Abstract:Functional particles that respond to external stimuli are spurring technological evolution across various disciplines. While large-scale production of functional particles is needed for their use in real-life applications, precise control over particle shapes and directional properties has remained elusive for high-throughput processes. We developed a high-throughput emulsion-based process that exploits rapid vitrification of a thixotropic medium to manufacture diverse functional particles in large quantities.… Show more
“…In general, the higher the viscosity of the medium or the shear rate, the lower the mean diameter of the resulting grains, which is in agreement with existing emulsion processes. [58][59][60] When prepared at 500 rpm in a 3 wt% sodium alginate aqueous medium, the size distribution of the grains exhibits a normal distribution with a mean diameter of 162 µm (Figure 2B). Thermogravimetric analysis (TGA) reveals that each grain contains around 8 wt% PFD solvent when compared against neat silicone control spheres (Figure 2C), and cross-sectional scanning electron microscopy (SEM) images confirmed internal grain pockets where PFD was contained (Figure 2D).…”
Section: Resultsmentioning
confidence: 99%
“…Once the uncured silicone is broken down sufficiently, the shear‐thinning thixotropic properties of the aqueous sodium alginate enables static‐state solidification. [ 58 ] This double emulsion results in silicone grains with multiple solvent cores suspended in the aqueous continuous phase, which can be rinsed away after the silicone cures.…”
microscale actuation. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions. To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. Phasechange actuators are capable of inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. [29][30][31][32] Accepting the advantages of phasechange actuators, we now turn our attention to granular media. Granular assemblies, consisting of discrete grains, can be tuned to accomplish a variety of different responses. For example, favorable self-assembly of granular media can be achieved via modulating interparticle interactions and external stimuli, [33][34][35][36] and granular assemblies with different packing configurations have been shown to yield different bulk properties. [36][37][38][39] Furthermore, granular media can exhibit optimized changes in stiffness during jamming transitions, enabling tunable moduli. [16,[40][41][42][43][44] When unjammed, the bulk material is compliant, and when jammed, the bulk material can achieve the stiffness of the grains. Finally, when mixed into carrier fluids, granular media can impart thixotropic behavior on the carrier fluid due to dynamic jamming, [45][46][47][48] enabling 3D printing of previously unprintable materials. With all the advantageous properties granular media has to offer, the technology has seen application in soft robotics, [49][50][51][52] medical devices, [53][54][55] flexible airfoils, [56] and programmable aggregate architecture. [57] Here, we bring together the unique advantages of phasechange soft actuators and granular media to introduce soft granular actuators made of discrete, volumetrically expanding grains. A single active grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force microscopic actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Furthermore, agglomerates of active grains can self-assemble from disordered arrangements to conform around objects and exhibit variable moduli. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packing...
“…In general, the higher the viscosity of the medium or the shear rate, the lower the mean diameter of the resulting grains, which is in agreement with existing emulsion processes. [58][59][60] When prepared at 500 rpm in a 3 wt% sodium alginate aqueous medium, the size distribution of the grains exhibits a normal distribution with a mean diameter of 162 µm (Figure 2B). Thermogravimetric analysis (TGA) reveals that each grain contains around 8 wt% PFD solvent when compared against neat silicone control spheres (Figure 2C), and cross-sectional scanning electron microscopy (SEM) images confirmed internal grain pockets where PFD was contained (Figure 2D).…”
Section: Resultsmentioning
confidence: 99%
“…Once the uncured silicone is broken down sufficiently, the shear‐thinning thixotropic properties of the aqueous sodium alginate enables static‐state solidification. [ 58 ] This double emulsion results in silicone grains with multiple solvent cores suspended in the aqueous continuous phase, which can be rinsed away after the silicone cures.…”
microscale actuation. However, these material-based actuators are often slow or only expand or contract axially, preventing use in applications that require volumetric expansions. To address such limitations of existing actuating materials, actuators based on the liquid-to-gas phase change of solvent inclusions encapsulated in a hyperelastic matrix have been realized. Phasechange actuators are capable of inducing rapid volumetric expansion similar to fluidic actuators, without being tethered to an external fluid source. [29][30][31][32] Accepting the advantages of phasechange actuators, we now turn our attention to granular media. Granular assemblies, consisting of discrete grains, can be tuned to accomplish a variety of different responses. For example, favorable self-assembly of granular media can be achieved via modulating interparticle interactions and external stimuli, [33][34][35][36] and granular assemblies with different packing configurations have been shown to yield different bulk properties. [36][37][38][39] Furthermore, granular media can exhibit optimized changes in stiffness during jamming transitions, enabling tunable moduli. [16,[40][41][42][43][44] When unjammed, the bulk material is compliant, and when jammed, the bulk material can achieve the stiffness of the grains. Finally, when mixed into carrier fluids, granular media can impart thixotropic behavior on the carrier fluid due to dynamic jamming, [45][46][47][48] enabling 3D printing of previously unprintable materials. With all the advantageous properties granular media has to offer, the technology has seen application in soft robotics, [49][50][51][52] medical devices, [53][54][55] flexible airfoils, [56] and programmable aggregate architecture. [57] Here, we bring together the unique advantages of phasechange soft actuators and granular media to introduce soft granular actuators made of discrete, volumetrically expanding grains. A single active grain consists of multiple solvent cores encapsulated in a hyperelastic silicone shell (Ecoflex 00-30). At elevated temperatures, the encapsulated solvent vaporizes and increases the internal pressure of the hyperelastic shell, inducing volumetric expansion of the grain. The grains are independently capable of rapid, high-force microscopic actuation, and are also easily arranged into granular assemblies to form larger-scale bulk actuators. Furthermore, agglomerates of active grains can self-assemble from disordered arrangements to conform around objects and exhibit variable moduli. Finally, the use of grains suspended in a carrier solvent or resin enables compatibility with granular self-assembly and 3D printing techniques, offering the potential to print volumetrically expanding actuators into freeform patterns across scales.Recent work has demonstrated the potential of actuators consisting of bulk elastomers with phase-changing inclusions for generating high forces and large volumetric expansions. Simultaneously, granular assemblies have been shown to enable tunable properties via different packing...
“…Because the particles become fully solid during the cooling step, they can be handled directly without the need of any additional stabilizers or encapsulants that might be needed for liquid-core droplets. 22 Further, in contrast to Yup et al, 20 this method does not require any additional solvent or enzyme to release the particles from the medium as it does not form a polymerized gel. Indeed, with sodium alginate powder and water as the only required components, this process also carries the benefit of being inexpensive, simple to execute, and eco-friendly with sustainable by-products.Figure 1.Manufacture and characterization of FM particles.…”
Section: Methodsmentioning
confidence: 99%
“…All of this contributes to particles that are not only highly spherical, but also possess a very smooth surface. 20 The surface texture can be roughened as an additional processing step to improve bonding between the particles and the host matrix material. After the particles have been collected and dried, they are heated to approximately 80°C, thereby melting the FM particle cores while the oxide shell remains intact.…”
Phase-changing materials have recently found use as particulate fillers to engender stiffness-changing behavior in composites. In the transition from solid to liquid particles, the composite modulus can be significantly reduced, and then raised again as the particles are allowed to solidify. While the general effect of stiffness tuning in phase-changing composites has been demonstrated in a range of studies and applications, the most drastic stiffness ranges are provided by fusible metallic alloys, which are commonly subject to the formation of oxide shells on their surfaces. In this work, we examine the effect these oxide surfaces have on variable-stiffness performance, and investigate the corresponding role of particle size. In particular, we study particles of a low-melting-point metallic alloy, Field’s Metal, and we also present a facile method of manufacture that allows for control of particle size. We then manufacture a set of stiffness-changing composites using these particles and characterize mechanical performance, with the aim of controlling particle size to increase the total stiffness ratio while resisting material failure.
“…Fabrication of Spherical Particles: Spherical particles were manufactured via a process modified from ref. [43]. An ingot of FM (Roto144F, RotoMetals, Inc.) was submerged in a 500 mL glass beaker filled with 200 mL of a thixotropic mixture, made from an aqueous solution of confectionery sodium alginate powder with a loading of 2 wt%.…”
Variable stiffness in elastomers can be achieved through the introduction of low melting point alloy particles, such as Field's metal (FM), enabling on‐demand switchable elasticity and anisotropy in response to thermal stimulus. Because the FM particles are thermally transitioned between solid and liquid phases, it is beneficial for the composite to be electrically conductive so the stiffness may be controlled via direct Joule heating. While FM is highly conductive, spherical particles contribute to a high percolation threshold. In this paper, it is shown that the percolation threshold of FM particulate composites can be reduced with increasing particles aspect ratio. Increasing the aspect ratio of phase‐changing fillers also increases the rigid‐to‐soft modulus ratio of the composite by raising the elastic modulus in the rigid state while preserving the low modulus in the soft state. The results indicate that lower quantities of high aspect ratio FM particles can be used to achieve both electrical conductivity and stiffness‐switching via a single solution and without introducing additional conductive fillers. This technique is applied to enable a highly stretchable, variable stiffness, and electrically conductive composite, which, when patterned around an inflatable actuator, allows for adaptable trajectories via selective softening of the surface materials.
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