Interfacial Regulation On Individual And Collective Motion Of Dumbbell-shaped Hematite Colloidal Motors

Colloidal motors have shown great advantages such as simple fabrications,various structures,as well as controllability in propulsions and movements.As the best simplified physical models,colloidal motors can imitate the emergent swarming behavior of organisms in nature and play important roles in interdisciplinary fields including biomedicine,nanotechnology and environment management.Emerging a swarm by move cooperatively,colloidal motors could achieve complex tasks that are hard for their individual counterparts.However,the emergence and control of collective motion of colloidal motors are difficult to be realized by global manipulations,due to the unclear mechanism of swarm emergence of colloidal motors.Inspired by natural swarms of all scales,we introduced a strategy to regulate the local fluid field around colloidal motors,which could spontaneously change the individual movements,the interparticle communications and the collective motion of these colloidal motors,optimizing the cooperative motion of the colloidal motors in the swarm.In order to compare the mechanisms of interfacial regulation to the local fields of colloidal motors propelled by external field and chemical field,a hematite dumbbell-shaped colloidal motor was fabricated with inherent ferromagnetism and photocatalysis capability.Under different externally-applied physical fields,the tunable individual movements and interparticle communications of the colloidal motors with different propulsions were studied respectively.After the swarm emergence of these two kinds colloidal motors,the emerging condition and physical characteristics of such swarms were investigated.Furthermore,the interfacial regulation of emergent swarms was achieved.The dumbbell-shaped hematite colloidal motor with a length of 2.8±0.2μm and a diameter of 1.1±0.3μm was prepared by a hydrothermal method.Under a rotating magnetic field or a conical magnetic field of 50 Hz,5.4 m T,the magnetic drive colloid motor can realize rolling or kayak-like wobbling movement with a velocity of 3.72μm s^-1 and 10.36μm s^-1,respectively.These two movements of the magnetically-driven colloidal motors can be controlled by changing the intensity,frequency,and direction of the applied magnetic field.These magnetically-driven colloidal motors can move with high speed and accuracy along predefined tracks.The fluid field simulation results show moving mechanism of the magnetically driven colloidal motors.By combining the two driving modes of the colloidal motor,a single cell can be manipulated in a non-contact way.The simulation results show that an pressure difference near the cell was induced by the asymmetric fluid field around colloidal motor,and account for the capability of the colloidal motor to manipulate cell.Based on the established magnetically-driven movements of the dumbbell-shaped hematite colloidal motors,the influence of interface on the individual movements of the colloidal motor was studied,and a swarm of these colloidal motors emerged near the interface.The interface regulation to the movement of colloidal motors was investigated by using sloped and vertical microfluidic boundaries.The simulation results explained that the changes in movement of the colloidal motor near a boundary result from the changes of fluid flow between the colloidal motor and the boundaries.The communication behavior of the colloidal motors near the vertical interface was studied and the influence of fluid field overlay on the motion speed of the magnetic drive colloidal motor was investigated.Based on the interfacial regulation of the colloid motor,a propagating swarm of colloidal motors was achieved near the interface,and the emergent speed increase was further investigated.Accordingly,the sequential traverse of a microfluidic maze was achieved by a swarm of colloidal motors,and a multi-motor-based conveyor for cell transportation could be formed.A chemically self-propelled dumbbell-shaped hematite colloidal motor with tunable speed and controllable moving direction was constructed,the individual movement and communications in between were then investigated.Experimental and simulating results show that the self-propulsion was induced by a light-activated diffusio-osmosis effect based on a photocatalytic reaction.When two light-activated colloidal motors approach each other to a critical distance of around 4.5μm,the communication between colloidal motors occurs.The simulation results and experimental data were accordant to show that the communication behavior between individual light-activated colloidal motors is influenced by both a short-ranged magnetic dipole-dipole interaction and a long-ranged chemical-induced fluid field effect.A phototactic swarm of the light-activated dumbbell-shaped hematite colloidal motor was emerged.The emerging condition,mechanism and regulation of such swarm were then studied.A threshold surface density fraction（4.1%）of the emerging process was found,with the experimental results of the swarm emergence.The analysis of the fluid field shows that the fluid changes,caused by the light-activated chemical concentration gradient,dominates phototaxis process of the colloidal motors.According to the local surface density and active colloidal ribbon size which were increased emergently in the swarming process,motion-induced phase separation and giant number fluctuations were shown as two typical physical characteristics of the non-equilibrium system.Under structured light illumination,a patterned swarm was emerged according to the phototactic direction of the colloidal motors.By applying the magnetic field in a direction of diffusion,the reversibly assemble-disassemble control of the light-activated colloidal motor with multiple cycles is realized.The improvement of this chemically propelled swarm of colloidal motors by using passive particles was also studied.To sum up,this thesis uses the magnetically-driven and light-activated dumbbell-shaped hematite colloidal motors,to study the communications between individual colloidal motors based on local fluid fields,the regulation of collective behavior by a"bottom-up"way,and the cluster coordinated movement of the interface control mechanism for different type colloid motor cluster emergence and the conditions of research,to gel motor cluster more widely used in the complex environment laid a theoretical foundation.