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

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.

Colloid system are composed of at least two phases, with one phase (the colloids) dispersed in another one. The dispersed phase are tiny particles with sizes in the range of 1-1000 nm. Colloidal particles are very common in daily life and can be found for example in milk, cream, paint and detergent. An important feature of colloidal systems is that the particles are in the size range where they can be observed in real space and time by for example optical microscopy, yet are small enough to exhibit Brownian motion. Another important feature is that colloidal particles can spontaneously assemble into various amorphous ordered structures, which is called self-assembly. Self-assembly is an important method to create new materials by nature. In the past, hard spherical colloids have primarily been used as the building blocks to investigate self-assembly into superstructure. However, spherical particles limit the application in fabricating more advanced structures due to their isotropic interactions. Recently, researchers started to explore the use of small clusters of particles with shapes that look like molecular models of real molecules, so-called colloidal molecules, as advanced building blocks for this purpose. These colloidal molecules are believed to have a significant potential as a next generation of building blocks for the fabrication of more complex structures through self-assembly.

Figure 1: Schematic illustration of the temperature-induced collapse transition for PNIPAM microgels, the collapsed temperature for the microgel is 32 ◦ C.

In this thesis, we employ microgels as the building blocks to fabricate such colloidal molecules. Microgels are a particularly interesting class of responsive colloids, where the particles exhibit a specific response to various external parameters such as pH, temperature and others. One type of microgels used in this thesis is swollen in water at room temperature and collapsed when the temperature is increased above 32 ◦ C (Fig. 1). This allows us to tune the microgel size using temperature.

Microfluidics has emerged as a new technology to precisely manipulate small volumes of liquids (nL-pL) at the microscale. Microfluidic systems offer numerous advantages including faster analysis, low reagent consumption and miniaturization. The vision

vii oil

water-in-oil droplet

aq. phase

oil

200 휇m

Figure 2: a micrograph of droplet generation in the droplet device. The aqueous phase and the oil phase (HFE-7500) meet at the constriction to form water-in-oil droplets.The variation in size is low with a measured coefficient of variation of 2.2%.

is to integrate different components in microfluidic systems to the realization of all essential functions in an entire laboratory in a microfluidic platform, a so-called“Labon-a-chip”.

flow

large particle

small particle

Figure 3: Schematic illustration of a small area of a DLD device: the flow is from right to left. Two particles flow through the gap of pillars and are divided by their sizes.

In the thesis we demonstrate the preparation of colloidal molecules using droplet microfluidics. Two types of microgels are encapsulated in highly monodisperse water-in-oil droplets generated by the droplet device (Fig. 2). 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 cross-linked in order to create stable colloidal molecules, and redispersed into water phase. The two types of microgels have different collapsed temperatures, one is 32 ◦ C and the other one is 45 ◦ C. The thermoresponsive behavior of the mixed colloidal molecules was studied by confocal microscopy at different temperatures. At 32 ◦ C, both types of microgel interaction sites possess a repulsive interaction behavior, and individual colloidal molecules were observed. In contrast, at 35 ◦ C, one type of microgels in the

viii

separation clusters undergo a collapse transition, and their interaction potential changes from soft repulsive to attractive. The other microgel still possess a repulsive interaction. Thus, the attractive particles can act as 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.

We then developed Deterministic Lateral Displacement (DLD) devices to separate colloidal molecules according to their sizes. A typical DLD device consists of micro-pillars shifted laterally in each subsequent row. Particles suspended in a liquid that flows in such a device then interact with the fixed posts and follow different streamlines depending on their sizes. Larger particles are displaced into the adjacent streamline, while smaller particles are following the same streamline and to the end of the array, as shown in Fig. 3.

Figure 4: Schematic illustration of droplets are captured in the trap of a PhaseChip.

We finally described the design and fabrication of the PhaseChip (Fig. 4). This device enables us to investigate the droplets containing particles and or colloidal molecules in traps for long time. Moreover, the device provides control over the shrinkage of each droplet, which allows for an efficient variation of the volume fraction in the individual droplets. By integrating heat sources into the PhaseChip it would allow us to efficiently map out the phase behaviour and the self-assembly of mixtures of colloidal clusters as a function of concentration and temperature.