Advanced Imaging, Folding, and Assembly of Colloidal Molecules (ACE-T-9)

The Advanced Imaging, Folding, and Assembly of Colloidal Molecules (ACE-T-9) investigation involves the imaging, folding, and assembly of complex colloidal molecules within a fluid medium. This set of experiments not only prepares for future colloidal studies, but also provides insight into the relationship between particle shape, colloidal interaction, and structure. These so-called “colloidal molecules” are vital to the design of new and more stable product mixtures.

Delivery to the International Space Station via NG-16

Although decades of study using colloids as molecular mimics have shed significant light, the range of particles employed to date has been limited to relatively simple symmetries. The full diversity of fundamentally and technologically relevant phases is yet to be accessed. The key difference between conventional colloidal models and real molecules is the former usually lacks directional interactions. Pushing beyond this limitation helps to develop colloidal building blocks that better model natural molecules with varying composition, complex architecture, and external field-responsive interactions. The fabrication and subsequent assembly of these "molecules" allow the study of outstanding questions with more realistic models with profound technological impact.

The Advanced Imaging, Folding, and Assembly of Colloidal Molecules (ACE-T-9) research tests the microscopy imaging capability on the colloidal molecules (colloidal chains, dimers, and lock-and-key particles) created in space. This lays the foundation for subsequent investigation of them under electric fields when the electric cell module becomes available in near future. The colloidal chains are fabricated by combining both magnetic fields and Michael-addition reaction. An external magnetic field is used to align functionalized superparamagnetic spheres into linear chains of different lengths, and the Michael-addition reaction chemically links neighboring monomers in the same chain. The mechanical properties of those chains change as a function of reaction temperature, time, and linker length. The Light Microscopy Module (LMM) images the dynamics of single colloidal chains in bright field to capture the real-time, (quantitative [without confocal]) three-dimensional movement of monomers on single chains. The impacts of chain flexibility on dynamics are studied by both designing different chains, and varying salt concentrations.

The second objective of this research is to study the self-assembly of colloidal dimers and lock-and-key particles. Colloidal dimers have two lobes fused together, but one lobe has distinct interfacial, compositional, or physical properties from the other. Lock particles are colloidal spheres with well defined cavities synthesized from monodisperse emulsions. Key particles are spheres that possess the right curvature to match the size of the lock particles. Lock and Key particles can assemble, at the right experimental conditions, due to the depletion-driven lock-and-key interactions. During the past few years, dimer particles with combined geometric and interfacial anisotropies, such as the metallodielectric dimers and dimers with quadrupoles have been successfully fabricated.

It has been found that colloidal structures assembled from dimers sensitively depend on the relative orientation between neighboring dimers. In this research, the relative strength of double layer repulsion and hydrophobic attraction on both dimers and lock-and-key particles are tuned, by adjusting the salt concentration in solution. The three-dimensional self-assembled structures and the binding equilibrium constants under microgravity environment are imaged/measured using both bright field and fluorescence microscopy.

Large colloidal molecules, such as linear chains with tens to hundreds of monomers, settle quickly on Earth. This work expands the capability and lays the foundation to assemble complex colloids into three-dimensional structures, under the microgravity environment.

This work tests the imaging capability for large and complex colloidal molecules. It also probes the combined impacts of particle shape and colloidal interactions on assembly. This understanding can lead to improved control of non-covalent assembly of molecules, efficient tailoring of lattice symmetries, and scalable processing of nano-structured materials.

ACE-T9 in Space Station Research Explorer