Directed Assembly of Microparticles in Flowing Viscoelastic Fluids

Numerous consumer and intermediate products consist of some kind of complex fluid and in many cases; particles of different sizes and shapes are dispersed in these fluids. The flow behavior of the fluids is then complicated further. Complex fluids generally display a number of non-linear rheological properties such as shear thinning of the viscosity and elasticity, which cause time-dependent behavior and memory effects. On the one hand, the presence of particles and the entailed disturbances of the flow field and hydrodynamic interactions, strongly affect fluid behavior. The interactions between suspended particles are, on the other hand, also strongly affected by the rheological constitution of the suspending medium. The inherent correlation of all effects and the high degree of non-linearity cause the understanding and possibility to predict of the occurring phenomena to be challenging. Due to the widespread presence of these systems and the vast number of possible future applications, a further understanding will prove very useful. This work is aimed at investigating particle interactions in flowing viscoelastic media that lead to an assembly of these particles into a structure, such as a train. This formation of particle chains in viscoelastic fluids under shear was discovered a long time ago, but due to the complexity of the problem of suspensions in non-Newtonian fluids mentioned above, a complete explanation for the phenomenon has not been given yet. Experiments have been done to determine which fluid properties and geometrical circumstances are necessary, beneficial or detrimental for particle assembly in trains. The first alignment experiments were done using polymer solutions that display both elasticity, as exemplified by normal stress differences, and a shear thinning viscosity. Newtonian fluids, on the other hand, cannot be used to align particles in bulk. A first question that arises is which one of both complex fluid properties is determining for obtaining strings under shear, or whether a combination is required. From alignment experiments in Boger fluids, it was previously shown that the presence of elasticity of the fluid, but without a shear thinning viscosity, is insufficient to create particle strings. The opposite experiments were performed in the present work, using an fd-virus solution that displays no measurable normal stress differences, but does exhibit a shear thinning viscosity. Since alignment of particles proved to be possible, but less pronounced in this fluid, it can be concluded that elasticity is a fluid property strongly promoting string formation. It is, however, not a part of the driving mechanism, as strings can also be found in a non-elastic medium. Shear thinning, on the other hand, has turned out to be a conditio sine qua non for alignment of particles in flowing fluids. Regarding geometrical constraints and concentration effects, a confining effect by walls or other particles shows to be factor that improves alignment. Particles have a higher probability of encountering each other and in elastic fluids, normal stresses can help keeping structures that were formed together. An uneven distribution of the particles has shown to be unfavorable for alignment. The next question arising is how exactly flow-oriented strings nucleate in flowing viscoelastic fluids and how they are maintained without tumbling, like ellipsoidal particles would do when they move on Jeffery orbits at low to moderate Deborah numbers (De). From comparison with elongated particles, it can be deduced that the strings of spheres do not act as a single, rigid body. They would tumble as a result of the torque exerted by the liquid under shear. It has turned out that they maintain an individual rotating motion instead and a thin layer of fluid keeps existing between the individual building blocks of a string. Shear tinning is the key element here, as the small gaps between the rotating particles locally entail high shear rates and thus low viscosities. Those low viscosities are exactly the circumstances that allow the individual rotating motions of the aligned particles. The pressure in the gap, on the other hand, is also less affected in a shear thinning fluid. The lower pressure drop between approaching particles allows for the particles to stay separated by a fluid layer so tumbling is avoided. Depletion experiments have confirmed that particles need to stay separated in order to obtain a flow-oriented, aligned structure. The experiments in this work are done at an intermediate range of Reynolds numbers (Re), and the conclusion must not be generalized. Flow oriented ellipsoids have been observed at higher shear rates, and it is thus very well possible that another alignment mechanism occurs in other flow regimes, for instance where the string does act as a rigid, elongated body. Strings oriented in the vorticity direction have also been observed, and another mechanism is also expected to act here. This is for important when considering possible applications. One could think of micro- or even nanowires from all kinds of building block that are assembled using direct contact during flow-induced alignment, but the gap existing between the particles prevents any application of this kind from being workable. High shear rates or vorticity oriented strings can then be a solution, as a gap between particles is not expected to be present there. Effects of surface roughness of the particles have been investigated using home-made particles that are less smooth and less spherical in shape than the model latex particles. The results here have confirmed that the development of a flow profile in the gap between the particles in a string is crucial for alignment. This flow profile is disturbed by factors such as large-scale surface roughness effects, non-spherical shapes of the particles and size polydispersity. In the case of the home-made particles, these properties showed to be detrimental for the formation of strings. A possible application has been tested in the development of protein fibers composed of globular proteins that are used in the food industry for improving the texture of certain products. As mentioned before, the gap between particles in a flow-oriented train is a major technology kill, since there is no way the structure can be made permanent by linking the particles. For forming the desired structures in this case, the vorticity-oriented structures turned out the be the answer, as long and thin fibers could be formed this way. These structures also appeared to have a permanent nature. Finally, a modified colloidal probe microscope was developed to investigate the effect of viscoelasticity on lubrication forces, which are obviously very relevant in the final approaching stage of particles that are being used for alignment. It turned out here that the decrease in lubrication force due to shear thinning is even more pronounced when looking at the experimental results than predicted by numerical methods taking the flow index into account. This way, a new explanation for flow-induced alignment, bringing forward the critical role of shear thinning, has been formulated. It must be remembered, however, that other mechanisms can also occur in different shear rate regimes and that these other mechanisms may potentially be more useful when developing applications due to the possibility to permanently link the particles in order to fixate the structure.status: publishe