A microfluidic toolbox to fabricate, sort and characterize thermoresponsive colloidal molecules and their assemblies

Abstract

In this thesis we discuss the assembly of thermoresponsive microgel particles into small clusters, and their use as colloidal molecules with specific and directional interactions. The underlying goal is to use these particles as advanced building blocks for new types of self- assembled structures with novel or vastly improved properties, and as model particles for investigating fundamental problems in condensed matter physics. For this purpose, we have developed a microfluidic toolbox to prepare, sort and characterise colloidal molecules and their interactions and self-assembly.

In the first part of this thesis, two types of thermoresponsive colloidal particles with different collapse transition temperatures (VPTT) are synthesized based on poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM), respectively. The mixture of these microgels are encapsulated with a narrow distribution into highly monodisperse water-in-oil droplets generated by developed droplet- based microfluidics. The evaporation of the water inside the droplets leads to clustering of the microgels and the formation of microgel-based colloidal molecules that can then be harvested. These clusters are subsequently crosslinked in order to create stable colloidal molecules, and redispersed into a water phase. The thermoresponsive behavior of these mixed colloidal molecules was studied by confocal microscopy at different temperatures. At 20 °C, both types of microgel interaction sites possess a repulsive interaction behavior, and individual colloidal molecules were observed. In contrast, at 35°C, the PNIPAM-based microgels in the clusters undergo a collapse transition, and their interaction potential changes from soft repulsive to attractive. The PNIPAM particles then act as attractive patches or binding sites, inducing the formation of reversible bonds between individual clusters. These observations confirm that temperature could be used as an external stimulus to control the interactions in these patchy colloidal molecule systems in a highly selective way. Other attractive methods to fabricate large quantities of colloidal molecules are to use electrostatics-driven assembly and a microgel-pickering emulsion approach. However, these strategies used to create colloidal molecules resulted in a mixture of microgel clusters and excess individual microgels. Therefore, we developed and employed microfluidic Deterministic Lateral Displacement (DLD) devices to efficiently separate colloidal molecules from the large background of individual satellite particles. While microgel synthesis can easily be upscaled to obtain large quantities of individual particles, the fabrication of well-defined colloidal molecules and the investigation of their temperature-dependent interactions and the resulting phase diagrams make the question of the individual sample size still an important point to consider. Here we again resort to droplet microfluidics, where we use larger water-in-oil emulsion droplets as our sample container that can be visualised and investigated with a confocal microscope. We have thus designed and fabricated a so-called PhaseChip with multiple storage trap arrays. This device allows us to investigate the interactions between individual particles or patches on different clusters and their assembly into larger superstructures as a function of temperature, pH, ionic strength and particle concentration for a large number of individual samples, while keeping the amount of material required to a minimum.

Abstract (Swedish)

In this thesis we discuss the assembly of thermoresponsive microgel particles into small clusters, and their use as colloidal molecules with specific and directional interactions. The underlying goal is to use these particles as advanced building blocks for new types of self- assembled structures with novel or vastly improved properties, and as model particles for investigating fundamental problems in condensed matter physics. For this purpose, we have developed a microfluidic toolbox to prepare, sort and characterise colloidal molecules and their interactions and self-assembly.

In the first part of this thesis, two types of thermoresponsive colloidal particles with different collapse transition temperatures (VPTT) are synthesized based on poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM), respectively. The mixture of these microgels are encapsulated with a narrow distribution into highly monodisperse water-in-oil droplets generated by developed droplet- based microfluidics. The evaporation of the water inside the droplets leads to clustering of the microgels and the formation of microgel-based colloidal molecules that can then be harvested. These clusters are subsequently crosslinked in order to create stable colloidal molecules, and redispersed into a water phase. The thermoresponsive behavior of these mixed colloidal molecules was studied by confocal microscopy at different temperatures. At 20 °C, both types of microgel interaction sites possess a repulsive interaction behavior, and individual colloidal molecules were observed. In contrast, at 35°C, the PNIPAM-based microgels in the clusters undergo a collapse transition, and their interaction potential changes from soft repulsive to attractive. The PNIPAM particles then act as attractive patches or binding sites, inducing the formation of reversible bonds between individual clusters. These observations confirm that temperature could be used as an external stimulus to control the interactions in these patchy colloidal molecule systems in a highly selective way. Other attractive methods to fabricate large quantities of colloidal molecules are to use electrostatics-driven assembly and a microgel-pickering emulsion approach. However, these strategies used to create colloidal molecules resulted in a mixture of microgel clusters and excess individual microgels. Therefore, we developed and employed microfluidic Deterministic Lateral Displacement (DLD) devices to efficiently separate colloidal molecules from the large background of individual satellite particles. While microgel synthesis can easily be upscaled to obtain large quantities of individual particles, the fabrication of well-defined colloidal molecules and the investigation of their temperature-dependent interactions and the resulting phase diagrams make the question of the individual sample size still an important point to consider. Here we again resort to droplet microfluidics, where we use larger water-in-oil emulsion droplets as our sample container that can be visualised and investigated with a confocal microscope. We have thus designed and fabricated a so-called PhaseChip with multiple storage trap arrays. This device allows us to investigate the interactions between individual particles or patches on different clusters and their assembly into larger superstructures as a function of temperature, pH, ionic strength and particle concentration for a large number of individual samples, while keeping the amount of material required to a minimum.

• author: Peng, Feifei
• supervisor: Peter Schurtenberger; Jonas Tegenfeldt
• opponent: Professor Isa, Lucio, Eidgenössische Technische Hochschule Zürich (ETHZ), Zürich, Switzerland
• organization: Physical Chemistry; NanoLund: Centre for Nanoscience
• publishing date: 2019-05-24
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: Microgels, Thermoresponsive Colloidal Molecules, Microfluidics, Self-Assembly, Microgels, Thermoresponsive Colloidal Molecules, Microfluidics, Self-Assembly
• pages: 162 pages
• defense location: Lecture Hall B, Kemicentrum, Naturvetarvägen 14, Lund
• defense date: 2019-09-04 13:15:00
• ISBN: 978-91-7422-671-3
• ISBN: 978-91-7422-670-6
• language: English
• LU publication?: yes
• id: 9a7d1613-84af-4861-8c3b-568b06942e75
• date added to LUP: 2019-05-23 13:15:31
• date last changed: 2020-12-03 09:05:48