Colloidal molecules in microgravity assembled by critical Casimir forces

Space-based research can provide fundamental insight into assembly processes that are of central importance in physics, material science and biology. Colloidal particles have served as a model system to study the assembly at convenient time and length scales, and microgravity research has played an important role in studying these assembly processes in their purest form without disturbance by sedimentation. Recently, anisotropic particles have emerged, opening the door to the study of complex structures; specifically, patchy particles enabling directed bonding promise the assembly of structures that could mimic those of molecular compounds.

Based on previous space research on tunable attractive particles, here we explore the use of patchy colloidal particles for studying the assembly of complex structures in space. The particles interact along their patches via temperature-tunable critical Casimir interactions with bond energies of several (ten) k_BT, the thermal energy. The tunability and reversibility of these critical Casimir forces allows convenient remote control of the colloidal interactions from the ground, enabling the repeated formation and break-up of the structures without much intervention by the astronaut. Together with the confocal microscope extension of the Light Microscopy Module on board the International Space Station, this offers unique opportunities for investigating complex structure formation in real space without disturbance by gravity.