This year we finally had another group excursion. We went hiking and did Via Ferrata climbing near Hirschbach in the Hersbrucker Alb. Great fun!

Crossing the Noris-Brettl.
In front of the Noristörl.

This look at the geometry of crystals beyond the constraints of chemistry accomplished by computer simulation has been 8+ years in the making:

“Which crystal structures are possible if the restrictions of the quantum realm are lifted? Our knowledge of ordered particle geometries was previously restricted to the kinds of structures observable in hard condensed matter—on the atomic scale. Here, we use freely tunable computational models to represent particles with variable properties, and we determine the crystal structures into which they self-assemble. The resulting arrangements often correspond to structures known from atomic-scale materials; however, we discover a comparable number of previously unknown crystal structures with different local coordination motifs, incompatible with the limitations of the chemical bond. Our results can be used to engineer soft condensed matter with unprecedented, ordered geometries, paving the way toward materials with potentially novel properties.”

Read this work here:

Julia Dshemuchadse, Pablo F. Damasceno, Carolyn L. Phillips, Michael Engel, Sharon C. Glotzer
Moving Beyond the Constraints of Chemistry via Crystal Structure Discovery with Isotropic Multiwell Pair Potentials
Proceedings of the National Academy of Sciences 118, e2024034118 (2021)

You need soap to remove dirt from your skin. The surfactant molecules it contains squeeze their way into the surface area between the dirt and the skin and help to dissolve the dirt in water. Researchers at FAU and Heinrich-Heine-Universität Düsseldorf (HHU) have observed the same phenomenon with rotating microrobots. Microrobots rotating in a clockwise direction separate from those rotating in an anticlockwise direction to form two cohesive groups clearly separated from each other, just like water and oil. By linking the microrobots to make chains, researchers were able to observe various effects: the chains are capable of mixing the groups and acting like surfactants to create new structures, the same as what happens with soap and soap bubbles.

3D-printed microrobots are driven to rotate on a vibrating table. The angle of the legs determines the rotation direction. Below, two oppositely-rotating robots are chained together.
If the chain consists of oppositely rotating robots, then the chain spontaneously closes; the start and end interlock irreversibly. We call this structure a `rotelle’.

Read about this work here:

Christian Scholz, Anton Ldov, Thorsten Pöschel, Michael Engel, Hartmut Löwen
Surfactants and Rotelles in Active Chiral Fluids
Science Advances 7, abf8998 (2021)
Highlighted in: FAU Research

A new virtual seminar series called GEOMPACK (geompack.com) aims to bring together researchers from a range of disciplines (physics, materials, biology, mathematics, computer science). The series focuses on problems in geometry and packing in materials and biology, and provides an avenue to share new research and promote discussion.

Seminars will take place in spring every two weeks and are scheduled for Wednesdays at 4:30 pm (London time). The first seminar will be Wednesday, March 24, from Sabetta Matsumoto (GA Tech), “Twisted topological tangles or: the knot theory of knitting”.

Other speakers this semester are Marjolein Dijkstra (Universiteit Utrecht), Sasche Hilgenfeldt (University of Illinois), Lisa Manning (Syracuse University), Vinothan Manoharan (Harvard University), and Giuliana Indelicato (University of York).

To see the line up of speakers and to subscribe to the seminar announcements, visit geompack.com.

FAU will receive funds to establish a National Center for High Performance Computing (NHR@FAU). It will be part of a nationwide network with (initially) seven other centers. The federal and state governments will provide a total of up to 625 million € in funding for the entire project over the next 10 years. Scientific support for broad application groups, promoting the further development of HPC techniques and tools, and training and education activities will also be funded in addition to HPC systems and operating costs.

This is very exciting news for all computationally working research groups in Erlangen. Congratulations to everybody involved!

Read more about this development here.

What happens when you etch nanoparticles? This process has now been recorded in real time and in situ by our collaborator Xingchen Ye using a small droplet sandwiched between two graphene sheets. Alberto Leonardi resolved details of the anisotropic kinetics of their gradual dissolution using molecular dynamics and lattice Monte Carlo simulations. Together, experiment and simulation help understanding the mechanism of etching at atomistic resolution, which is important to design more stable catalysts.

Schematic illustration of a graphene liquid cell encapsulating a solution of Pd@Au nanocubes and oxidative etchants. Carbon atoms of graphene sheets are enlarged for clarity purpose.

Read about this work here:

Lei Chen, Alberto Leonardi, Jun Chen, Muhan Cao, Na Li, Dong Su, Qiao Zhang, Michael Engel, Xingchen Ye
Imaging the Kinetics of Anisotropic Dissolution of Bimetallic Core-Shell Nanocubes using Graphene Liquid Cells
Nature Communications 11, 3041 (2020)

There is an observation that has been puzzling the colloid community for years: Experiments with binary mixtures of quasi-hard colloids and free energy calculations predicted binary crystals. But simulations of binary hard sphere systems never confirmed this phenomenon. Is there a discrepancy between experiment and simulation? Our manuscript, now published in Physical Review Letters, directly answers this question.

In brief: No, there is no discrepancy. With the right simulation method and good order parameters it is in fact possible to detect crystalline order in binary hard spheres. As we show in detail and also quantitatively for Laves phases, diffusion in the fluid is the reason why crystallization is much slower than in systems of identical spheres.

The results also lead to new scientific insights by demonstrating the existence of a transition from a nucleation and growth regime to a spinodal decomposition regime. The findings add to an active discussion in the glass physics community. Finally, state diagrams are reported as reference for future research.

Read about the research here:

Praveen K. Bommineni, Marco Klement, Michael Engel
Spontaneous Crystallization in Systems of Binary Hard Sphere Colloids
Physical Review Letters 124, 218003 (2020)

Greetings from this week’s group meeting!

The coronavirus stopped in-person teaching and work in the buildings of FAU but our research continues remotely. We work from home and partially from the lab at low occupancy. We write codes and papers, submit and analyze simulations. Meetings are conducted via Zoom.