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Scanning tunneling microscopy (STM) offers unparalleled sub-molecular resolution for visualizing surface-bound molecular assemblies. We developed a custom 3D-printed liquid cell that enabled stable, long-duration liquid-phase STM imaging of a metallocene dimer assembled on a highly oriented pyrolytic graphite (HOPG) substrate. High-resolution images revealed two distinct molecular packing structures. However, STM alone is difficult to pinpoint the detailed molecular arrangements, resonance Raman spectroscopy (RRS) was used to provide complementary information. Aided with density functional theory (DFT) calculated RRS, a cis conformer of the metallocene dimer was identified as the more probable form in both crystal and surface-bound states. These findings led to assemblies with cyclopentadienyl rings pointing towards the HOPG, and the carbonyl groups towards the water. This work demonstrates the synergistic power of integrating STM, RRS, and DFT in elucidating molecular assembling structures at the solid–liquid interface. 

Microtubule-kinesin active fluids consume ATP to generate internal active stresses, driving spontaneous and complex flows. While numerous studies have explored the fluid's autonomous behavior, its response to external mechanical forces remains less understood. This study explores how moving boundaries affect the flow dynamics of this active fluid when confined in a thin cuboidal cavity. Our experiments demonstrate a transition from chaotic, disordered vortices to a single, coherent system-wide vortex as boundary speed increases, resembling the behavior of passive fluids like water. Furthermore, our confocal microscopy revealed that boundary motion altered the microtubule network structure near the moving boundary. In the absence of motion, the network exhibited a disordered, isotropic configuration. However, as the boundary moved, microtubule bundles aligned with the shear flow, resulting in a thicker, tilted nematic layer extending over a greater distance from the moving boundary. These findings highlight the competing influences of external shear stress and internal active stress on both flow kinematics and microtubule network structure. This work provides insight into the mechanical properties of active fluids, with potential applications in areas such as adaptive biomaterials that respond to mechanical stimuli in biological environments. 

Microtubule-kinesin active fluids consume ATP to generate internal active stresses, driving spontaneous and complex flows. While numerous studies have explored the fluid's autonomous behavior, its response to external mechanical forces remains less understood. This study explores how moving boundaries affect the flow dynamics of this active fluid when confined in a thin cuboidal cavity. Our experiments demonstrate a transition from chaotic, disordered vortices to a single, coherent system-wide vortex as boundary speed increases, resembling the behavior of passive fluids like water. Furthermore, our confocal microscopy revealed that boundary motion altered the microtubule network structure near the moving boundary. In the absence of motion, the network exhibited a disordered, isotropic configuration. However, as the boundary moved, microtubule bundles aligned with the shear flow, resulting in a thicker, tilted nematic layer extending over a greater distance from the moving boundary. These findings highlight the competing influences of external shear stress and internal active stress on both flow kinematics and microtubule network structure. This work provides insight into the mechanical properties of active fluids, with potential applications in areas such as adaptive biomaterials that respond to mechanical stimuli in biological environments. *We acknowledge support from the National Science Foundation (NSF-CBET-2045621). This research is performed with computational resources supported by the Academic & Research Computing Group at Worcester Polytechnic Institute. We acknowledge the Brandeis Materials Research Science and Engineering Center (NSF-MRSEC-DMR-2011846) for use of the Biological Materials Facility. 

Abstract Active fluid droplets surrounded by oil can spontaneously develop circulatory flows. However, the dynamics of the surrounding oil and their influence on the active fluid remain poorly understood. To investigate interactions between the active fluid and the passive oil across their interface, kinesin-driven microtubule-based active fluid droplets were immersed in oil and compressed into a cylinder-like shape. The droplet geometry supported intradroplet circulatory flows, but the circulation was suppressed when the thickness of the oil layer surrounding the droplet decreased. Experiments with tracers and network structure analyses and continuum models based on the dynamics of self-elongating rods demonstrated that the flow transition resulted from flow coupling across the interface between active fluid and oil, with a millimeter–scale coupling length. In addition, two novel millifluidic devices were developed that could trigger and suppress intradroplet circulatory flows in real time: one by changing the thickness of the surrounding oil layer and the other by locally deforming the droplet. This work highlights the role of interfacial dynamics in the active fluid droplet system and shows that circulatory flows within droplets can be affected by millimeter–scale flow coupling across the interface between the active fluid and the oil. 

Active matter consumes local fuels to self-propel. When confined in a closed circular boundary, they can self-organize into a circulatory flow. Such coherence originates from the interactions between the active matter and boundaries, and boundary conditions play an important role on self-organization of active fluid. Herein, we probed how fluid boundaries influenced the self-organization of active fluid. The fluid boundaries were created by confining the active fluid in a compressed water-in-oil droplet. Due to surface tension, the droplet shaped into a cylinder-like geometry. Since water and oil were both fluids, their interface was fluid. We systematically probed how droplet shapes and the amount of oil surrounding the droplet influenced the development of circulation. We found that the formation of circulatory flows depended on the thickness of the oil layer surrounding the droplet, implying that the fluid dynamics between the active fluid within the droplet and the oil outside the droplet were coupled. We used a 3D COMSOL-based simulation successfully reproduced such oil-layer dependence. Finally, we developed two milli-fluidic devices to deform the droplet and alter the oil layer thickness manually to trigger and suppress the intra-droplet circulatory flow in real time. 

Fluid dynamics of conventional passive fluid are known to be affected by boundary condition. For example, flow rates in a pipe depend on slipperiness of pipe surface. Similarly, active fluid, which consumes fuels locally to flow spontaneously, was reported to self-flow along a meter-long tubing with the flow rate depending on tubing geometry. However, how boundary condition influences fluid dynamics in an active fluid system remains poorly understood. Here, we investigated how a fluid boundary influenced self-organization of confined active fluid by establishing a 3D COMSOL-based nemato-hydrodynamic simulation platform where active fluid was confined in a compressed cylindrical water-in-oil droplet. Since the droplet interface was fluid, the fluid dynamics within and outside the droplet were coupled. Our simulation demonstrated that flow behaviors of intra-droplet active fluid were influenced by the amount of oil that surrounded the droplet: Without altering the droplet geometry, expanding the volume of oil could induce a circulatory flow within the droplet, which resembled our experimental observation. Our work suggested the feasibility of controlling the fluid dynamics of a confined active fluid system across a fluid interface.