Heterogeneous nucleation and microstructure formation in binary colloidal systems
Final Report Abstract
Many properties of solids depend on their microstructure which evolves during the solidification process of the corresponding melt. In case of particle mixtures, this process is additionally complicated by phase separating mechanisms which can largely influence the later stages of the structure of the solid. Although details may vary amongst different materials, the general scenario is not limited to atomic and molecular systems but also applies to much larger objects, such as colloids with sizes ranging between 10nm - 10µm. Accordingly, such systems have been established as convenient model systems for structural studies related to material science. This is because their motion can be monitored in real space and real time with optical techniques e.g. digital video microscopy. Therefore, the motivation of this project was to employ colloidal suspensions to obtain a deeper understanding of phase separation and structure formation of heterogeneous systems. An important ingredient to induce such phenomena is the presence of effective cohesive forces which can be induced in different ways. Contrary to previous studies which used non reversible effects by adding depletion agents or ions to the suspension the aim of this project was to explore the possibility whether cohesive forces could be created in a fully reversible manner because this would facilitate experimental studies in this field. In our project we have pursued two different approaches. In the first, such forces have been induced by rotating magnetic fields which in case of superparamagnetic particles indeed lead to an aggregation process. Although this approach indeed allows inducing nucleation of colloidal clusters starting from a colloidal liquid, this approach is rather sensitive to residual magnetic stray fields which are difficult to eliminate. As a result, the effective pair interaction is not uniform which then leads to a strong bias in the structures forming at later times. Therefore, we did not follow this approach further but instead used critical Casimir forces to induce cohesive forces in colloidal systems. Such forces arise in liquid solvents close to the critical point due to the spatial confinement of critical concentration fluctuations of the liquid. Attractive and repulsive critical Casimir interactions can be easily realized by employing binary colloidal suspensions of silica particles which are different in the adsorption preference for the two components forming the liquid mixture (i.e. water and lutidine). This has been achieved by the functionalization of the surface of one particle species with alkyl chains, changing their adsorption preference from hydrophilic to hydrophobic. When such a binary colloidal system is immersed in a water-lutidine mixture at critical composition and the temperature gradually increased to the critical point of the solvent, the colloidal system undergoes a spinodal demixing process. The results are in good agreement with numerical calculations by Prof. Andrew Archer (Loughborough University, UK) who confirmed who calculated the phase behavior of a binary colloidal suspension in the presence of critical Casimir forces. Our work demonstrates a novel route to induce phase separation in colloidal systems by application of critical Casimir forces. Critical Casimir forces currently receive considerable attention to tune interactions in colloidal systems since they are distinguished by an exquisite temperature dependence. In addition, they are extremely susceptible to small changes in the surface properties which results in either attractive or repulsive pair interactions. In this project we have demonstrated that critical Casimir forces can be used to continuously and fully reversibly change the phase behavior in colloidal suspensions. Our results suggest that a critical point in a solvent can also lead to a critical point in phase diagram of colloidal particles suspended in such a mixture, i.e. that critical phenomena at different length scales can be closely related to each other. To our knowledge, such connection between critical points in different subsystems has not been reported before and we expect that this will motivate further theoretical studies in this field.
Publications
- Tunability of critical Casimir interactions by boundary conditions, EPL 88, 26001 (2009)
U. Nellen, L. Helden, and C. Bechinger
