Summary

Overview and Motivation

This Cluster of Excellence “Smart Interfaces: Understanding and Designing Fluid Boundaries” focuses on phase interfaces at which fluidic phases (gas and/or liquid) interact with a solid surface. “Smart Interfaces” refers to fluid-solid boundary interfaces that have been designed or built for achieving a specific purpose, such as enhancement or controllability of mass, momentum or heat transfer. The major research goals of this Cluster encompass the understanding, design, development and application of Smart Interfaces, while integrating rapid advances in a variety of disciplines in the physical and engineering sciences.

Such fluid-solid interfaces and the associated interfacial phenomena are ubiquitous in our daily lives and represent key technologies in many established and emerging fields. Their economic importance and the enormous potential for innovation in the development and application of Smart Interfaces at fluid boundaries build the foundation of this Cluster of Excellence. Five interrelated Research Areas will be established in which high potential for innovation and promise for technology transfer to industrial applications can be recognized:

• Static and Dynamic Wettability
• Heat Transfer Enhancement
• Near-Wall Reactive Flows
• Near-Wall Multiphase Flows
• Drag and Circulation Control

In all of these areas there exist realistic expectations for dramatically improving the performance of conventional interfaces through passive, active or reactive means of influencing and/or controlling fundamental properties of the surface. Depending on the particular features involved, such improvements can result in completely new applications, decreased energy consumption, extended lifetime, enhanced output, improved quality or ecological compatibility.

Research on the subject of Smart Interfaces demands a highly interdisciplinary approach, combining fluid mechanics with numerous other fields of research, including material science, thermodynamics, physical chemistry, plasma physics, mathematics, optimization theory, production and forming technology, adaptronics, etc. One major research and structural goal of the proposed Cluster is therefore to facilitate communication and exchange of knowledge among the key disciplines involved, not just at the TU Darmstadt, but on an international scale.

Scientific Discourses Addressed

The high expectations for significant advancement in this subject area are well founded. For example, the understanding of near-wall fluid mechanics has improved dramatically over the past years due to various technological advancements:

• Static and Dynamic Wettability
  Recent advances in understanding the influence of surface morphology on static wettability (superhydrophobicity) will be extended to dynamic wetting in which inertial forces may also be significant. Optimization of morphology for given wettability features and the fabrication of such structures will be addressed. The mathematical modeling and prediction of dynamic contact angle is of central interest and will accompany experimental and numerical investigations of contact angle phenomena of complex liquids (non-Newtonian). Another clear objective is to achieve spatially and/or temporally switchable wettability or capillary forces.
• Heat Transfer Enhancement
  An order of magnitude enhancement or control of heat transfer can be expected by cleverly combining surface structure, wettability and fluid-to-surface motion with wide applications in film evaporation and condensation, high performance heat pipes, spray cooling and spray deposition, high-speed cutting, micro-combustion with fuel pre-evaporation, significant increases in critical heat flux. A general long-term objective is the possibility of active heat flux switching by means of active and reversible surface manipulation.
• Near-Wall Reactive Flows
  Structuring and tailoring of surfaces (morphology, porosity, dispersity and chemical composition) are one means to control the governing features for reactions and to achieve process intensification or to optimise the reaction performance. Such improvements are especially beneficial in micro-reactors, where the surface-to-volume ratios increase greatly. Typical catalytic reactors to be considered are micro-combustors, fuel processors, falling film reactors and thin-film reactors with combined reaction evaporation or for photochemical purposes.
• Near-Wall Multiphase Flows
  The scientific objectives relate to one of three flow classes: dispersed flows, discontinuous flows and flows with phase change. Emphasis will be placed on novel strategies for influencing flow features: wetting, instabilities, flow patterns, solidification, etc. The model systems to be examined include drops and spray collisions with surfaces, flows in films and rivulets, near-wall phase changes and liquid/gas shear or pressure driven flows over structured surfaces. Flows of non-Newtonian liquids will also be considered. Among the long-term objectives of the research area are the atomization of tailored sprays, the possibility to actively control the stability, patternation and dryout of liquid films in real-time, or the development of controllable and switchable multiphase microfluidic systems.
• Drag and Circulation Control
  This Research Area focuses on the active or passive manipulation of the velocity field in the near-wall region to achieve benefits in drag and/or circulation. The topics chosen include manipulation through small scale wall morphology, adaptive structures, and active boundary-layer control. A special emphasis has been placed on improving system operation under unsteady flow conditions with applications targeted at off-design flow conditions in fluid machinery, energy extraction technologies, flapping wing and maneuver aerodynamics. Furthermore, flow control at the micro-scale is particularly interesting for all other Research Areas of the Cluster.