In this area we investigate topics such as aqueous particle stabilized foam systems, particle-stabilized (Pickering) emulsions, active colloids near polymer functionalized substrates or hybrid microgels.

Research topics

Picture: AG von Klitzing

Foams

Aqueous particle stabilized foams can be found in many technical applications and food products. In these systems the particles adsorb at the air-liquid interfaces of the foam and stabilize them. When altering the properties of the particles like their hydrophobicity, the properties of the produced foams can be changed. Typical particles are modified silica nanoparticles, but also soft colloidal particles like proteins or polymer based microgels.

Foams are a complex, hierarchical material with structures ranging from the nanometer (foam stabilizers) to the millimeter (individual bubbles) range. Because of this complexity, a full understanding of foams requires multiple experiments and techniques.We study the macroscopic air-water interface, individual foam films or bubbles and macroscopic foams. Probing the internal structure of macroscopic foams is very difficult, because of the randomly distributed internal surfaces and the high difference in refractive indices between the liquid and gas phase. To overcome this problem, we use neutron scattering, where the contrast between the two phases can be modified by controlling the isotope composition.

Selected publications:

  • A.-L. Fameau, A. Carl, A. Saint-Jalmes, R. von Klitzing, Responsive aqueous foams, ChemPhysChem, 2015, 16, 66-75.
  • A. Carl, A. Bannuscher, R. von Klitzing, Particle Stabilized Aqueous Foams at Different Length Scales: Synergy between Silica Particles and Alkylamines, Langmuir, 2015, 31, 1615-1622.
  • A. Carl, J. Witte, R. von Klitzing, A look inside particle stabilized foams – particle structure and dynamics, J. Phys. D: Appl. Phys, 2015, 48, 434003.

Funding:

Funding by the Federal ministry of education and research (BMBF) in the framework of the project “FlexiProb: Flexible Probenumgebungen für die Untersuchung weicher Materie zur Implementierung an der ESS” is acknowledged.

Contact:

Matthias Kühnhammer

Picture: AG von Klitzing

Pickering emulsions for catalysis

Pickering Emulsions (PEs) describe emulsions stabilized by solid or soft (nano-) particles. The use of PEs as reaction environment for catalytic reactions gained increasing interest over the last years in research due to their high stability and comparably low temperature sensitivity.

The interactions between each component play a key role when designing these type of emulsions. For example, nanoscale particle properties such as hydrophobicity, shape, size and surface charge dictate the resulting PE regarding droplet size and stability. Object of research is the quantization of these interactions and the investigation of their impact on the reaction behavior.

Selected publications:

  • Stehl, Dmitrij; Milojević, Nataša; Stock, Sebastian; Schomäcker, Reinhard; Klitzing, Regine von (2019): Synergistic Effects of a Rhodium Catalyst on Particle-Stabilized Pickering Emulsions for the Hydroformylation of a Long-Chain Olefin. In: Ind. Eng. Chem. Res. 58 (7), S. 2524–2536 https://doi.org/10.1021/acs.iecr.8b04619.
  • Stehl, Dmitrij; Hohl, Lena; Schmidt, Marcel; Hübner, Jessica; Lehmann, Maren; Kraume, Matthias, Klitzing, Regine von (2016): Characteristics of Stable Pickering Emulsions under Process Conditions. In: Chemie Ingenieur Technik 88 (11), S. 1806–1814 https://doi.org/10.1002/cite.201600065.

Funding:

Funded by CRC/Transregio TRR 63 subproject B6 (DFG)

Contact person:

Sebastian Stock

Picture: AG von Klitzing

Active colloids near polymer functionalized substrates

Microswimmers move autonomously by converting the energy of their environment into directed motion. Sperm and E. coli are some examples of biological microswimmers. Their propulsion is in the realm of low Reynolds number.

Inspired by biological microswimmers, active colloidal particles have been developed, where depending on the structure and chemical nature of particles, various mechanisms[1] could induce their self-propulsion, such as diffusiophoresis, thermophoresis[2], etc.

Janus particles, due to their asymmetrical structures, are promising candidates to induce self-propulsion. Au-PS particles exhibit thermophoretic self-propulsion once heated with laser λ=532 nm. Our interest is the interaction between the particle and the substrate. We functionalize the substrate with various polymer coatings and investigate the impact of physico-chemical properties of the substrate on the active motion of the particles. In particular, polymer brushes have been chosen as polymer coatings, where their thickness hence their mechanical and surface properties can be easily tuned.

Publications:

[1] Bechinger, C.; Leonardo, R. Di; Reichhardt, C.; Volpe, G.; Volpe, G. Active Particles in Complex and Crowded Environments. 2016. https://doi.org/10.1103/RevModPhys.88.045006.

[2] Bregulla, A.; Yang, H.; Cichos, F. Stochastic Localization of Particles by Photon Nudging. ACS Nano 2014, 8 (7), 6542–6550.

Contact person:

Mojdeh Heidari

Picture: AG von Klitzing

Hybrid microgels (acoustics and magnetics)

The poly(N-isopropylacrylamide) (PNIPAM) stimuli-responsive microgels were the subject of studies so far where their reaction to changes in temperature in aqueous solutions and under confinement has been systematically investigated.

Microgels shrink in the solution above their volume phase transition temperature (VPTT) due to the breaking of hydrogen bonds between PNIPAM and the solvent.

The combination of microgels with other components or devices leads to interesting and novel applications and research fields. Such a combination can be obtained by using microgels as a responsive media to acoustic waves or load the microgels with non-organic particles such as gold/silver or magnetic nanoparticles.

Acoustic waves, can transfer a certain amount of energy by penetrating into a liquid solution. As an acoustic wave propagates into the microgel solution, it attenuates due to the absorption of its energy by viscous damping. The absorbed energy may break the PNIPAM hydrogen bonds in the medium and make them shrink while the liquid temperature maintains below its VPTT. Therefore, the amount of energy enough for breaking the hydrogen bonds, that is not high to deteriorate the polymer chains, is a crucial parameter in this context. The shrinking/swelling behavior, dynamic stiffness, as well as streaming pattern of microgels concerning the solvent, are subjected to analyze. This phenomenon opens a new field of research with a wide range of applications introducing acoustic waves as a new stimulus for responsive microgels.

The combination of PNIPAM microgels with gold or magnetic nanoparticles opens up new application through additional stimuli. Gold nanoparticle microgels are one example. Here the VPT can be triggered by light without changing the temperature of the outside of the microgels. This VPT is triggered by the heating of gold nanoparticles through plasmon coupling. Where the gold nanoparticles are behaving as local hot spots. Furthermore, replacing the gold nanoparticles with magnetic ones the VPT may as well be triggered by high frequency magnetic fields. Also, the magnetic nanoparticles are behaving as local hotspots. Additionally, these magnetic nanoparticles are responsive to external magnetic fields (DC) and therefore the magnetic microgels become responsive to magnetic fields. This leads to a deformation of the magnetic microgels on surfaces under magnetic fields in field direction. Furthermore, the magnetic microgels can be moved in solution by external magnetic fields.

Selected publications:

S. Backes, M. Witt E. roeben, L. Kuhrts, S. Aleed, A. M. Schmidt, R. v. Klitzing: Loading of PNIPAM Based Microgels with CoFe2O4 Nanoparticles and Their Magnetic Response in Bulk and at Surfaces, J. Phys. Chem. B 2015, 119, 12129−12137

M. Witt, S. Hinrichs, N. Möller, S. Backes, B. Fischer, R.v.Klitzing: Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based Microgels of Different Cross-linker Distributions, J. Phys. Chem. B 2019, 123, 2405−2413

Acknowledgment:

The Ingenium support at TU Darmstadt, for providing the ‘Future Talents Postdoctoral Scholarship’ program is acknowledged.

Contact person:

Amin Rahimzadeh

Marcus Witt