Magnetophoretic Assembly of Anisotropic Colloids for Spatial Control

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Magnetophoretic Assembly of Anisotropic Colloids for Spatial Control of Reinforcement in Composites Ahmet Faik Demirörs J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05935 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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The Journal of Physical Chemistry

Magnetophoretic Assembly of Anisotropic Colloids for Spatial Control of Reinforcement in Composites Ahmet Faik Demirörs∗ Complex Materials, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland E-mail: [email protected] Phone: +41 44 632 6431

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Abstract Anisotropic particles have attracted interest for decades and have been studied for many aspects, ranging from fundamental phase behaviour to photonic properties. In addition, magnetic fields have been heavily used for external manipulation of colloidal particles for separation, assembly and even for photonic applications. Here we use magnetic field microgradients established in a paramagnetic fluid to act as templates for the assembly of both non-magnetic and magnetic anisotropic colloidal particles. We embed the assembled structure in a polymer matrix in order to obtain a composite where the spatial distribution of the reinforcing particles are preprogrammed. By using a mixture of paramagnetic and diamagnetic particles with different mechanical strengths, a periodical modulation of reinforcement by the variation of the particle type at different locations is achieved. Furthermore, we introduce a similar method for assembly of paramagnetic particles, where we use magnetic gradients of permanent magnet arrays to obtain field gradients and modulate spatially the particle concentration, thus reinforcement, through the macroscopic samples in three dimensions.

Introduction Interest in assemblies of colloidal particles has been motivated by their applications in many fields in a range from photonics 1–3 to sensors 4,5 and electronics 5–7 . Anisotropic colloids, either crystalline and readily available (natural), cheap but polydisperse 8,9 or synthesized by bulk synthesis methods, monodisperse, and possess better functional properties (photonics, electronics 10 ) but available in smaller quantities 10–12 , have both received considerable attention in past decades. Anisotropic particles were studied for fundamental phase behaviour 13,14 but also for their mechanical aspects as reinforcing elements in synthetic 15 and biological composites 16 . Natural anisotropic particles are grown to their anisotropic shape as a result of different crystal facets, which grow at different rates. Such anisotropic rods or platelets are usually single crystalline and exhibit different mechanical strength on their different facets 17 , which makes these particles a key constituent of synthetic and natural composites to reinforce the matrix 18–20 . Usually controlling the orientation, position and concentration/gradient of ACS Paragon Plus 1 Environment

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these elements is key for optimum mechanical properties 18,21,22 . Nature also uses such strategies like orientation and spatial-distribution control of reinforcers to enhance the mechanics of bio-materials 23 . Although rare, controlling orientation of discontinuous stiff reinforcers was addressed recently 18 thus we focus here on the spatial control of the reinforcing elements, which we demonstrate by controlling the position of monocomponent and (simultaneously) binary particle systems. In a recent work 22 , we managed to control the spatial distribution of reinforcers with electric field gradients, however this was demonstrated for a monocomponent system and lacked control over the whole space as we could only control the reinforced domains but non-reinforced parts of the composite relied solely on the mechanics of the polymer matrix. Therein, we showed that such mechanical modulation considerably improves the wear resistance of the composite films 22 . In addition to wear resistance, locally varying mechanical properties have been recently reported as an important crack-arresting mechanism in biological materials 24 , which enhances fracture toughness of such materials. Note also that, structures with periodically modulated local mechanical properties were observed in biological materials, as in the mussel threads 25 , spicules of sea sponges 24,26 and dental enamel 19 to improve the mechanics. Despite the fact that well defined periodic variations of local mechanical properties are expected to strongly regulate the global mechanical performance of composites, assembly tools to enable spatial control of reinforcing elements are rare. Furthermore, having a bimodal control of local surface mechanics is expected to enhance the surface mechanics and the level of control even further. Here, we use magnetic field gradients and a mixture of paramagnetic and diamagnetic particles that allow for a bimodal spatial control over the composite matrix. As reinforcing constituents we use different types of anisotropic particles i.e. rods and platelets.

We employ a general self assembly method for creating self assembled structures and extend its use for several anisotropic particle systems by using magnetic fields to manipulate either paramagnetic or diamagnetic colloids or both simultaneously. The magnetostatic potential experienced by a particle of χpart −χmed |H(r)|2 , where radius a in an applied field H at location r is given by UMag (r) = −2πa3 µ0 χpart +2χmed +3

µ0 is the magnetic permeability of free space, and χpart and χmed are the magnetic susceptibilities of the particle and of the dispersing medium, respectively. Although the magnetic field influence on



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the diamagnetic particles is typically low, this limitation can be overcome by adjusting the magnetic susceptibility contrast (χpart − χmed ) between the particles and the medium. In particular, it is known 27 that by utilizing solutions of paramagnetic salts (e.g. Ho(NO3 )3 ), one can tune the magnetic mag susceptibility of the medium such that χdia part < χmed < χpart . This will allow one to adjust the

magnetophoretic forces, FM ag = −▽UMag , to manipulate both paramagnetic and diamagnetic colloids by attracting the paramagnetic particles to the regions of high field strength and expelling the diamagnetic particles from these high field strength regions. This effect, together with patterning of magnetic fields on commensurate length scales, can be used to position both paramagnetic and diamagnetic particles. We implemented this idea to anisotropic particles, which aims to reinforce the mechanical properties of polymer matrices. Thus, we first made assemblies of colloids, either paramagnetic or diamagnetic, and then embedded these assemblies into polymer matrices to fabricate composites, where we demonstrate a first example of binary microspatial control on the reinforcement of the composite by controlling the positions of the reinforcing particles.

Experimental Particles: Monodisperse silica rods (4.65 ± 0.4 µm long and 752 ± 117 nm-thick ) used in this study were synthesized according to the synthesis of Kuijk et al 12 . For magnetic platelets we used natural alumina platelets, 8.3 ± 4 µm long, 403 ± 139 nm-thick (Merck KGaA, Germany, white sapphire grade) and modified them by electrostatically adsorbing oppositely charged superparamagnetic iron oxide nanoparticles (SPIONs) on their surface 18 . Fluorescent alumina particles were made by following a method described by Libanori et al. 8 Diamagnetic 3 ± 0.8 µm thick and 18 ± 6.8 µm long calcium sulfate hemihydrate rods were synthesized according to the procedure of Wang et al. 28 . These particles were suspended in a 0.2–0.4 M solution of the paramagnetic salt Ho(NO3 )3 in DMSO:water mixtures. To prevent aggregation of the particles by van der Waals forces, the suspensions were refractive-index-matched by the addition of an appropriate amount of DMSO/water. Exceptionally calcium sulfate hemihydrate rods were suspended in ethanol as they are partially water


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soluable. Finite Element Analyses: Simulations of the magnetic fields above nickel grids were performed using COMSOL Multiphysics software with relative magnetic permeabilities of air (µr (air)=1) and of nickel (µr (nickel)=200), and with magnetization of the permanent magnet equal to 105,000 A/m. Grids: Nickel grids were fabricated by standard photolithography, dissolution of the exposed photoresist regions, e-beam evaporation of the 200-nm nickel layer, and lift-off of the masking photoresist by sonication in acetone. Vickers Indentation: A Wolpert Microhardness tester MXT- was used to measure Vickers hardness of the samples. Measurements were carried out using an HV 0.01 (0.0981-N load) indenter and a dwell time of 10 s.

Results and Discussion The strategy we used to pattern the magnetic field was to fabricate nickel grids of desired geometry/dimensions and typical periodicity of L ≈ 10 to 100 µm. These grids were made by conventional photolithography methods followed by evaporation of nickel and lift-off the nickel (for details, see Supplementary Information, SI, Figure S1). These grids placed onto a permanent magnet of typical strength 0.44 T. Under these circumstances, the ferromagnetic nickel film modulates the otherwise uniform field of the magnet. Finite-element calculations (COMSOL) in Figure 1a and 1b illustrate the modulation of the magnetic field over such a nickel grid. Such modulations result magnetic gradients over the grid, which causes local magnetophoretic forces attracting paramagnetic particles onto the nickel regions and pushing the diamagnetic particles onto the voids of the grid 27 . We demonstrated this assembly scheme first by using fluorescently labeled monodisperse silica rods, obtained according to the synthesis method of Kuijk et al 12 . Figure 1c shows confocal microscopy image of diamagnetic fluorescent silica rods that assemble in voids of the nickel grid, which is a square array of round wells. Inset in Figure 1c is an SEM image of the assembly after drying. Figure 1d demonstrates a similar assembly of fluorescent rods over a hexagonal array of 5 µm sized nickel discs where the rest of the area is nickel free. In both template geometries silica rods are driven



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Figure 1: (a) Magnetic flux density fluctuations over a line on the ferromagnetic grid placed on a magnet. The fluctuation vary depending on the position of the line, whether on the bars (red line in (b)) or over the wells (green line). Red and green spheres denote where the magnetic and nonmagnetic particles will be assembled, respectively. (b) Magnetic flux density calculations over a square grid, which depicts the domains of field variations. (c) Diamagnetic silica rods assembled on the wells, where the field is lowest. (d) Diamagnetic silica rods assembled over an array of round nickel discs.

towards the low field regions and yield the shown assemblies, i.e. in Figure 1c, particles sitting in round voids and in Figure 1d, particles fill the hexagonal network of nickel free regions. Note in Figure 1d most rods lie down but there are few standing (seen as ring-shaped fluorescence). To demonstrate the possibility of assembling anisotropic paramagnetic particles, we used natural alumina platelets, 8.3 µm long, 400-nm-thick (Merck KGaA, Germany, white sapphire grade),


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Figure 2: (a) Paramagnetic alumina platelets arranged on a square nickel grid and attracted to the grid bars where the field strength is high. (b) Diamagnetic alumina particles with a fluorescent silica shell that assemble on the voids of the grid where the field strength is low. (c) A mixture of paramagnetic alumina particles modified with a fluorescent silica shell (red) and 5 µm long fluorescent silica rods (green) assembled simultaneously over a square nickel grid .

which provides an easily accessible particle system for this purpose. Although, alumina is naturally diamagnetic, it can easily be modified to obtain paramagnetic particles by electrostatically adsorbing oppositely charged superparamagnetic iron oxide nanoparticles (SPIONs) on the surface of the platelets. It is known that a low surface coverage of such nanoparticles is enough to obtain magnetically responsive alumina platelets 18 For instance, minor SPION concentrations in the range of 0.01–0.5vol% allows for platelet alignment in magnetic fields as low as 1mT. We slightly modified the recipe of Erb et al 18 and doubled the superparamagnetic iron oxide particle concentration during the magnetization of the platelets to enhance the magnetic repsonse of the platelets. Magnetically modified alumina platelets and diamagnetic alumina platelets (unmodified) were separately assembled



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to account for the assembly of both species using the magnetophoretic assembly scheme. Figure 2a demonstrates the assembly of paramagnetic alumina particles over a square grid where all the platelets organise over the grid bars, where the field strength is higher. As an example of a diamagnetic particle system we employed the unmodified alumina particles and fluorescently labeled them for imaging purposes according to the method described by Libanori et al. 8 . The fluorescently-labeled alumina particles were assembled on a square nickel grid and were observed by using a confocal microscope, see Figure 2b. Here, it is clear that the particles were driven to the voids of the grid. Figure 2a and 2b demonstrate that we can manipulate both diamagnetic and paramagnetic particles by using the modulating magnetic field that arises from the patterned nickel grid. It is straightforward that if the monocomponent manipulation of diamagnetic and paramagnetic particles are possible, then simultaneous assembly should be within reach. In order to simultaneously assemble paramagnetic and diamagnetic particles, we used a binary mixture of anisotropic paramagnetic and diamagnetic particles that were made from different materials. The first example to such assemblies was the mixture of magnetically modified 8.3 µm long, 400-nm-thick alumina platelets 18 and diamagnetic 5 µm long and 200 nm wide, fluorescently labeled silica rods. We simultaneously assembled the mixture of these particles over a square nickel grid and imaged the assembly with a fluorescent microscope. Figure 2c shows the assembly of this binary system where the two types of particles are segregated depending on their magnetic properties. Here, the red particles are the paramagmetic alumina platelets assembled on the grid bars and the green ones are the silica rods, that are driven to the grid voids. Composite materials with reinforcing constituents are demanded due to their low weight and cost while having competitive mechanical properties. Such materials are frequently fabricated by using anisotropic reinforcers, which are harder than the polymer matrix and enhances the mechanics of the material. It is long known that 29 higher aspect ratio materials exhibit higher flexural strength and toughness compared to lower aspect ratio reinforcers. Thus, anisotropic particle assemblies obtained by magnetic assembly can be employed to fabricate mechanically enhanced composites. To explore mechanical properties of such composites we designed another particle system with modified alumina platelets and diamagnetic 1µm thick calcium sulfate hemihydrate rods synthesized according to Wang


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et al. 28 . These two different shape and material types were selected to improve identification of the two species by means of characterization techniques. A mixture of these particles were dispersed in a 0.2 M Ho(NO3 )3 ethanol solution and left sedimenting on a nickel grid placed over a permanent magnet. The assembled structure was dried and infiltrated with an epoxy polymer (Norland optical adhesive NOA 61) to obtain a composite material. To identify the self assembled structure and address the binary nature of the assembly within the composite, we performed elemental mapping by Energy-dispersive X-ray spectroscopy (EDX). Elemental maps clearly demonstrate the segregation of the diamagnetic calcium sulphate particles from the paramagnetic alumina particles. Figure 3a shows an SEM image of the composite where the grid lines and large calcium sulphate rods are visible. Figure 3c and 3d shows the EDX elemental maps for the Al and Ca atoms over the sample, respectively. Aluminum map in Figure 3c, which is nearly a replica of the nickel grid beneath and indicates that alumina predominantly assembles on Ni grid bars. Ca map in Figure 3d indicates the positions of the calcium sulphate rods and demonstrates the assembly of these rods on the grid voids where the field strength is low. Such atomic mapping indicates the binary assembly of the alumina platelets (major components Al and O) and calcium sulphate rods (major components Ca, S and O, see SI for the map of S) at different locations. Note that, binary assembly allows to spatially adjust the reinforcement of the composite matrix as a result of different types of particle loading at different locations of the composite. To demonstrate the difference of reinforcement by these two types of particle systems we performed Vickers hardness measurements on the composite surface at different positions. Vickers hardness over the alumina regions was 276 MPa whereas the hardness went down to 60 MPa over the calcium sulphate regions. Considering that bulk Vickers hardness values of calcium sulphate and alumina are 80 MPa 30 and 15 GPa 31 , respectively, it is comprehensible that the alumina rich sites are harder compared to the calcium sulphate rich sites. In other words alumina is a stronger reinforcer compared to calcium sulphate rods, see Figure 3b. To acknowledge the segregated distribution of the two types of particles we took a zoomed SEM image given in Figure 3e, which clearly demonstrates the locations of the calcium sulphate rods and alumina platelets. Note that the assembly methods assure the segregation of two different types of particles within the composite, while using anisotropic



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particles enhance the mechanical properties together with the modulating strength of the composite.

Figure 3: A composite made by simultaneously assembling binary particles of magnetic alumina platelets and nonmagnetic calcium sulphate rods. An SEM image of the composite assembled on a square grid is given in (a). Magnetic alumina particles lie on the magnetic grid bars whereas the nonmagnetic calcium sulphate rods fill the magnetic wells of the grid with lower magnetic field. The composite made by simultaneously assembling binary particles from magnetic alumina platelets and diamagnetic calcium sulphate rods, is mechanically tested by Vicker‘s indentation and the hardness for the two distinct regions are given in (b). An EDX map that locates the aluminum atoms, i.e. alumina platelets on the grid bars (c). A similar EDX map that shows the location of calcium atoms, thus the calcium sulphate rods, is given in (d). Location of the magnetic and diamagnetic component are totally dictated by the underlying magnetic field gradient that causes the magnetophoretic forces the particles experience. A zoomed-in SEM image depicts the positions of the alumina platelets on the bars and the calcium sulphate rods located between the grid bars (e).

Another strategy to spatially control the reinforcement of the composites is using arrays of permanent magnets to modulate the field strength in space. This method provides 200 µm to few mms ACS Paragon Plus 9 Environment

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scale control over the position/concentration of the paramagnetic platelets and enables easy fabrication of large scale macroscopic samples with particle content modulations. Finite element analysis of how magnetic field strength modulates over an array of small cube magnets is given in Figure 4. Here, it is observed that magnetic field strength is highly dependent to the distance from the surface of the magnets. The height dependence of the magnetic field is shown in Figure 4 via different plots in panels 4a, b and c. Magnetic field strength decreases in magnitude globally with elevation but also the modulation of the field strength spatially varies at different distances from the magnet surfaces. Figure 4a shows magnetic flux density modulation surfaces over a 3 x 3 array of cube magnets at three different elevations, namely 0.1, 0.3 and 0.5 times h that is the size of the cube magnet. Closer to the magnet surface the highest field strength localises to the edges of the magnets whereas the voids of four adjacent magnets form the local minima. Away from the magnet surface highest field strength moves towards the center of the magnet while the minima stays to be the voids between the magnets. Figure 4b depicts the top view of magnetic flux density surface at three elevations, where we observe the trend, especially the switch of the highest field from the edges towards the centers of the magnets. Figure 4c gives the plots of the flux density fluctuations along the diagonal of the 3 x 3 array magnets and compares the plots for different elevations starting from 0.1 h towards 0.7 h. Note here, that the highest field region moves from the edge to the center of the magnet from 0.1 h to 0.2 h. When a composite with reinforcing magnetic platelets is made, one would expect platelets to follow the field modulations and higher reinforcement would be expected at regions of high field strength. To prove this hypothesis we fabricated a composite of paramagnetic alumina platelets in an epoxy matrix (Sikadur 300, Sika) under the influence of a magnet array. We first dispersed 10 % vol alumina particles in the epoxy matrix and casted it over the magnet array. Dispersion with the magnets beneath was kept at 60 ◦ C overnight for polymerization. The variation of Vickers hardness on the composite surface was tested by applying the Vickers test to various domains of the field strength. As one can already observe in the field strength graphs in Figure 4b and from the images of the assembled composites in Figure 5, there are three easily distinguishable regions of the composite, i) the center of the magnet, ii) the edge of the magnet and



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iii) the void between four adjacent magnets. Magnetic field strength follows the same sequence as given above. After an elevation of 0.1 h the highest field regions are the centers of the magnets, which are followed by the edges of the magnets and lowest field strength regions are the voids between the magnets. We expect to observe the same sequence in the amount of platelets collected by the field and thus in the surface hardness of the composite. To test the influence of magnetic field, at different elevations from the surface, we polished the sample 0.5 mm deep and repeated the hardness test at the new surface at the same regions of the composite to prove for the varying spatial reinforcement (see Supplementary information Figure S2). We observed that hardness globally decreases after the polishing. Note that at an elevation away from the magnet, relatively lower magnetic fields are experienced, thus, less particles concentrate at an elevated location and causes a global lower hardness, which we observe in our measurements. The magnetic field strength variations in 3D and the distribution of the paramagnetic particle and the reinforcement (in parallel with particle concentration) correlates well with the Vickers hardness results (see Supplementary information Figure S2). Note that, experimentally we do not observe the trend given in Figure 4a at an elevation of 0.1 h. This is simply due to the space we have between the magnet array and the polymer casted beaker, which is around 1 mm and this brings the system to an elevation of 0.2 h at the bottom of our composite. Figure 5 demonstrates the images of the composite surfaces facing the magnet after a 0.5 mm and 1.5 mm polishing depths. The data given on the left of these images are the bar plots of the Vickers hardness of the composite after 1.5 mm deep polishing, taken at various positions of the composite with in accordance with the templating magnets. The trend of the hardness data follows the magnetic field strengths, as predicted. Note also the size of the void diameter increases form 0.5 mm to 1.5 mm. We attribute this change to the depth and width of the magnetic field modulations at the void at different elevations, see Figure 4c. The depth of the field minima becomes shallower as the elevation increases and the width of the minima increases with elevation, see also the widening of the blue region in Figure 4b.


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Figure 4: Magnetic flux density surface plots over a 3 x 3 array of cube magnets at three elevations 0.1, 0.3 and 0.5 (in units of height of the magnet). At an elevation 0.1 h, field strength localizes to the edges of the magnets and the voids between four adjacent magnets form the local minima. Away from the magnet surface highest field strength moves towards the center of the magnet while the minima are still the voids between the magnets. Top views of magnetic flux density at these three elevations are given in (b). Plots of the flux density fluctuations along the diagonal of the 3 x 3 magnet array at different elevations starting from 0.1 towards 0.7 times the height of the cube magnet(c). Note here, the movement of the highest field region from the edge to the center of the magnet.

Figure 5: Composites patterned by underlying magnet array, the surface images at polishing depths of 0.5 mm and 1.5 mm are given. The Vickers hardness bar plots are the data from 1.5 mm deep polishing.

Conclusions Directed-self assembly of anisotropic particles, namely silica rods, alumina platelets and calcium sulphate rods were obtained by using magnetic field microgradients. The number of particle sysACS Paragon Plus 12 Environment


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tems we can manipulate with such scheme proves the generality of the method. Such microgradients provide micrometer precision control over the spatial distribution of these anisotropic particles, either paramagnetic or diamagnetic, as long as they posses a magnetic susceptibility contrast with the dispersed medium. Note that, for diamagnetic particles the contrast should be provided by a paramagnetic dispersing medium. Furhermore, we used our assembly technique to simultaneously assemble paramagnetic and diamagnetic particles. We embeded the assembly of the particles in a polymer matrix and obtained a composite, which allowed to control the mechanical properties of the surface as a result of reinforcer selection and the magnetic template. By using a mixture of hard paramagnetic platelets and softer diamagnetic rods in our assembly we fabricated a composite with spatial control of reinforcement, i.e. regions of soft rods in the composite are softer and regions of hard platelets are harder. Here, we do not benefit from particle anisotropy in designing our assemblies or changing the assembly and order, however, we use the mechanics of these anisotropic particles in order to assure that the composite outperforms mechanically. Anisotropic reinforcers together with mechanical modulations, provided by our method, are expected to enhance the toughness, wear and flexural mechanics of the composite. The micrometer precision control of assembly is reflected in the precise reinforcement control. Note also that we focused here at the mechanics of these two components, however, with our method surface properties can be decorated with any quality, i.e. hydrophilicity/hydrophobicity, biofouling/anti-biofouling, conductivity or transparency, which may lead to many applications in fields from engineering 32 to optics 33 .

Another strategy where we provided mm-scale control over the content of the composite in 3D is provided by using an array of mm-sized magnets. The underlying principles and the expected results over the spatial manipulation of the reinforcing particles, which nicely correlates, are given by finite element analysis and by experiments. All in all, we have mediated magnetic field gradients in order to spatially control the reinforcing elements within a composite at a previously unprecedented level of control and complexity.


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Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Figures showing the segragation of reinforcers in a binary reinforced composite and local reinforcement as a result of local magnetic field strength.

Acknowledgements

Author acknowledges Dr. Thijs Besseling for providing the rod-like silica colloids and Dr. Wen Yang for the calcium sulphate rods, Prof. Andre Studart for discussions and support, the Swiss National Science Foundation (Ambizione grant, number P Z00P 2− 148040) for financial support.

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