Session: 16-01-01: NSF-funded Research (Grad & Undergrad)

Paper Number: 77380

Start Time: Wednesday, 02:25 PM

77380 - Disorder-Order Transition in Colloids of Ellipsoidal Particles in Microgravity 

Motivation: Colloidal particles can be considered as blocks for building materials, like atoms for forming molecules and crystals. Understanding at the particle level of how a disordered arrangement of colloidal particles self-assembles into a three-dimensional ordered structure is needed to provide the capability for manufacturing of structured materials, ranging from photonic crystals and stretchable electronics to liquid crystals and optical metamaterials. The major challenge is due to kinetic limitations - as particles can be trapped into metastable configurations for a long time due to the lower mobility of multi-particle structures. The rich variety of transitions from a disordered arrangement of particles to an ordered state observed in numerous terrestrial experiments could also have been influenced by gravity effects, such as particle sedimentation, convection and jamming, during the slowly evolving structure formation. A requirement for precise matching of densities between suspended particles and a host liquid in order to avoid undesirable gravity effects severely limits the possibilities to study the evolution of colloidal structures on the Earth. Moreover, a difference between coefficients of thermal expansion of particles and a host liquid would generate positive and/or negative buoyancy forces in a spatially non-uniform thermal field, even though this suspension is neutrally buoyant at a certain temperature. Long duration microgravity on the International Space Station (ISS) offers a unique opportunity to conduct experiments on assembly of colloidal particles in non-buoyancy-matched suspensions without the masking effects of gravity.

Methodology: We present results of the project Advanced Colloids Experiment with Temperature control (ACE-T-Ellipsoids) carried out on the ISS. The main objective of these microgravity experiments was to study at the particle level how a disordered arrangement of colloidal particles of ellipsoidal shape self-organized into a three-dimensional ordered structure. This project was supported by the joint NSF-CASIS program. The year-long experiments were conducted on fluorescent PMMA micro-particles of ellipsoidal shape with the aspect ratios 2.8 and 4.5. These monosized particles were synthesized by Prof. Andrew D. Hollingsworth, New York University, and suspended in a decalin/tetralin mixture to match the particle refractive index that enabled colloids to appear transparent rather than milky white. A colloidal sample was loaded in a capillary through a filling port that was then sealed. The capillary was mounted in a copper thermal bridge equipped with thermistors to control a temperature gradient across the capillary. The experiments utilized the ISS Fluids and Combustion Facility (FCF) Fluids Integrated Rack (FIR) and the Light Microscopy Module (LMM) housed in the FIR. The LMM is an automated Leica DM-RXA confocal light microscope equipped with a sample stage and a digital low noise scientific camera for video microscopy. The LMM uses a 532-nm frequency-doubled Nd:YAG laser, a Nipkow Spinning Disk Technique for scanning and a digital CCD camera. The LMM involves on-orbit operations consisting of the ISS crew activity and powered operations performed remotely from the NASA GRC Payload Operations Center (GIPOC).

Results and conclusions: The particles used in the ISS experiments are huge in size when compared to liquid crystals, which are too small to be seen with an optical microscope. One of the important findings is that our microscopic ellipsoids mimic these molecular-sized structures, along with their defects, commonly called ‘hedgehogs’ in the liquid crystal community. We also found that the application of a temperature gradient to a colloid generated Marangoni-like flows around a bubble formed in the sample. This observation holds the promise to use Marangoni-like flows to control and manipulate particles in dense colloids in microgravity where conventional methods used in terrestrial experiments are of limited use.

The work was supported by the NSF CBET grants 1832260 and 1832291.

Presenting Author: Qian Lei New Jersey Institute of Technology

Authors:

Qian Lei New Jersey Institute of Technology
Boris Khusid New Jersey Institute of Technology