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.

The German Research Foundation (DFG) approved a new Collaborative Research Centre (CRC) ‘Design of Particulate Products’ to start in January 2020. The CRC will be coordinated by FAU and its researchers are set to receive around 11 million euros in funding for nanoparticle design.

The research team, including the Engel Lab, are planning a novel approach by developing models to design and optimise the nanoparticles before they are produced in the laboratory, a technique that has been made possible by close collaboration between mathematics and particle technology.

For more information, read the FAU Press Release and visit the Webpage of CRC 1411.

Logo of CRC 1411

New work with the Vogel lab, this time on structural color:

“Micrometer‐scale crystalline colloidal clusters are produced by confined self‐assembly in emulsion droplets. Structural color is used to characterize icosahedral, decahedral, and face‐centered cubic clusters. Their color motifs arise from internal grain arrangement, which gives rise to circle, strips, bowtie patterns, and so on. Monitoring color evolution provides information on the dynamics of rotation and the colloid crystallization in confinement in real time.”

Read about the research here:

Structural Color of Colloidal Clusters as a Tool to Investigate Structure and Dynamics
J. Wang, U. Sultan, E.S.A. Görlitzer, C.F. Mbah, M. Engel, N. Vogel
Advanced Functional Materials TBA, 1907730 (2019)

The following Saturday,

Oct 19, 2019, 18h–1h, in the foyer of IZNF (Cauerstraße 3)

our lab will participate at Lange Nacht der Wissenschaften, an established form of public relations activity in Germany. We will present our work in form of interactive particle simulations, 3d visualizations, and a small hands-on experiment involving a laser interacting with a colloidal monolayer.

Kommen Sie uns besuchen!

EngelLab Group poster
EngelLab Group poster (in German).
A high-resolution version can be viewed by clicking on the image.

Michael Engel attended the 50 year celebration of CECAM in Lausanne, Switzerland. CECAM (Centre Européen de Calcul Atomique et Moléculaire) is the longest standing European Institute for the promotion of fundamental research on advanced computational methods and their application to problems in frontier areas of science and technology.

As the conference demonstrated, simulations are more relevant than ever due to advances in computer hardware and, even more importantly, advances in simulation algorithms. Recent trends particularly well covered in Lausanne were the topics ‘Machine Learning’ and ‘Neural Networks’. Even though being a blind follower can easily lead astray, there are huge possibilities with the right strategy.

Lake Geneva from Lausanne harbor at sunset.

Our joint work with Nicolas Vogel and Erdmann Spiecker advanced the understanding of the structure, defect accumulation and thermodynamics of colloidal clusters on and off magic numbers and was awarded this month’s cover for ACS Nano. Congratulations Junwei and Chrameh!

Read about it here:

Free Energy Landscape of Colloidal Clusters in Spherical Confinement
J. Wang, C.F. Mbah, T. Przybilla, S. Englisch, E. Spiecker, M. Engel, N. Vogel
ACS Nano 13, 9005-9015 (2019)

Some pictures from the 2019 group hike to several caves near Muggendorf in the Franconian Switzerland.

At the end of the Mehlbeerensteig.
Exploration of the Kammerfelsen
Dripstone in the Doktorshöhle.