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Membrane filtration processes are best known for their application in the water, oil, and gas sectors, but also in food production they play an eminent role. Filtration processes are known to suffer from a decrease in efficiency in time due to e.g., particle deposition, also known as fouling and pore blocking. Although these processes are not very well understood at a small scale, smart engineering approaches have been used to keep membrane processes running. Microfluidic devices have been increasingly applied to study membrane filtration processes and accommodate observation and understanding of the filtration process at different scales, from nanometer to millimeter and more. In combination with microscopes and high-speed imaging, microfluidic devices allow real time observation of filtration processes. In this review we will give a general introduction on microfluidic devices used to study membrane filtration behavior, followed by a discussion of how microfluidic devices can be used to understand current challenges. We will then discuss how increased knowledge on fundamental aspects of membrane filtration can help optimize existing processes, before wrapping up with an outlook on future prospects on the use of microfluidics within the field of membrane separation.

Clogging of porous media by soft particles has become a subject of extensive research in the last years and the understanding of the clogging mechanisms is of great importance for process optimization. The rise in the utilization of microfluidic devices brought the possibility to simulate membrane filtration and perform in situ observations of the pore clogging mechanisms with the aid of high speed cameras. In this work, we use microfluidic devices composed by an array of parallel channels to observe the clogging behavior of micrometer sized microgels. It is important to note that the microgels are larger than the pores/constrictions. We quantify the clog propensity in relation to the clogging position and particle size and find that the majority of the microgels clog at the first constriction independently of particle size and constriction entrance angle. We also quantify the variations in shape and volume (2D projection) of the microgels in relation to particle size and constriction entrance angle. We find that the degree of deformation increases with particle size and is dependent of constriction entrance angle, whereas, changes in volume do not depend on entrance angle.

Cake layer formation in membrane processes is an inevitable phenomenon. For hard particles, especially cake porosity and thickness determine the membrane flux, but when the particles forming the cake are soft, the variables one has to take into account in the prediction of cake behavior increase considerably. In this work we investigate the behavior of soft polyacrylamide microgels in microfluidic model membranes through optical microscopy for in situ observation both under regular flow and under enhanced gravity conditions. Particles larger than the pore are able to pass through deformation and deswelling. We find that membrane clogging time and cake formation is not dependent on the applied pressure but rather on particle and membrane pore properties. Furthermore, we found that particle deposits subjected to low pressures and low g forces deform in a totally reversible fashion. Particle deposits subjected to higher pressures only deform reversibly if they can re-swell due to capillary forces, otherwise irreversible compression is observed. For membrane processes this implies that when using deformable particles, the pore size is not a good indicator for membrane performance, and cake formation can have much more severe consequences compared to hard particles due to the sometimes-irreversible nature of soft particle compression.

Plant‐based proteins, such as those derived from potatoes, are increasingly used to stabilize oil‐in‐water (O/W) emulsions due to their higher sustainability. Their interfacial activity is strongly influenced by their structure. Given the sub‐millisecond timescales at which emulsion droplets are generated at large scale, this activity would need to be measured within this time range to be in a position to connect to eventual emulsion stability. This necessitates dedicated analytical tools. In this study, a custom‐designed microfluidic device was used to investigate the coalescence stability of O/W droplets stabilized by two commercial potato protein fractions, patatin and protease inhibitor, across a range of pH values relevant for food (3–7) and protein concentrations (0.1–1 g/L). Droplet stability was assessed 11 milliseconds after formation and shows a strong pH dependence. Both patatin and protease inhibitor exhibited high interfacial stabilization near neutral pH, while higher coalescence levels were observed at lower pH. This is correlated with protein secondary structure changes (e.g., α‐helix, β‐sheet content) and surface‐exposed hydrophobicity. This study highlights the utility of this microfluidic technique in capturing early‐stage interfacial phenomena and provides insights into the pH response of plant proteins. Oil droplets were generated at the T‐junction where the continuous (protein solution) and dispersed phase (oil) channels meet. The droplets then enter the coalescence channel, and the images were recorded at both the inlet of the coalescence channel and the outlet at various pH and concentrations. The adsorption time (the time between the formation of oil droplets at the T‐junction, and their entering in the coalescence channel) is in milliseconds.

Increasing the particle density of a suspension of microgel colloids above the point of random-close packing, must involve deformations of the particle to accommodate the increase in volume fraction. By contrast to the isotropic osmotic deswelling of soft particles, the particle-particle contacts give rise to a non-homogeneous pressure, raising the question if these deformations occur through homogeneous deswelling or by the formation of facets. Here we aim to answer this question through a combination of imaging of individual microgels in dense packings and a simple model to describe the balance between shape versus volume changes. We find a transition from shape changes at low pressures to volume changes at high pressures, which can be explained qualitatively with our model. Whereas contact mechanics govern at low pressures giving rise to facets, osmotic effects govern at higher pressures, which leads to a more homogeneous deswelling. Our results show that both types of deformation play a large role in highly concentrated microgel suspensions and thus must be taken into account to arrive at an accurate description of the structure, dynamics and mechanics of concentrated suspensions of soft spheres.

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 wellknown 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.

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.