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.

Fractional crystallization is crystal formation out of chemical mixtures or solutions. In this process, the growing crystal typically has a different composition than the fluid. This makes fractional crystallization an important method for separating or purifying substances based on differences in solubility. In geology, fractional crystallization is operating within the Earth’s crust and mantle during the formation of igneous rocks.

The simplest case of fractional crystallization in simulation is the crystallization of hard spheres. Praveen Bommineni, MAP student Nydia Varela-Rosales and Marco Klement in the group of Michael Engel now calculated the crystallization behavior of mixtures of hard spheres as a function of size-dispersity (composition) and packing fraction (density). The work was achieved using advanced statistical sampling to speed up simulation and access long times required for observing the crystallization phenomenon. The crystals discovered have relevance for the behavior of nanoparticles, micelles, and the structure of alloys and the elements.

Crystallization from size-disperse mixture of spheres.

Complex Crystals from Size-Disperse Spheres
P.K. Bommineni, N.R. Varela-Rosales, M. Klement, M. Engel
Physical Review Letters 122, 128005 (2019)

After two years of construction and several more years of planning, the new Interdisciplinary Center for Nanostructured Films (IZNF) is now ready for research groups. Our group moved in today!

The new lab is in the heart of the Technical Faculty of FAU next door to our experimental collaborators.

Streetview of our new building IZNF at Cauerstraße 3.
Alberto Leonardi and Chrameh Mbah testing the new workplace.

In a joint collaboration combining experiment (synthesis and self-assembly), analysis (electron microscopy including tomography), and simulation (molecular dynamics and free energy calculations), a team from FAU involving Junwei Wang and Chrameh Mbah reported magic number colloidal clusters:

“Clusters in systems as diverse as metal atoms, virus proteins, noble gases, and nucleons have properties that depend sensitively on the number of constituent particles. Certain numbers are termed ‘magic’ because they grant the system with closed shells and exceptional stability. To this point, magic number clusters have been exclusively found with attractive interactions as present between atoms. Here we show that magic number clusters exist in a confined soft matter system with negligible interactions. Colloidal particles in an emulsion droplet spontaneously organize into a series of clusters with precisely defined shell structures. Crucially, free energy calculations demonstrate that colloidal clusters with magic numbers possess higher thermodynamic stability than those off magic numbers. A complex kinetic pathway is responsible for the efficiency of this system in finding its minimum free energy configuration. Targeting similar magic number states is a strategy towards unique configurations in finite self-organizing systems across the scales.”

Read about it here:

Magic Number Colloidal Clusters as Minimum Free Energy Structures
J. Wang, C.F. Mbah, T. Przybilla, B.A. Zubiri, E. Spiecker, M. Engel, N. Vogel
Nature Communications 9, 5259 (2018)

A paper involving Michael Engel with coauthors Joshua Anderson and Sharon Glotzer from University of Michigan, Masaharu Isobe from Nagoya Institute of Technology, Etienne Bernard then from Massachusetts Institute of Technology, and Werner Krauth from École Normale Supérieure has been chosen as a Milestone Paper “that made significant contributions to their field” among all articles published in the journal Physical Review E in 2013.

Hard-disk equation of state: First-order liquid-hexatic transition in two dimensions with three simulation methods
Michael Engel, Joshua A. Anderson, Sharon C. Glotzer, Masaharu Isobe, Etienne P. Bernard, and Werner Krauth
Phys. Rev. E 87, 042134 (2013)