Periodic Reporting for period 2 - ELECTROFLUID (Conductive suspension flows for soft electronics)
Período documentado: 2022-07-01 hasta 2023-12-31
Wearables that sense our moves, hearth beats, or sugar levels, implants that adapt to soft body tissues, artificial skins that help to bring haptic feeling to prostheses, or soft robots that can reshape and adapt to the environment are just some examples of challenges that technology is addressing. For that, new materials must be developed. Classical electronic components are stiff. Today, two main alternatives are used to overcome this problem: liquid metals and composites.
Liquid metals are good conductors and able to adapt easily and fast during mechanical deformation retaining their electrical properties. However, their use is limited to Gallium and some of its alloys and their properties cannot be easily tuned, for example to create sensors. These aspects limit their use.
Composites combine an elastomer as matrix and conductive particles as fillers. Both electrical conductivity and mechanical elasticity are more limited than in the case of liquid metals. As the particles are embedded in a solid elastomer, they are limited in the degree of freedom to rearrange under mechanical deformation. However, by design, composites can be fabricated with tuned properties depending on the application.
Here, we propose a new concept: “ELECTROFLUID”, that conducts electrons while flowing. We mix conductive particles (carbon- and metal-based) in solvents with different polarity and viscosity to create a conductive fluid/paste. Similar to a classical composite but in this case, the matrix remains liquid. The number of particles is sufficient, so they are in contact forming a 3D network that ensures connectivity, and thus electrical conductivity. The contacts between the particles are transient, so they can move in the liquid and rearrange under mechanical deformation preserving the electrical conductivity of the material.
We study the particle-particle and particle-solvent interactions attending to the nature of interfaces (e.g. shape of the particles, surface-volume ratio, or surface chemistry). This helps us to understand the 3D network formation of the solid in the liquid. The structure of the network dominates both the conductivity and the viscosity of the material. We tune the properties of the Electrofluids by adjusting its components. The objective is to create electrical conductors with tunable mechanoelectrical properties and integrate them into devices. The specific aims of the project are: (i) to design highly concentrated suspensions that form transient percolating networks, (ii) to use this knowledge and synthesize fluids with tunable electrical conductivity at low viscosity, (iii) to demonstrate that Electrofluids can be curtailed for targeted applications.
Liquid metals are good conductors and able to adapt easily and fast during mechanical deformation retaining their electrical properties. However, their use is limited to Gallium and some of its alloys and their properties cannot be easily tuned, for example to create sensors. These aspects limit their use.
Composites combine an elastomer as matrix and conductive particles as fillers. Both electrical conductivity and mechanical elasticity are more limited than in the case of liquid metals. As the particles are embedded in a solid elastomer, they are limited in the degree of freedom to rearrange under mechanical deformation. However, by design, composites can be fabricated with tuned properties depending on the application.
Here, we propose a new concept: “ELECTROFLUID”, that conducts electrons while flowing. We mix conductive particles (carbon- and metal-based) in solvents with different polarity and viscosity to create a conductive fluid/paste. Similar to a classical composite but in this case, the matrix remains liquid. The number of particles is sufficient, so they are in contact forming a 3D network that ensures connectivity, and thus electrical conductivity. The contacts between the particles are transient, so they can move in the liquid and rearrange under mechanical deformation preserving the electrical conductivity of the material.
We study the particle-particle and particle-solvent interactions attending to the nature of interfaces (e.g. shape of the particles, surface-volume ratio, or surface chemistry). This helps us to understand the 3D network formation of the solid in the liquid. The structure of the network dominates both the conductivity and the viscosity of the material. We tune the properties of the Electrofluids by adjusting its components. The objective is to create electrical conductors with tunable mechanoelectrical properties and integrate them into devices. The specific aims of the project are: (i) to design highly concentrated suspensions that form transient percolating networks, (ii) to use this knowledge and synthesize fluids with tunable electrical conductivity at low viscosity, (iii) to demonstrate that Electrofluids can be curtailed for targeted applications.
We fabricated electrofluids using carbon-based, conductive particles such as Carbon Black (CB), Carbon Nanotubes (CNTs), and Graphene Flakes (GFs). These conductive materials (fillers) are suspended in liquid matrices with different polarity and viscosity.
A protocol for the electrofluid characterization has been elaborated. We systematically investigate the agglomeration state of the filler in the matrix at low concentrations, the electrical percolation at increasing filler concentrations, the rheological behavior by means of both rotational and oscillatory tests, and the mechanoelectrical properties of the electrofluids.
One of the main outcomes of the project is the great difference observed in the required concentration of filler to achieve percolation depending on a) the nature of the liquid matrix for a given particle filler and b) the shape of the filler for a given matrix. Consequently, the resulting electrofluids present a wide variety of rheoelectrical properties.
We have proposed network structures for the fillers in polar and non-polar solvents that help to explain the impact of mechanical deformations (shear and tensile) on the electrical response of the materials.
As a step further towards the integration of electrofluids as soft electronic components, uniaxial electromechanical tests were performed. We observed a large difference in the electrical response of, for example, CB-PDMS electrofluid (with high sensitivity towards strain) respect to the CB-glycerol electrofluids, which retain the electrical conductivity during over 1000 strain cycles. Based on these results, we designed a prototype glove with integrated strain sensing and interconnecting parts, showcasing successful movement detection via combination of electrofluids (see Figure).
We started to work with more complex electrofluids that includes the use of additives or complex matrices such as the combination of two immiscible liquids. We studied the CB-PDMS mixtures with ionic liquids (ILs) as additives as part of the collaboration with Prof. Dr. Tobias Kraus. The resulting material shows more elasticity at the cost of small decrease of the electrical conductivity. We performed rheological measurements of the mixtures, which helped to propose a model of the system. The results are described in the publication: L. Zhang et al., Adv. Mater. Technol., 2022, 2101700.
For the use of metal particles and flakes as conductive filers, we established surface modification protocols in order to functionalize the commercial silver powders with conductive polymers that will, on one hand bring stability to the suspensions in different solvents, and, on the other hand, render the electrofluids conductive.
A protocol for the electrofluid characterization has been elaborated. We systematically investigate the agglomeration state of the filler in the matrix at low concentrations, the electrical percolation at increasing filler concentrations, the rheological behavior by means of both rotational and oscillatory tests, and the mechanoelectrical properties of the electrofluids.
One of the main outcomes of the project is the great difference observed in the required concentration of filler to achieve percolation depending on a) the nature of the liquid matrix for a given particle filler and b) the shape of the filler for a given matrix. Consequently, the resulting electrofluids present a wide variety of rheoelectrical properties.
We have proposed network structures for the fillers in polar and non-polar solvents that help to explain the impact of mechanical deformations (shear and tensile) on the electrical response of the materials.
As a step further towards the integration of electrofluids as soft electronic components, uniaxial electromechanical tests were performed. We observed a large difference in the electrical response of, for example, CB-PDMS electrofluid (with high sensitivity towards strain) respect to the CB-glycerol electrofluids, which retain the electrical conductivity during over 1000 strain cycles. Based on these results, we designed a prototype glove with integrated strain sensing and interconnecting parts, showcasing successful movement detection via combination of electrofluids (see Figure).
We started to work with more complex electrofluids that includes the use of additives or complex matrices such as the combination of two immiscible liquids. We studied the CB-PDMS mixtures with ionic liquids (ILs) as additives as part of the collaboration with Prof. Dr. Tobias Kraus. The resulting material shows more elasticity at the cost of small decrease of the electrical conductivity. We performed rheological measurements of the mixtures, which helped to propose a model of the system. The results are described in the publication: L. Zhang et al., Adv. Mater. Technol., 2022, 2101700.
For the use of metal particles and flakes as conductive filers, we established surface modification protocols in order to functionalize the commercial silver powders with conductive polymers that will, on one hand bring stability to the suspensions in different solvents, and, on the other hand, render the electrofluids conductive.
We successfully formulated electrically conductive suspensions with tunable rheological properties. We found that the interactions between the filler particles and those between the particles and the solvent determine the degree of agglomeration and, therefore, the structure of the 3D network. Carbon Black in non-polar matrices tend to disperse better than in polar ones. Thus, the percolation threshold can vary in one order of magnitude.
For the first time, rheoelectrical measurements were performed of such suspensions. Their analysis confirmed (a) the formation of a strong network (gel-like behavior) of the filler in the matrix for concentrations of CB above the percolation threshold, (b) the breaking of the network at strains above 10%, and (c) the correlation between the breakdown of the network and the increase in the electrical resistance of the material.
We have spotlighted mixtures with different electromechanical behavior: while the electrical conductivity of CB-PDMS mixtures remains unchanged under at least 10% strain, large changes can be observed in the case of CB-glycerol mixtures. This means that, by changing the system (the solvent in this case), the resulting material is a stable conductor or a good strain sensor. We expect to exploit these differences by combining other fillers and solvent mixtures and be able to build a demonstrator to show the applicability of Electrofluids.
For the first time, rheoelectrical measurements were performed of such suspensions. Their analysis confirmed (a) the formation of a strong network (gel-like behavior) of the filler in the matrix for concentrations of CB above the percolation threshold, (b) the breaking of the network at strains above 10%, and (c) the correlation between the breakdown of the network and the increase in the electrical resistance of the material.
We have spotlighted mixtures with different electromechanical behavior: while the electrical conductivity of CB-PDMS mixtures remains unchanged under at least 10% strain, large changes can be observed in the case of CB-glycerol mixtures. This means that, by changing the system (the solvent in this case), the resulting material is a stable conductor or a good strain sensor. We expect to exploit these differences by combining other fillers and solvent mixtures and be able to build a demonstrator to show the applicability of Electrofluids.