Product and process have for a long time been two separate entities. This way of thinking is valid as long as simple products are produced for which the product properties are not so much affected by the process, e.g. classical base chemicals. For complex products, like whipped cream, cheese, detergents, membranes and drug formulations, processing strongly determines the structure of the product which in turn affects the processing. It has therefore been recognized that products, processes and their interactions need to be studied as a whole, as is evidenced by the emergence of fields like ‘product-driven process design’. In line with this trend, we perform research on the relation between the structure and processing of soft matter products. The structure and behavior of soft matter is, by definition, determined by relatively weak interactions and thus particularly for soft matter products, processing is likely to have a large impact on their structure and behavior. Vice versa, the processing of soft matter products is often relatively mild (e.g. only to a limited extent involves chemical reactions, does not take place at very high temperature) and thus the interactions taking place within the soft matter product are likely to affect processing.
Research takes place in the following three fields.
Emulsion formation and stability
The formulation of emulsions and foams in industry still relies more on practical experience than on science. This leads to suboptimal situations. In the food industry for example spray-dried emulsions generally contain about two times as much emulsifier as needed to cover the interface of the emulsion droplets. In metalworking, the search for more environmentally friendly lubricating emulsions is hindered by a lack of understanding . A major reason why science is only limitedly used in the formulation and processing of emulsions has been the inability to study the behavior of the interface of emulsions at relevant time scales. Important processes like droplet deformation and coalescence generally take place on a time scale of 10-4 s, while conventional means of studying the interfacial rheology cannot measure processes faster than 1 s . Very recently practically relevant time-scales have become accessible through the use of microfluidics [3,4]. We plan to use microfluidics to mimic the different flow regimes that determine the practical functionality of an emulsion while using practically relevant surface-active materials like proteins and phospholipids. For example, droplets can be made to impact each other with different force and with different amount of adsorbed proteins using microfluidic systems (figure 1) such as the one described by Krebs et al . This way we can mimic the situation inside industrially used high pressure homogenizers where droplets initially collide at high impact while their surfaces contain low amounts of adsorbed surface-active material whereas in later stages collisions take place at lower impact with their droplet surfaces containing a high amount of adsorbed material. The result of these studies will be design rules to come to a homogenizer flow profile that is optimized for the type of emulsion to be processed. Previous studies have already shown that for different emulsions different optimal homogenizer designs exist . The planned microfluidic studies can be extended to include coalescence between droplets and a liquid-gas interface. This type of coalescence determines if emulsion droplets are properly encapsulated during spray-drying and is thus of great practical importance. Through these studies we will be able to apply science for a rational formulation of emulsions and foams.
Rheological behavior of complex fluids during processing
In recent years great progress has been made in understanding the rheological behavior of non-Newtonian fluids such as (bio)polymer solutions and concentrated dispersions . These fluids are widely used in food, pharmaceutical and chemical industries. The rheological behavior of non-Newtonian fluids is relevant in several industrial processes, such as liquid dosing, membrane filtration and atomization in e.g. coating and spray-drying. It has been recently shown for example that concentrated dispersions often jam in confined flow . This may explain the difficulty in atomizing dispersions such as concentrated protein dispersions, ceramic slurries and pigments. This knowledge may also be used to better understand the blockage of membranes during filtration, which is an important practical problem that to our knowledge has never before been described based on the extensional rheological behavior of the product to be filtrated. A possible solution to this jamming may be the application of the right pre-shearing, but the effects of pre-shearing are still poorly understood . We plan to investigate the flow of relevant concentrated model dispersions under flow conditions relevant to atomization and filtration. Also here, use can be made of microfluidics to mimic relevant flow conditions while allowing direct observation of the flow. This has already been done successfully to study the break-up of droplets in porous media . The above mentioned jamming transitions have so far been only observed for relatively large, non-Brownian particles such as starch whereas many practically relevant particles are smaller, Brownian particles, e.g. proteins. The behavior of such particles in confined flow may be studied by combining microfluidics with in situ observation of the flow using methods like Diffusing Wave Spectroscopy or by adding tracer particles.
In the past years, we produced several new soft matter structures, such as Pickering stabilized water-in-water emulsions, non-microfluidically produced core-shell double emulsions, stable antibubbles and jammed bi-continuous structures from phase-separated polymer solutions [11-14]. There is still a lot of fluid mechanic and soft matter science to be explored in these topics. For example, water-in-water emulsions are a topic that is gaining interest for example for drug delivery . However, how Pickering stabilization opposes coalescence in water-in-water emulsions and how Pickering-stabilized water-in-water emulsion droplets break up has not been studied yet. Also, we have shown that we can produce remarkably strong bicontinuous structures from phase-separated polymer solutions using jamming colloidal particles (figure 1) . These structures are of interest for applications as diverse as cheese making to catalysis and membrane adsorption processes. Nevertheless, how to control the formation of these structures has not yet been studied.