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.

Keywords: colloids; composites; dielectrophoresis; self-assembly; wear resistance.

Fig. 1.

Sample cell design and electric field strength simulations. (A) Sample cell design with microfabricated electrodes. (B) Sketch of the one-sided microfabricated electrode cell and the corresponding micrograph of the electric field strength variations within the cell. (C) Force field diagram depicting the negative DEP force applied to particles at a distance 0.30× the void size over a patterned electrode. (Inset) Sketch of the micropatterned electrode. The blue ring highlights the location of highest field gradient, where the force exerted onto the particles is largest. (D) Plot of the electric field strength along the electrode at an elevation of 20% of the pitch size. (E) Electric field strength oscillations given at different elevations to demonstrate the height dependence of the DEP forces. The indicated elevation refers to the height from the electrode surface relative to the void size of the micropattern.

Fig. 2.

Directed self-assembly of fluorescently labeled spherical colloids using negative DEP. (A) Fluorescent colloids are found to spread all over the sample cell 1 min after the field is turned off. (Inset) Zoomed-in image of the edge of the void. (B) When an electric field of 1 V_rms/µm is applied particles move to the low-field regions, corresponding to the voids of the electrode. (C) The assembly spans the volume between the two electrodes and forms a 3D pillar that becomes thinner toward the unpatterned electrode. The aspect ratio, h/d, of the pillar is ∼10, with a height h of 100 µm and a diameter d of 10 µm (here “g” denotes the direction of the gravity). (D and E) SEM images of assemblies fixed within a photosensitive polymer matrix while the field was on. Structures shown in D and E were obtained using micropatterned electrodes with round (D) and square (E) voids, respectively. Assembly is consolidated within the photocurable polymer matrix. (All scale bars are 20 µm except the one in the inset E, which is 10 µm.)

Fig. S1.

Confocal microcopy images of colloidal silica particles suspended in between micropatterned electrodes with and without electric field. These confocal images were taken from a sample with 10-µm void size. The panels show the reversible directed assembly using electric fields. Notably, the particles assemble when the electric field is on.

Fig. S2.

Confocal microcopy image reconstruction that illustrates the 3D assembly. This is a 3D reconstruction of the confocal image series scanned in z axis. It nicely demonstrates the organization of the assembly in 3D. The 3D shape results from the force gradient generated by the electric field and gravity. As shown in Fig. 1E (main text) the electric field gradient decreases as a function of distance from the surface of the patterned electrode. As the DEP forces depend on the electric field gradient, forces experienced by the particles lower at higher distances from the patterned electrode surface. Thus, the amount of particles gathered decreases away from the bottom electrode, which causes the shape of the assembly shown above.

Fig. 3.

Various designs of electrodes resulting in complex 3D self-assemblies of colloidal particles. (A) C-shaped electrodes assemble particles into a 3D Greek theater shape. (Inset) The 3D structure constructed from a stack of confocal images. (B) Another assembly on a star-shaped electrode design. (Insets) Zoomed image (Top) of a single star, which shows the crystalline order of the particles in the assembly, and (Bottom) the 3D reconstruction of the confocal images. (C) A cross electrode design results in an assembly of 3D cross shapes. (Insets) Zoomed image (Top) and a 3D construction image of the assembly (Bottom). Note that the bottom shape dictated by the electrode design evolves to a rounder shape as it grows in height toward the pattern-free electrode. (Scale bars, 50 µm.)

Fig. S3.

Confocal microcopy image of particle assemblies that are concentrated by DEP forces and form crystals. As a result of DEP forces, particles concentrate in the electric field wells. We observe a phase change from the initially liquid phase of colloids to a crystal phase as the threshold volume fraction for phase transition is achieved. Brownian colloids are known to form a hexagonal close-packed (hcp) phase with the hexagonally ordered 111 surface parallel to the substrate surface (52). We observe such crystal phases of hcp crystals especially at lower field strengths. However, we also observe crystals structures lying on a square lattice, as shown above. The DEP forces concentrate the particles to crystallize and can also induce phase changes due to dipolar interactions between the colloids.

Fig. 4.

Combination of magnetic and electric fields for the self-assembly of anisotropic colloidal particles. A combination of electric and magnetic fields was used to direct the position and the orientation of silica-coated alumina platelets, respectively. (A) A rotating magnet, as sketched, orients the magnetized platelets in the plane of rotation, whereas the electric field positions the particles due to DEP forces. (B) SEM images of a composite where platelets are assembled in a square array of round islands with vertical alignment. (C) A composite film with horizontally aligned platelets in a square array of square islands. (Scale bar is 100 µm in B and C but 50 µm in small panels of C.) Top image in C has the same alignment as bottom (C). The square depicts the plane of alignment (horizontal/in-plane alignment) whereas oblique square indicates that the plane of alignment is perpendicular to the image (vertical/out-of-plane alignment).

Fig. 5.

Vickers hardness and wear resistance of composites with spatially and orientationally controlled platelets. (A, Top) Sketch indicating the distribution of platelet-containing islands within the matrix and the spots where Vickers indentation was performed. (A, Bottom) SEM image of a sample with islands of horizontally aligned platelets after being locally probed with several Vickers indents. (B) Local Vickers hardness of islands containing horizontally and vertically aligned platelets compared with the nonreinforced surrounding matrix. (C) Two SEM images of the same region of the sample taken with different beam voltages to identify the location of the islands containing vertically aligned platelets underneath the composite surface (see also Fig. S4). (D and E) Wear behavior of the composites depending on (D) the alignment of platelets within the compartments and (E) the compartmentalization of the reinforcing elements. (F) Wear volumes given for randomly oriented (V), noncompartmentalized (IV), and compartmentalized ordered (I–III) structures after 300 wear ball rotations.

Fig. S5.

Sample preparation for the Vickers hardness and wear resistance tests. The samples for Vickers hardness and wear tests were prepared as shown above. First the particles were assembled and fixed within the sample cell, followed by consolidation and drying of the matrix. As the composite film is about 30 µm thin, we deposited an extra epoxy layer (about 300 µm thick) on top of the sample. The Vickers hardness and wear resistance tests were performed on the surface that was in contact with the bottom electrode. Composite films were carefully peeled from the surface of the micropatterned electrode using a razor blade.

Fig. S6.

Periodic variation of the surface properties for composites containing in-plane and out-of-plane aligned platelets. (A) Vickers hardness on the surface of composites reinforced with horizontally aligned platelets. (B and C) Nanoindentation hardness and elastic moduli values for composites reinforced with in-plane (B) and out-of-plane (C) platelets.

Fig. S7.

Wear volume for compartmentalized and noncompartmentalized samples with different platelet orientations after 300 ball rotations. Compartmentalization and out-of-plane (vertical) alignment of the embedded platelets significantly improve the resistance of the composite against wear. The gray bar on the far left is the wear measurements from randomly oriented sample where no spatial or orientation control was implemented. Note that the vertically aligned and compartmentalized samples (dark green/khaki, orange) improve the wear resistance of the composite up to 2.3-fold.

Fig. S8.

Electric field strength plots on the microfabricated electrode. (A) Electric field strength surface plot slightly above the micropatterned electrode. (B) A 1D plot of the field strength along the line shown in A. (C) Negative derivative of the plot in B, which indicates the force profile that the particles experience in case of negative DEP. Note that a particle will likely assemble at/close to force = 0, where the curve shows a negative slope. This condition provides a counteracting force when a particle moves to the right or to the left.

Fig. S4.

Vickers indentation imprints on composites exhibiting controlled spatial distribution and orientation of micrometer-sized platelets. SEM images of the same area of interest using different beam voltages are shown. The exact penetration depth depends on the type of material. For high beam voltages, as in the SEM image on the right, more information from the bulk of the sample is collected, which enables identification of the position and orientation of the platelets. Note that the larger indentations are located in between the platelet islands, whereas the smaller ones sit on top of the regions locally reinforced by the platelets. The dashed frames are drawn to guide the eye. The red circle is used to provide a reference for the two images.