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Fundamental physics of thin films

THIN LIQUID films are ubiquitous in our daily lives in a variety of settings, ranging from agricultural sprays adhering to leaves, water on a windshield, coatings and paints, to the film of tears which protects the cornea of the eye.

Thin films are critical in a wide spectrum of technological and scientific applications such as food, cosmetics, petrochemical and pharmaceutical foams and emulsions, optical and microelectronic coatings, antiglare and antistatic coatings for TVs and computer monitors and stealth coatings in warfare.

In most thin film applications, the ability to form or deposit and maintain a stable smooth film of uniform thickness is critical.

The problem is that thin liquid films are usually inherently unstable and spontaneously develop defects, such as craters and holes.

These defects separate the film from the substrate, a process known as dewetting.

The consequences can range from mild to devastating as in the development of haze and distortion in optical coatings, short circuits in microelectronics, reduced shelf life of food, and potentially vision threatening and difficult-to-manage dry eye syndromes.

For over a decade, my research group in the Department of Chemical Engineering at Indian Institute of Technology, Kanpur, has been probing the fundamental physics of thin films.

Our aim has been to examine simple everyday phenomena and thereby to develop insights into the behaviour of materials on very small scales.

For example, how do liquid droplets and thin films spontaneously de-wet a non-wettable surface? What patterns are spontaneously formed during de-wetting and can this pattern formation be controlled to create well-ordered structures?

What can be learnt about the otherwise experimentally elusive intermolecular forces from the thin film patterns? What happens to perfectly smooth surfaces of soft materials when they come into contact with each other or are pulled apart?

Such questions are relevant not just from the scientific perspective, but have practical applications too. The exciting new field of polymer film based opto-electronics, for example, requires the ability to manipulate and pattern thin polymer films. If polymers can be induced to form nanoscale patterns (about 1,000 times smaller than the thickness of a human hair) through a process of self-assembly or self-organization, it would reduce the cost of making polymer-based electronics.

The current multistep process used in manufacture of silicon-based devices is both complex and expensive.

In addition, the miniaturisation of components to nanoscales has advantages of speed too, allowing faster computer processors to be made. Polymer based opto-electronics are also widely expected to provide the flexible, low power, light-weight displays of the future.

My then doctoral student, Dr. Rajesh Khanna (now at IIT Delhi), and I were able to show that thin films spontaneously breakdown into holes and droplets due to the action of weak forces which act between molecules of the thin films.

Although these intermolecular forces are very weak, we found that they gather enough collective muscle when a large number of molecules are involved.

It also turns out that the number of droplets or holes per square millimetre is a unique signature of the intermolecular forces responsible for the film's destruction.

These intermolecular forces cannot be directly measured and so, in essence, de-wetting patterns becomes a "thin-film force microscope" to peer into the highly enigmatic realm of the intermolecular forces!

In collaboration with Dr. Guenter Reiter of Institut de Chimie des Surfaces et Interfaces in France, we carried out further experiments on the de-wetting of polymer films that directly confirmed all the theoretical expectations; the key feature being a statistical distribution of holes that correlates with intermolecular forces.

Conventional wisdom had held that localised defects, such as very tiny dirt particles and cavities, were responsible for the break-up of the film. But if our view of intermolecular forces is correct, then usual remedies, such as cleaning, polishing the substrates, and the preparation of film in clean rooms, will not stop thin film from self-destructing!

Research carried out along with Dr. Kajari Kargupta (now at Jadavpur University in West Bengal) and Dr. Rahul Konnur (now with Tata Research Design and Development Centre, Pune) has suggested ways to control the apparently random de-wetting patterns.

Using chemical pattern etched onto substrates, called templates, we have shown that the de-wetting can be controlled and manipulated.

Instead of being a random event, controlled de-wetting transforms thin films into well-ordered structures. Understanding the precise conditions under which ordered, complex structures can self-organize on the sub-micron to nanometre scales (one-thousandth to one-millionth of a millimetre) would be of immense use in creating polymer based electronic devices, electromechanical systems and sensors of various hues. Recently, my fellow collaborators (Dr. Vijay Shenoy at the Indian Institute of Science, Bangalore, Dr. Manoj Chaudhury at Lehigh University in the United States and Dr. Animansu Ghatak at Harvard University) and I have discovered that surfaces of soft solids films, such as rubber, become spontaneously rough when brought in contact with another solid surface.

The microscopic well-ordered cavities thus formed at the interface manifest as long bridges when the two surfaces are pulled apart.

The same surface instability is responsible for sticky threads that are seen when lady's finger and jackfruit are cut and separated as well as when adhesive is peeled. My group, composed of four doctoral and five M.Tech. students, is currently focusing on tailoring thin coatings using electric fields and other innovative means, nano-scale patterning of soft materials including polymers for opto-electronic applications, mechanics of failure of soft adhesives, and on detergency or the science of cleaning of surfaces.

Ashutosh Sharma

Professor & Head of
Chemical Engineering
IIT, Kanpur

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