Ultrathin Films: Patterns, Instabilities and Wetting
We are investigating the stability, dynamics, dewetting and morphology of ultrathin (< 100 nm) films and nano-particles in order to understand the surface property--structure relationships in areas as diverse as coatings, adhesives, wetting, adsorption and heterogeneous nucleation. We address these problems by a combination of theoretical, numerical and experimental techniques. In particular, we currently have a strong Computational Fluid Dynamics effort underway to elucidate completely the problems of 3 - D nonlinear dynamics and pattern formation in thin films.
In particular, the following questions are being investigated currently in my group.
What are the general classes of the short and long range interactions responsible for the surface instability and defect-formation in a variety of thin films, e.g., aqueous films on hydrophilic and hydrophobic substrates, polymer films with chain-adsorption/grafting?
We have found that the aqueous films almost always rupture due the long range hydrophobic interaction, and the van der Waals force in the most instances has a stabilizing role. In collaboration with Dr. Gunter Reiter at CNRS, Mulhouse, we have discovered another enigmatic long range force in polydimethyl siloxane (PDMS) polymer films sandwiched between water and silicon substrates. This force mimics the decay behavior of nonretarded van der Waals force, but is stronger by about three orders of magnitude compared to the conventional estimates of the Hamaker constant. It is strong enough to rupture some 100 nm thick highly viscous PDMS films within a few minutes, whereas the accepted theories of thin film stability would show the time scale for the destruction of such films to be of the order of months or even years! Search is on to probe the origin and the meaning of this force, as well as to refine our theoretical understanding of thin film breakup.
What factors cause the rupture and dewetting of ultrathin films?
What are the correlations between the interfacial properties (interactions) and the film-morphology?
There is a long standing debate about the mechanism of instability and dewetting in thin films of various systems. Films devoid of preexisting defects on homogeneous surfaces rupture by the so called spinodal mechanism, for which we have for the first time discovered the complete 3 - D nonlinear pathways of morphological evolution. Relatively thin films to the left of the minimum in the spinodal parameter evolve by the formation of an undulating bicontinuous structure composed of long hills and valleys, which fragment rapidly into droplets before the onset of dewetting. Relatively thick films on the other hand, breakup by the formation of isolated circular holes, which expand and coalesce to form either a giant polygonal network or long liquid threads, which fragment or retract to eventually resolve into droplets. The pattern selection may be viewed as a "morphological phase inversion", i.e., liquid drop-in-air ® hole-in-liquid, as the film thickness, which is an analog of concentration, is increased. These simulations have also clarified that the formation of circular holes is not necessarily indicative of "nucleation" by preexisting defects and heterogeneity of the substrate.
The macro-scale patterns which form spontaneously in thin films are a facile probe of the underlying nano-scale interfacial interactions. The theory has the potential to develop into a novel "Thin Film Force Microscopy" technique for the measurement of intermolecular forces based on matching of observed patterns with simulations.
Another pathway of dewetting by hole formation due to "nucleation" has also become apparent in a variety of experimental thin films, e.g., polystyrene and liquid crystal on silicon wafers, evaporating aqueous films. We are currently developing a theoretical understanding of this mechanism with the aid of simulations for chemically and physically heterogeneous substrates. A surprising finding has been that even small wettability contrasts arising from chemical heterogeneities engender a gradient of chemical potential strong enough to rupture relatively thick films within a few seconds. We are exploring the unique fingerprints of the patterns arising from the chemical heterogeneity, which will aid in differentiating the "spinodal dewetting" from "nucleation".
What are the important factors and phenomena not accounted for by the classical theories of thin films?
Importance of several non-classical effects has become apparent from our recent ongoing experimental studies, as well as of several other groups, on the instability, dewetting and pattern formation in thin polymer films on a variety of tailored surfaces. The most prominent of these phenomena include strong slippage at the solid-polymer interfaces, surface heterogeneity, evaporation/condensation, visco-elasticity and pre-existing stresses in solid like films, pattern formation in completely wetting films, anisotropic patterns, entropic effects at solid-polymer interfaces and anomalous long range forces in aqueous systems. Efforts are currently underway in my group, in collaboration with Dr. Gunter Reiter, to model and quantify these phenomena by using a variety of theoretical and experimental approaches.
Biosurfaces: Surface Interactions in the Cornea and Tear Film
Dry eye sufferers contribute over a billion dollars to the economy worldwide. While traditionally this problem has been addressed by the clinicians and by the biomedical researchers of various hues working with animal models, I initiated a different approach to our understanding of the tear film based on Interfacial Science.
An aqueous tear film about 10 m m thick covers the cornea in normal eyes, but a rapid tear film breakup within the interblink period is frequently witnessed in the so called "dry eye" syndromes, despite no apparent lack of aqueous tears. We are investigating the causes of the tear film breakup, and of corneal dehydration and damage, based on the surface properties of the cornea and tear film in health and disease. Based on the measurements of the apolar (Lifshitz-van der Waals) and polar (acid-base) surface properties of the corneal cells, their extracellular proteins and associated mucus coating in normal and pathological conditions, we have, for the first time, presented a coherent view of the physico-chemical mechanisms responsible for the corneal wetting, hydration, tear film breakup, genesis of dry eye disorders, and defense against the invading hydrophilic and hydrophobic bacteria. I, as well as Dr. John Tiffany at Oxford, independently found that the normal corneal cells with their glococalix are as hydrophilic and wettable by tears as the mucus coating of the cornea derived from the conjunctival goblet cells. This discovery successfully challenged a long standing myth that the mucus coating of the cornea is directly responsible for wetting of the cornea by the tear film. However, we found an indirect significant role for the corneal mucus in the maintenance of the corneal wettability. The polar (acid-base) surface properties of the mucus make the adsorption, absorption and removal of hydrophobic nonwettable contaminants, bacteria and cellular debris possible. It was also shown that in the absence or discontinuity of mucus coating of the cornea, nonwettable contaminants should readily adhere to the corneal epithelium, which may trigger the tear film breakup.
We are currently strengthening the above themes and further exploring the subtle surface pathways and their interactions in order to arrive at a holistic understanding of the physiology of the tear film.
Surface Interactions in Membranes
Our focus in this area is on predictions of flux, rejection, compatibility and fouling based on the long and short range surface interactions between the solute and membrane pores. Our approach in this area combines both the pore level and membrane level modeling strategies, statistical mechanics, interface science and macroscopic membrane level experiments. Some of this work has been in collaboration with Dr. Prashant K. Bhattacharya of our department.
One of the fundamental problems in this area has been that the non-van der Waals interactions of macromolecules (e.g., proteins) and colloidal particles with the pore - surface could not be characterized either theoretically or experimentally. This information is crucial for the pore - level modeling of membrane separation processes like the ultrafiltration. We developed a new "Surface Element Integration (SEI)" technique, which is a generalized Deryaguin type approximation, to evaluate the apolar, polar and the electrostatic double layer interactions for particles interacting within highly curved or enclosed geometries, e.g., a particle in a pore - geometry. Like the celebrated Deryaguin approximation, this technique requires only that the corresponding interactions in a flat plate geometry be known. The flat plate energies of interaction could be obtained based on facile measurements of macro - scale surface properties by the contact angle goniometry. Incorporation of solute - solute and solute - membrane pore interactions in the transport models led to a unified prediction of rejection, flux, hindered transport, diffusivity and osmotic pressure in the ultrafiltration of proteins line the BSA.