Periodically microstructured composite films made by electric- and magnetic-directed colloidal assembly

Abstract

Living organisms often combine soft and hard anisotropic building blocks to fabricate composite materials with complex microstructures and outstanding mechanical properties. An optimum design and assembly of the anisotropic components reinforces the material in specific directions and sites to best accommodate multidirectional external loads. Here, we fabricate composite films with periodic modulation of the soft-hard microstructure by simultaneously using electric and magnetic fields. We exploit forefront directed-assembly approaches to realize highly demanded material microstructural designs and showcase a unique example of how one can bridge colloidal sciences and composite technology to fabricate next-generation advanced structural materials. In the proof-of-concept experiments, electric fields are used to dictate the position of the anisotropic particles through dielectrophoresis, whereas a rotating magnetic field is used to control the orientation of the particles. By using such unprecedented control over the colloidal assembly process, we managed to fabricate ordered composite microstructures with up to 2.3-fold enhancement in wear resistance and unusual site-specific hardness that can be locally modulated by a factor of up to 2.5.L ightweight composites offer an attractive alternative toward minimization of the energy demand of mobility systems from automobiles and airplanes to trains and ships. Continuous stiff fibers or discontinuous anisotropic particles have often been used as reinforcing phase to enhance the specific mechanical properties of such lightweight polymer-based composites. Although the fabrication costs of long-fiber reinforced composites remain prohibitive for the large-scale production of high-performance materials, the processing of polymer-based composites reinforced with discontinuous stiff elements can often be adapted to the automated manufacturing processes commonly used in the polymer industry, such as injection molding. However, these fabrication processes usually offer a limited control over the alignment and spatial distribution of the reinforcing particles, preventing a full optimization of the microstructural design in composite materials. Thus, the development of new strategies to assemble microstructured composites with tailored reinforcing architectures is crucial for the design and fabrication of even lighter and cheaper structural materials.Control over the spatial distribution and orientation of particles in composites can be achieved using colloidal assembly tools that guide/template the organization of the reinforcing particles while the material is still in a fluid state. Recently, we have shown that control of particle orientation using an external magnetic field can lead to a significant increase in the materials' resistance against deformation, wear, and crack initiation, widening their range of applications (1, 2). Independent of orientation control, tuning the spatial distribution of particles through magnetic fields, centrifugal forces, or film welding techniq...

External fields for the fabrication of highly mineralized hierarchical architectures

“…To avoid this flow and maintain a homogeneous concentration of ions in the suspension, AC fields at frequencies between 50 kHz and 1 MHz are necessary. 69,70 Under these conditions, anisotropic particles, with homogeneous charge, will align their long axis along the field direction [ Fig. 5(C)].…”

“…76 To control the positioning of particles in complex patterns, arrays of positive and negative electrodes can be used to modulate the electric field strength in 2-D and in 3-D. In this case, positively charged components will be dragged toward the negative electrodes while the negatively charged components will be attracted to the positive ones 70 [Fig. 5(D)].…”

“…68 In some particular cases, it might even be beneficial to couple different external fields to have access to an even greater range of design freedom. 63,70,84 For example, 2D vertical alignment of discs can be realized by combining 1D electric field with a perpendicular 1D magnetic field. 66 To create composite materials with significant mineral content, the manipulation of the particles has to take place already in concentrated suspensions [ Fig.…”

“…(A) (a) Acoustic waves generated by a piezoelectric actuator can create a standing pressure wave in a channel of finite dimensions; (b-d) Anisotropic particles suspended in the channel will orient and concentrate at the pressure nodes of the wave64 (Copyright © 2015 ASA). (B) (a and b) AC electric fields generated by applying an AC voltage will orient anisotropic particles along the field lines69 (copyright © 2009 RSC); (c-e) arrays of electrodes alternate the electric field strength in the plane and allows for the local concentration of particles in selected areas70 (copyright © 2016 PNAS). Scale bars 20 lm.…”

Bioinspired Wear‐Resistant and Ultradurable Functional Gradient Coatings

“…Inspired by this design principle of nature, functional gradient coatings (FGCs) with the material compositions and mechanical properties gradually varying along the coating thickness direction have been developed . With improved mechanical compatibility between the coating and the substrate, FGCs can achieve simultaneously good interfacial integrity and high surface hardness/strength and have been applied extensively for thermal, wear, corrosion, and damage barriers …”