Category: Publication

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.

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

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)

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)

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.

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

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)