Linking microgel particle properties to filtration behavior through microscopic observations

Soft particles are present in our daily lives and differently from their hard counterparts, they can change conformation and composition when experience an external source of stress. This specific characteristic of soft particles can make it more challenging to predict their behavior in processes such as filtration and centrifugation. More information on the specific behavior of soft particles under external stress is still lacking on current literature and can be useful to different areas of application. The aim of this thesis is to provide information that will contribute to the understanding of soft particle behavior under pressure such as in pore clogging and cake formation in membrane processes. We use micrometer-sized microgels as model particles in this work due to their tunability and ease of production. Also, using micrometer-sized microgels we can consider colloidal interactions negligible, what simplifies our system and allows us to focus on individual particle behavior. In the first two experimental chapters (Chapters 2 and 3), we focus on microgel packings (static conditions). The packings were produced by osmotic stress with controlled, varying applied pressure. In Chapter 2, we focus on the collective behavior of microgels in packings in static conditions and we describe the behavior of the microgel packings in term of well-known polymeric theories such as the Flory-Rhener theory. We found that suspensions of dextran microgels start to resist compression at volume fractions close to random close packing of hard spheres with the same size distribution. For volume fractions between random close packing and 1, the resistance increases similarly to that of a dextran solution of the same concentration. From image analysis followed that microgels are deformed but internal concentration remains the same. At volume fractions ‘higher than 1’, microgels are forced to expel solvent and deswell. In Chapter 3, we explore our observation from Chapter 2 that individual particles will respond to stress in different ways according to the applied pressure. For that we use microgel packings containing a mixture of fluorescent and non-fluorescent microgels with an excess of non-fluorescent microgels. We observe the packing using fluorescence microscopy and are able to observe single fluorescent particles surrounded by non-fluorescent particles (non-visible). We found that both deswelling and deformation occur simultaneously when soft particles are under pressure and we describe a theory to predict their behavior according to the pressure applied to the system. In Chapters 4 and 5, we use microfluidic devices to observe the behavior of soft particles in dynamic systems. In Chapter 4, we focus on the collective behavior of particles. For that we use a microcentrifuge coupled with an optical microscope to investigate the reversibility of soft particle deposits according to the applied force. We found that, for the particles used, total reversibility of deposits is possible as long as there is water available for particle reswelling. Also in Chapter 4, we use microfluidic devices composed of an array of parallel channels as a model membrane for filtration experiments. In this device, we observe the clogging behavior of soft particles in filtration, focus on cake layer formation and assess cake reversibility. We found that the propensity of a particle to clog is dependent on the applied pressure and that at low pressures, microgels are more likely to clog a pore. As pressure increases, microgels are more likely to be pushed all the way through the pores or block deeper in the pore. We also found that a microgel deposit layer (cake layer) that formed on top of the model membrane can be compressed up to 30% but the compression is totally reversible. After focusing on collective behavior of soft particles, in Chapter 5 we focus on what is happening at individual particle level. We observe single particles going through pore constrictions and assess deformation and deswelling of the particles. We then correlate the observations with particle and system properties such as particle size and applied pressure. We found that higher pressures promote clogging deeper in the channels but most of the microgels will still clog at the first constriction. We also observe a shift in particle size of microgels that clog the pores with increasing applied pressure, as could be expected from previous chapters: particles decrease their size with increasing pressure and are more likely to pass a pore. The degree of particle deformation is dependent on channel entrance angle whereas changes in volume are not. Finally, in Chapter 6, we discuss our main findings and their implications in real life situations and processes. The results presented in this thesis are of importance in many areas involving packings and concentration of soft particles such as membrane filtration and chromatography.