ABOUT BIOMIMETIC SURFACES
We study how surface roughness affects adhesion with water, organic liquids, bacteria, ice, concrete and other materials. Among all forces of nature, surface forces are worst understood by students. Imagine, for example, a water droplet suspended on a ceiling by the surface forces. Where the force is applied, is it at the 1D perimeter of the droplet or at the 2D contact area? How can a force be applied to a 1D three-phase line, rather than to a material body? Many students have difficulties answering these questions.
One prominent manifestation of the surface forces is the Lotus effect. The ancient Hindu poem Bhagavad Gita says about the seeker of truth “Having abandoned attachment, he acts untainted by evil, just as a lotus leaf is not wetted.” Lotus leaf emerges clean from dirty water due to special hierarchical microstructure of its surface, and scientists mimic superhydrophobic and self-cleaning properties of the lotus effect. Furthermore, surfaces that repel various substances, from oil to bacteria, to ice are being developed by scientists.
Besides wetting, hydrophobicity is crucial for many important effects, such as the “hydrophobic effect” and hydrophobic interactions. For two hydrophobic molecules (e.g., hydrocarbons) placed in water, there is an effective repulsive hydrophobic force due to their interaction with the water medium. The hydrophobic effect, entropic in its nature, is responsible for folding of proteins and other macro-molecules and has wide application in many physical, biological, and chemical processes. We are investigating the similarities between the superhydrophobicity, icephobicity and adhesion on various levels, from the thermodynamic entropic nature of these interaction, to the parallelism between snowflake formation, protein folding and frictional stick-slip, to engineering applications such as superhydrophobic nanoengineered concretes and self-cleaning materials for water industry.
There are many interesting effects in how biological materials and tissues are organized (e.g., their multiscale hierarchical organization) which are now well understood by biologists and the knowledge should be transferred into engineering. In the past, we suggested that thermodynamic methods of analysis of self-organized patterns and structures can be applied to the materials with embedded self-healing mechanisms. Such methods will allow to relate the structure (including the micro- and nanostructure) of such materials and composites to their self-healing properties. The structure-property relationships help designing the optimized structure and serve as a guidance for synthesizing such materials, for which currently the trial-and-error approach is usually employed.
HOW BOUNCING DROPLETS AND INDIAN ROPE RELATED TO NOVEL MATERIALS
Vibrations are temporal periodic patterns, while surface microtopography often introduces spatial patterns. Both types of patterns can affect properties of materials. Small vibrations are equivalent to an effective force, which can stabilize, for example, an inverted pendulum or bouncing droplets.
We call such stabilizing effective force caused by small vibrations a “levitation force” as a trubute to Isaac Newton, who studied secretly levitation in his lab at Trinity College while officially working on the laws of gravitation. Effectively, small vibrations “freeze” the droplets. Vibrations also can make granular material flow like a liquid or effectively change phase properties in other ways (liquid to solid, soft to hard, granular to liquid etc.). Similarly, a spatial micro/nanopattern can affect liquid properties (“texture-induced phase behavior”), such as wetting, formation of vapor films, and freezing (icephobicity). The idea of controlling phase properties of materials by applying spatial or temporal micropatterns is very promising and may have a wide range of applications. However, these effects are also of fundamental importance for science, since they involve interesting mathematical techniques (the parametric resonance and Mathieu equations, separation of motion and small parameter methods), related to fundamental physics. The phenomena can also be used as an effective demonstration for students.
Below, a PhD student Rahul Ramachandran demonstrates the famous “Indian Rope Trick”, which was solved by mechanicians mathematically only several years ago. In the classical version of the trick, an Indian fakir or magician demonstrate a soft rope, which suddenly becomes hard, rising up and levitating. A monkey would climb up the rope. Rahul demonstrates a small-scale version of the trick, where small vibrations are equivalent to a stabilizing (“levitation”) force: