II, A, 2, 9.1: Curvature Directed Assembly of Particles into Reconfigurable Structures at Fluid Interfaces
Abstract
CURVATURE - DIRECTED ASSEMBLY OF PARTICLES INTO RECONFIGURABLE STRUCTURES Scientific Objectives: In this research, we use geometry, including bounding vessel curvature and particle shape, to develop new means to direct assembly of colloids using elastic energy in confined nematic liquid crystals. In materials science, micro and nanoscale building blocks are routinely assembled into fixed, ordered, close packed structures via bottom-up strategies or top down approaches. However, to fully exploit the possibilities afforded by the growing library of micro and nanoscale building blocks, strategies are needed to direct the assembly of reconfigurable structures. We use energy stored in confined nematic liquid crystals to direct the assembly of colloids to particular docking sites and with control over their trajectories, orientation, and proximity to boundaries using geometric cues and non-singular director fields. The energetic landscape in these systems depends on the spatial arrangement of the nematic liquid crystal molecules as reflected in the director field. Nematic liquid crystals respond to external electro-magnetic fields, and are inherently reconfigurable. Furthermore, colloids also respond to external fields. Thus, the strategies we develop can be combined with external fields to make reconfigurable and actuatable structures. Methods to be employed: We fabricate vessels with particular geometries and anchoring conditions. By confining the nematic liquid crystal in these vessels, we impose the energy landscape in which the colloids will assemble. By placing particles with well defined anchoring conditions within these vessels, the colloids interact within the host director field to form companion defects and to assemble into structures. We use geometry and confinement to direct the motion and preferred loci of colloids. Colloid trajectories are imaged using optical microscopy. The director fields are characterized using polarized optical microscopy. To guide experiment, director fields are simulated using a Q-tensor based Landau-de Gennes model. We study colloid-boundary interactions and colloid structure formation as a function of boundary geometry, colloid shape and anchoring conditions of the liquid crystal on the colloids and boundaries. By defining geometries with strong deformations that complement those made by the colloids, we will define where these assemblies begin, and where they terminate. By understanding the forces on colloids within the domain, we define colloid trajectories from unstable loci, and develop domains with multistable states. To avoid trapped states, we focus on non-singular director fields defined by vessel boundary geometry and anchoring conditions. Significance of the Proposed Effort to the Advancement of Knowledge: We develop a general means to direct colloid motion and assembly. Interactions of colloids and boundaries in nematic liquid crystals depend boundary geometry, texture, and chemistry. These are independent of the colloid and vessel materials. Thus, these methods are applicable across materials platforms. Via reconfiguration of the nematic liquid crystal, the use of external fields, or the use of reconfigurable boundaries, these methods should allow actuation and reconfiguration of the structures formed. Control over colloid trajectories and development of multistable states have implications in microrobotics and devices for optical manipulation.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 25, 2019
- Source ID
- W911NF1610288
Entities
People
- Kathleen J. Stebe
Organizations
- Army Contracting Command
- United States Army
- University of Pennsylvania