Research on colloids is motivated by several factors. They can be used to answer fundamental questions related to the assembly of materials, and they also provide the capability for manufacturing of structured materials and devices in electronics, photonics, and life sciences. However, the rich variety of colloidal structures observed on the Earth can be influenced by the effects of gravity that leads to particles settling and the motion of the surrounding fluid. To suppress the gravity effects, experiments on concentrated colloids of fluorescent ellipsoidal particles were carried out in the Light Microscopy Module onboard the International Space Station. Due to the absence of gravity, sedimentation did not exist and the 3D colloidal structure formed by the particles did not sink to become a 2D sediment, as happens on Earth.The presented study was supported by the NASA Advanced Colloids Experiment (ACE) program. The project ACE-T (with temperature control) was exploring the behavior of six concentrated colloids (&gt;30% particle volume fraction) of micron size ellipsoidal particles with different aspect ratios. The particles were dispersed in a decalin/tetralin mixture to match the particle refractive index that enabled colloids to appear transparent rather than milky white. Each colloidal sample was loaded in a rectangular cross-section glass capillary with the 200-micrometers gap between the upper and bottom plates. Thermal effects in the colloids were studied by exposing the capillary upper and bottom plates to different temperatures (a temperature gradient across the capillary gap) or exposing the capillary ends to different temperatures (a temperature gradient in the plane parallel to the capillary upper and bottom plates). A remotely operated 3D confocal microscope was used to scan 0.3 micrometer-thick slices of colloids in the plane parallel to the capillary upper and bottom plates and take stacks of 0.3 micrometer-thick serial image slices in the direction across the capillary gap between the upper and bottom plates. Image J and Matlab codes were used for image processing and then for computing spatial and angular correlation functions of the particle arrangement.We will present observations of nematic ordering of particles – useful for controlling light, such as in liquid crystal displays, and discuss the effect of a temperature gradient on the colloidal structure. We will also present observations of the particle behavior caused by the presence of a large bubble developed within a middle of one of the capillaries. Exposure of the upper and bottom plates of this capillary to different temperatures generated an intensive particle motion near the bubble surface due to changing the bubble size and/or generating flow in its vicinity (possibly by Marangoni forces). In capillaries without a bubble in the middle, we did not see intensive particle movement caused by the temperature gradient, referred to as thermophoresis or a Soret effect.&nbsp;The work was supported by the NSF CBET grants 1832260 and 1832291 and the NASA grant 80NSSC19K1655.

Temperature Gradient Effects in Colloids of Ellipsoidal Particles Under Microgravity.

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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