Microscale collective behavior: Catalytic micropumps and reversible colloidal assembly

open_access
Open Access
Author:
Altemose, Alicia
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 15, 2019
Committee Members:
Ayusman Sen, Dissertation Advisor/Co-Advisor
Ayusman Sen, Committee Chair/Co-Chair
Thomas E Mallouk, Committee Member
Christine Dolan Keating, Committee Member
Darrell Velegol, Outside Member
Scott A Showalter, Program Head/Chair
Keywords:
Active matter
Collective behavior
Catalytic micropumps
Colloidal motors
Oscillatory dynamics
Schooling
Abstract:
Since their discovery in 2004, catalytic nano- and micromotors have sparked interest in researchers from many disciplines. These small-scale machines have potential applications as drug delivery vehicles, ultrasensitive chemical sensors, and smart materials. In addition to becoming increasingly popular in research over the years, nano- and micromotors have also become more complex in both their fabrication and behavior, raising many questions about propulsion mechanisms and interactions of motors with each other and the surroundings. This dissertation focuses on the areas of coordinated micropump systems, controllable collective behavior of motors, motor-driven annealing of colloidal crystals, and near-wall behavior of active and passive matter. Chapter 1 involves a review of the various forms of nano- and micromotors and their different mechanisms of motion, as well as the motivation for studying such systems. It also examines the types and applications of non-pressure driven micropumps, which originated as a study of momentum transfer by immobilized catalytic micro-machines. Investigating the motion of individual motors and the driving mechanisms of isolated catalytic micropumps is necessary for understanding and controlling the collective behavior of active microswimmers and the coordinated action of catalytic micropumps in series, which are the focus of the subsequent chapters. Chapter 2 deals with the use of interacting catalytic micropumps to induce unidirectional fluid flows in microchambers, as a means of improving sensors and other microdevices that operate by diffusion alone. The directed transport of nano- and microscale objects in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these species. Using experiments, it has been shown that catalytic surface reactions can be used to deliver nano- and microscale cargo to specified regions in microchambers. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended species until the reagents are consumed by the catalytic reactions. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local species concentration in a desired area, e.g. at a detector, highly sensitive assays can be performed efficiently and rapidly. Chapter 3 focuses on an autonomous oscillatory micromotor system in which active colloidal particles form clusters whose size changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. The oscillatory behavior is governed by an electrolytic self-diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag3PO4 particles form a non-oscillating core with an oscillating shell composed largely of silica microparticles. Chapter 4 looks at a synthetic system that exhibits chemical communication between small active and larger inactive particles which drives the reversible assembly of the latter into colloidal crystals. Autonomous hexagonal packing of inert silica particles is a result of the oscillatory behavior of neighboring silver phosphate micromotors under UV light in the presence of hydrogen peroxide. The colloidal crystals are formed under UV illumination and relax into a disordered state when the light is turned off. Furthermore, oscillatory waves generated by the active particles cause “autonomous annealing”, a phenomenon resulting in the elimination of defects between crystal boundaries of the silica colloidal crystals. Finally, Chapter 5 focuses on an intriguing characteristic of active matter systems: the ability of these microswimmers to overcome entropic and hydrodynamic limitations in order to interact with the walls of a confined system. This behavior is of critical importance for applications in drug delivery and 3D printing, for example. Motors must interact with capillary walls and must rheotax within printing nozzles in order to perform these tasks effectively. This study is still in progress, but the preliminary experimental results show that passive particles are excluded from near-wall regions in narrow microchannels, while active particles, e.g. bimetallic nanorod motors and Janus micromotors, have no restrictions on motion within the channels. The observed passive particle behavior has been predicted theoretically, but this study ultimately aims to extend these theories to active matter systems.

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