GESTIÓN DE RECURSOS

Micro assembly is the manipulation and assembly of sub-millimeter sized parts with high- resolution capabilities. Micro assembly has applications in many fields including micro surgery, microrobotics, biological sciences, micro manufacturing and soft matter. The abil- ity to manipulate with microscale precision to assemble components is important in all of

Water Oil

(a) (b) (c)

Figure 39: Magnetic robots can be used to direct assembly of passive micro structures using capillary effects at fluid interfaces. (a) The actuated robot (the triangle) can selectively assemble passive structures (shown as circles) to predetermined locations. (b) The curvature of the fluid interface created by the robot and the passive particle result in an attractive force that binds the robot and particle. (c) Top view of robots. Differently shaped robots will have different types of effect on the surface curvature and can direct particles with specificity to assemble to corners or more randomly around a circle.

these applications. To address these challenges in micro manipulation, a wide range of meth- ods have been explored including probing payloads using a micro manipulator [10], using microfluidics to manipulate cells [33] and using capillary interactions to build tissues [24].

In recent years, magnetic manipulation has attracted increasing attention because of several key associated advantages. Magnetic manipulation allows control of the robot with high degrees-of-freedom, without the need for line-of-sight interaction or tethers. Addition- ally, magnetic manipulation allows selective addressability, in contrast to electric fields. This allows the robot to function in small and difficult to reach places. Additionally, magnetic fields are well known to be safe to biological cells and tissue, and are widely used in the medical field [25].

A variety of different strategies for magnetic micro actuation in fluidic environments using magnetic fields generated by electromagnetic coils have been demonstrated [2]. Image based position control of a single magnetic particle at an water-air interface is explored in one-dimension by Dkhil et al. [22] and in a plane by Keuning et al. [37]. In work that combines magnetic manipulation and capillary interaction, Grosjean et al. [31] use three soft ferromagnetic beads powered by externally applied magnetic fields to swim across a water-air interface. The beads were collectively controlled along trajectories by oscillating the magnetic field to break the symmetry in the system. One of the advantages of this system is that the structure is self assembled by capillary interactions, making scaling the system to

micro-scale applications feasible, as precise manipulation for assembly of the system is not required. This system differs from our work in that all particles in the system are magnetic and locomotion is based on non-reciprocal motion.

Micro manipulation and assembly of passive parts using magnetic actuation has been demonstrated using a variety of methods. Ng et al. [49] combine a mixture of magnetic and non-magnetic gears assembled by capillarity at a perfluorodecalin-air interface. An exter- nal magnetic source is used to rotate the magnetic gears and torque is transferred to the non-magnetic gears by a combination of mechanical interaction, hydrodynamic shear and capillarity. Martel et al. [43] use a swarm of magnetotactic bacteria to manipulate micro bricks to assemble structures. While this method may be effective, precise control of the micro bricks is challenging. Diller et al. [78] used a magnetic microrobot to manipulate building blocks submerged in fluid in 2D and 3D settings. In this example, heterogeneous building blocks of rigid and soft materials were combined to create complex functional ma- terials. However, while the structures could be reconfigured with a resolution of tens of microns, the robot was teleoperated, and manipulation of the building blocks was challeng- ing, as only pushing motions were possible, and no grasping or pulling of objects could be achieved with the cube shaped microrobot.

In soft matter, assembly of colloids and other microscale objects lends fundamental insight into collective behaviors that emerge from many-body interactions, and provides a route to form reconfigurable materials. Magnetized ferrofluids comprising suspensions of ferromagnetic particles have been widely studied in this context [45, 80]. There are other means of organizing colloids that do not rely on the usual electromagnetic fields. One important means exploits the energy stored by distortions made by particles in their soft matter hosts. Capillary interactions are an example; particles attached to fluid interfaces deform those interfaces. The energy stored from this deformation is the product of the surface area of the distortion and surface tension. When surface distortions from neighboring particles overlap, particles interact and assemble [7]. More generally, when particles distort curved interfaces, they move along principle axes to sites of high curvature to lower the

surface area, and hence the energy, of the interface via curvature capillary migration [13, 64, 87]. The particles themselves can be arbitrarily complex, for example, cell-laden microgel particles trapped at liquid interfaces were assembled by Du et al.[24] into biomimetic 3D tissues.

In this paper, we demonstrate that an untethered, independently actuated magnetic robot can serve to control the surface deformation source at a liquid interface, Figure 39. Passive particles located at that interface interact with the robot via curvature capillary migration, and are attracted to its edges. Moreover, by tailoring robot geometry, preferred sites for particle assembly are defined, e.g. at robot corners, where associated interface curvatures are high, Figure 39c. The ability to move the robot allows specific particles to be selectively assembled at predictable sites in a way which would not occur without the ability to manipulate the robot. Furthermore, the bulk motion of the assembly of passive particles can be manipulated precisely as a result of being docked to the magnetic robot.