Particle suspensions that conduct electricity while flowing
Electrofluids research group investigates new alternatives as soft, electrical components. We use conductive particles that form electrical and mechanical networks in liquid matrices. This novel approach combines classical percolation of particles in composites and fluid dynamics. Our research encompasses particle characterization and agglomeration studies, surface functionalization, electrical and rheological measurements, 3D printing, and device integration
The research on Electrofluids is supported by the European Research Council (ERC)
Team Members
Research
Electronic components and robotic actuators are traditionally built from metals and semiconductors. The Junior Research Group Electrofluids investigates “Electrofluids” as a liquid alternative: suspensions that conduct electrons while flowing as liquids. Fluids in elastic enclosures can replace solid leads and enable truly soft devices. Sufficient conductivity requires highly concentrated suspensions of conductive particles. They form transient conductive networks at manageable viscosity. We study suspensions of common conductive materials such as carbon, silver, gold, and copper, and avoid specialized low-melting alloys of gallium or other expensive elements.
The group studies the interplay between particle-particle friction, contact resistance, percolation, bulk resistance, and suspension viscosity. We use both custom-synthesized and commercial particles in a size range of tens of nanometres to few microns and with different shapes, modify their surfaces with conventional and p-conjugated surfactants and formulate concentrated suspensions that exhibit large conductivity at low viscosity. The combination of different particle sizes, shapes, and fluids enables tuning the properties of the fluid towards specific application cases, for example to create highly flexible leads for logic signals versus high-power connections for the connection of actuators.
The specific aims of this research group are:
- to design highly concentrated suspensions that form transient percolating networks,
- to use this knowledge and synthesize fluids with tunable electrical conductivity at low viscosity,
- to demonstrate that Electrofluids can be tailored for particular applications.
News
We launch our NEWS section!
Here, we want to share with you our last announcements and achievements, so please stay posted!
Congratulations and “welcome”, Niclas Hautz!
Niclas Hautz has successfully defended his Master thesis in Material Science entitled „Effect of water on electromechanical properties of polymer-based carbon black composites“. A work carried out in collaboration between Electrofluids group and Structure Formation group at INM. Congratulations! We are twice as happy because, far from being a goodbye, this brings more time together: Niclas starts today as PhD student in the Electrofluids group. We wish you all the best!
Goodbye 2022!
We had last week the traditional Christmas party together with the Structure Formation Group, a fun way to say goodbye to the year. Some of us went first play laser tag, unfortunately, Electrofluids (in red) could not beat SFG (in blue)…but they had 1 more player! After that we spent some time at the traditional Christmas Martket in Saarbrücken followed by a nice German dinner. We all wish you a happy Christmas and a good start into the year and are looking forward to more science and fun in 2023!
Dr. Lola González-García is now associated Junior Professor at the Saarland University!
Congratulations to our group leader Jun.-Prof. Dr. Lola González-García, who is now associated Junior Professor at the Department of Materials Science and Engineering of the Saarland University.
Proudly winners of the Corn Hole tournament!
Congratulations to our group leader Jun.-Prof. Dr. Lola González-García, who is now associated Junior Professor at the Department of Materials Science and Engineering of the Saarland University.
We welcome Niclas Hautz!
Niclas Hautz is a material scientist. He received his bachelor’s degree from Saarland University in 2021. Currently, he is doing his master’s degree at the Saarland University in the field of Material Science. He joins the Leibniz INM to do his master thesis at the Electrofluids group on the topic of composite technology, with special interest in double-percolating systems. Welcome to Electrofluids, we wish you all the best!
We welcome Sergio Lago Garrido!
Sergio Lago is an industrial chemical engineer. He completed his Bachelor degree in the university from Huelva, Spain. Now, he is working on his Master degree in chemical engineering with the specialization of Food and Pharmaceutical Products Engineering. He joins the Leibniz INM to work on his thesis in the Electrofluids group with focus on carbon nanotubes soft conductors. Welcome to Electrofluids, we wish you all the best!
Congratulations to Hendrik Rolshausen for defending his Master Thesis!
Our Master Student, Hendrik Rolshausen, has defended successfully his thesis entitled “Surface Modification and Characterization of Silver Particles and their Use in conductive Materials”. Congratulations!
Electrofluids @Nano2022 in Sevilla (06-10 June)
Dr. Lola González-García will present the last results on Electrofluids (and beyond) in Sevilla at the 16th International Conference on Nanostructured Materials (Nano2022).
- Monday, 6th of June at 11.15 h, S7_1: Electrofluids: electronic flowing leads based on conductive particle suspensions.
- Monday, 6th of June at 18.00 h, S11_3: Direct nanoimprinting of metal nanostructures: a method to fabricate flexible, transparent electrodes.
- Tuesday, 7th of June at 18.00 h, S7_3: Sinter-free inks of metal-polymer hybrid particles for printed electronics.
Check our paper published in Advanced Materials Technologies
Our collaboration with the Structure Formation Group at INM (Tobias Kraus) has resulted in a very interesting work on „Microscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite“. You can find the article: LINK. Congratulations to all the authors!
We welcome Dr. Srishti Arora!
Dr. Srishti Arora is a soft matter physicist. She completed her Ph.D. from Laboratoire Charles Coulomb, Université Montpellier, France in 2017 followed by her first Postdoctoral studies at Northwestern University, USA. Her research work broadly encompasses the synthesis of colloidal particles, preparation of complex fluids, polymeric gels, etc., and understanding their behavior under extreme mechanical stress utilizing rheological techniques coupled with microscopy/or high-speed imaging. Currently, she is working on the development of electronic suspensions and investigating their rheoelectrical properties.
Publications
Kang, Dongjin | González-García, Lola | Kraus, Tobias
DOI:
Soft electronic devices enable new types of products for an ergonomic interaction of humans with a digital environment. The inkjet (droplet on demand) printing of electrically conductive ink on soft substrates such as paper, textile, and polymers is a promising route for the prototyping and small-scale production of soft electronics that is efficient, cost-saving, and provides a rapid turnaround due to its fully digital workflow. The choice of materials and processing parameters is challenging, however, due to the combined complexity of metal-containing inks, their dynamics during droplet ejection, the active role of the porous substrate, and possible post-deposition steps. This review focuses on recent developments in inkjet printing of metal inks onto soft, porous substrates and their applications. The first section discusses the general principles in the inkjet printing of metal inks, including drop formation and jetting, wetting, and post treatment processes. The second section deals with the effect that the porosity of substrates has on the drying, diffusion, and adhesion of inks. Finally, current challenges and achievements of inkjet-printed, metal-containing inks are discussed.
Bettscheider, Simon | Kuttich, Björn | Engel, Lukas F. | González-García, Lola | Kraus, Tobias
DOI:
We report on the dilution-induced agglomeration of ultrathin gold nanowires (AuNWs) into regular bundles. Wires with a metal core diameter of 1.6–1.7 nm surrounded by a ligand shell of oleylamine formed stable colloids in n-hexane and cyclohexane. Dilution with pure solvent induced the self-assembly into bundles with a regular, hexagonal cross-section. Small-angle X-ray scattering and thermogravimetric analysis indicated that bundles formed only if the ligand shell was sufficiently sparse. Dilution with pure solvent shifts the chemical equilibrium and reduces the ligand density, thus enabling agglomeration. We show that agglomeration is driven not by van der Waals forces but by the depletion forces of linearly shaped molecules. Linear solvent molecules or small amounts of unbound ligand align normal to the nanowire if the ligand shell is sparse. The resulting reduction in entropy creates a driving force for the AuNWs to bundle such that the low-entropy domains overlap and the overall entropy is increased. Dilution-induced nanowire bundling is thus explained as a combined effect of ligand desorption and destabilization by depletion.
Buchheit, Roman | Kuttich, Björn | González-García, Lola | Kraus, Tobias
DOI:
Abstract A new type of hybrid core–shell nanoparticle dielectric that is suitable for inkjet printing is introduced. Gold cores (dcore ≈ 4.5 nm diameter) are covalently grafted with thiol-terminated polystyrene (Mn = 11000 Da and Mn = 5000 Da) and used as inks to spin-coat and inkjet-print dielectric films. The dielectric layers have metal volume fractions of 5 to 21 vol% with either random or face-centered-cubic structures depending on the polymer length and grafting density. Films with 21 vol% metal have dielectric constants of 50@1 Hz. Structural and electrical characterization using transmission electron microscopy, small-angle X-ray scattering, and impedance spectroscopy indicates that classical random capacitor–resistor network models partially describe this hybrid material but fail at high metal fractions, where the covalently attached shell prevents percolation and ensures high dielectric constants without the risk of dielectric breakdown. A comparison of disordered to ordered films indicates that the network structure affects dielectric properties less than the metal content. The applicability of the new dielectric material is demonstrated by formulating inkjet inks and printing devices. An inkjet-printed capacitor with an area of 0.79 mm2 and a 17 nm thick dielectric had a capacitance of 2.2±0.1 nF@1 kHz.
Coupette, Fabian | Zhang, Long | Kuttich, Björn | Chumakov, Andrei | Roth, Stephan V. | González-García, Lola | Kraus, Tobias | Schilling, Tanja
DOI:
We examine network formation and percolation of carbon black by means of Monte Carlo simulations and experiments. In the simulation, we model carbon black by rigid aggregates of impenetrable spheres, which we obtain by diffusion-limited aggregation. To determine the input parameters for the simulation, we experimentally characterize the micro-structure and size distribution of carbon black aggregates. We then simulate suspensions of aggregates and determine the percolation threshold as a function of the aggregate size distribution. We observe a quasi-universal relation between the percolation threshold and a weighted average radius of gyration of the aggregate ensemble. Higher order moments of the size distribution do not have an effect on the percolation threshold. We conclude further that the concentration of large carbon black aggregates has a stronger influence on the percolation threshold than the concentration of small aggregates. In the experiment, we disperse the carbon black in a polymer matrix and measure the conductivity of the composite. We successfully test the hypotheses drawn from simulation by comparing composites prepared with the same type of carbon black before and after ball milling, i.e., on changing only the distribution of aggregate sizes in the composites.
Escudero, Alberto | González-García, Lola | Strahl, Robert | Kang, Dong Jin | Drzic, Juraj | Kraus, Tobias
DOI:
We describe the gram-scale synthesis of hybrid gold nanoparticles with a shell of conductive polymers. A large-scale synthesis of hexadecyltrimethylammonium bromide (CTAB)-capped gold nanoparticles (AuNP@CTAB) was followed by ligand exchange with conductive polymers based on thiophene in a 10 L reactor equipped with a jacket to ensure a constant temperature of 40 °C and a mechanical stirrer. Slow and controlled reduction of the gold precursors and the presence of small amounts of silver nitrate are revealed to be the critical synthesis variables to obtain particles with a sufficiently narrow size distribution. Batches of approximately 10 g of faceted AuNP@CTAB with tunable average particle sizes from 54 to 85 nm were obtained per batch. Ligand exchange with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) in the same reactor then yielded hybrid Au@PEDOT:PSS nanoparticles. They were used to formulate sinter-free inks for the inkjet printing of conductive structures without the need for a sintering step.