Structure and Dynamics of Soft Matter at Interfaces: Instabilities of Complex Liquids and Cell Adhesion
Final Report Abstract
The interplay between structure and dynamics at interfaces plays an essential role for soft matter on very small scales. Within the framework of this research project, instabilities of complex liquids have been studied in detail with the goal to gain a fundamental understanding of the fluidics of complex liquids on the nano- and micro-scale. The relaxation of perturbations at the surface of thin polymer films, a process that is purely driven by capillary forces, has been studied in experiments, numerical calculations and analytical theories. Combining theory and experiment, the levelling of stepped films has been established as a powerful nano-fluidic tool to characterise the rheology of complex liquids on small scales. For any nonvolatile fluid prepared as a stepped film it is now straightforward to precisely measure its capillary velocity, i.e. the ratio of surface tension and viscosity. Moreover, interfacial phenomena such as hydrodynamic slip and the mobility of complex liquids in confined geometries have been elucidated and quantified for a variety of liquids: For very long polymer chains confined in ultra-thin films evidence is found for an enhanced mobility. In case of hydrophobic substrates, capillary levelling at the surface of a stepped liquid film senses the hydrodynamic boundary condition at the solid/liquid interface. A fundamental understanding of the fluid dynamics on small scales, where such interfacial effects may dominate, has been obtained which is tremendously important for technological applications that are based on down-sizing of devices: Microfluidic devices are nowadays designed and widely applied as labs-on-chips in order to realise high-throughput and large-scale integration of biomedical or pharmaceutical studies and chemical reactions. The relaxation of more complex surface geometries such as rectangular trench perturbations of thin films has been explored in a second step. In addition to the relevance of the time scales of pattern formation for technological applications such as nano-lithography and high-density data storage, the morphological evolution of the film surface turned out to be also interesting from an applied mathematician’s point of view: The symmetric global boundary conditions lead to an analytic selfsimilar solution that differs from the self-similarity found for stepped films. Experimental profiles for small perturbations are perfectly described by a linear Green’s function attractor. After a finite time, a perturbed film surfaces on its journey towards an equilibrium flat film can be described by an intermediate asymptotic solution. This opens the possibility to derive general scaling laws for parameters such as time and geometry. The approach opposite to the levelling of initially perturbed surfaces has also been considered: A liquid in a cylindrical geometry is subject to a classic fluid instability, the Rayleigh-Plateau instability that can also be observed for water flowing out of a kitchen faucet, in order to reduce its surface energy. The rise of undulations and the formation of droplets has been be studied for a thin liquid film covering a cylindrical fibre, now involving a solid/liquid interface in addition to the free liquid surface. From the comparison of experiments on hydrophilic glass fibres to hydrophobic fibres, it is found that the dynamics and the growth rates of the amplitudes of undulations is influenced by the hydrodynamic boundary condition. While the link between interfacial properties and the dynamics of complex liquids is evident, the transition from the glassy to the liquid state of thin polymer films is controversial. In order to substantiate the role of interfaces on the glass transition temperature, freestanding thin polymer films, i.e. films exhibiting two free interfaces, have been studied before and after transferring them to a substrate. While the glass transition temperature of freestanding films is found to be significantly reduced, its bulk value is recovered after transfer to a substrate, representing the case of just one free interface. This result highlights the effect of free interfaces on the glass transition. We have shown that interfaces have large implications on the the dynamics of polymeric liquids. The last building block of this research project considers the dynamics of biological matter at interfaces, a topic that holds a large potential for medical applications such as the design of implant materials. The sophisticated detection of displacements of a micropipette represents an innovative experimental technique to study living matter in a liquid environment. For lipid vesicles, representing a model systems for cells, the adhesion and friction properties are found to be substantially altered on ‘slippery’ hydrophobic substrates.
Publications
- Self-Similarity and Energy Dissipation in Stepped Polymer Films", Phys. Rev. Lett. 109, 128303 (2012)
J.D. McGraw, T. Salez, O. Bäumchen, E. Raphaël, and K. Dalnoki-Veress
- “Capillary-Driven Flow Induced by a Stepped Perturbation atop a Viscous Film", Phys. Fluids 24, 102111 (2012)
T. Salez, J.D. McGraw, O. Bäumchen, K. Dalnoki-Veress, and E. Raphaël
- “Reduced Glass Transition Temperatures in Thin Polymer Films: Surface Effect or Artifact?", Phys. Rev. Lett. 109, 055701 (2012)
O. Bäumchen, J.D. McGraw, J.A. Forrest, and K. Dalnoki-Veress
- “Relaxation and Intermediate Asymptotics of a Rectangular Trench in a Viscous Film", Phys. Rev. E 88, 035001 (2013)
O. Bäumchen, M. Benzaquen, T. Salez, J.D. McGraw, M. Backholm, P. Fowler, E. Raphaël, and K. Dalnoki-Veress
