. I will then show that the dynamical response of flocks is hysteretic, and explain how to make use of this non-linear behavior to devise active-microfluidic oscillators. (ii)
I will finally discuss the robustness of colloidal flocks to spatial disorder, showing how the flow of polar flocks is channeled along sparse river networks. I will demonstrate that further increasing disorder suppresses collective motion in the form of a first-order phase transition generic to all polar active materials.