Ashutosh Sharma

Hydrodynamics of Meniscus-Induced Thinning of the Tear Film

A concave liquid meniscus is always formed rather rapidly when the surface of a liquid film meets a solid surface that displays partial wetting, that is, the equilibrium contact angle is less than 90°.1 The same phenomenon is commonly witnessed in the climbing of liquids even against gravity in narrow capillaries and around wettable surfaces placed in a pool of liquid. Similar lacrimal menisci are observed around foreign surfaces (e.g., contact lenses) placed in the tear film, and also along the upper and lower eyelids.2 After eyelid opening, the border between a lacrimal meniscus and the tear film thins due to Laplace pressure or a “capillary-suction” engendered by the concave meniscus. Continued local thinning adjacent to the meniscus results in the appearance of a “black line” when a fluorescein-stained tear film is viewed under blue light.2

Breakup and Dewetting of the Corneal Mucus Layer

The normal precorneal tear film usually remains intact between consecutive blinks, but holes begin to appear and grow at random spots in about 10-60 sec when blinking is prevented. Although the exact mechanism of the tear film breakup has eluded our understanding, it is certain that the breakup is secondary to the nonwettability of the corneal surface. We had earlier proposed a mechanism based on the possibility of the rupture and dewetting of the precorneal mucus layer due to the long-range van der Waals forces.l-4 The tear breakup was thought to be triggered by the hydrophobicity, or nonwettability, of the underlying corneal epithelial surface devoid of its mucus covering. The last ten years have seen significant advances both in the understanding of thin film stability and in characterization of the physico-chemical properties of the cornea-tear film system. Direct microscopic observations have shown that even extremely viscous (viscosities as high as 10,000 times water viscosity) polymer films as thick as 0.2 !-liD indeed dewet their substrates by formation of holes within a few minutes. 5-I I The rapid breakup of relatively thick viscous films by the long-range van der Waals forces is in part due to a strong slippage (lack of adherence) of the entangled polymers and gels on their substrates.6•1214 While these observations support the aforementioned mechanism of the tear film breakeup, the following facts should also be considered in a critical reappraisal and update of the mechanism. (i) The normal corneal epithelial surface sans mucus is now known to be as hydrophilic and wettable by aqueous tears as mucus itself. 15--18 (ii) The precorneal mucus layer now appears to be much thicker than what was thought to be the case previously. 1920 (iii) In the original model 1-4 of the mucus layer breakup, the exact numerical value of the Hamaker constant for the epithelium-mucus-aqueous tears system was

Surface-Chemical Pathways of the Tear Film Breakup

Numerous experimental 1-{i and theoreticae-8 studies have conclusively demonstrated that relatively thick(< 100 J.1m) fluid films on nonwettable substrates dewet spontaneously by the nucleation and growth of initial defects or dry spots. As expected, the thinner films on the less wettable substrates are more unstable. Interestingly, the theory also shows that the tear film breakup can be initiated by even an extremely small, mildly nonwettable corneal site of dimensions comparable to the size of a single superficial squamous cell. 9 Over two decades ago, the seminal work10 11 of Holly and Lemp suggested that the conversion of a completely wettable corneal surface to a partially wettable surface during the interblink period may engender the precomeal tear film breakup. While this view is in accord with the modem understanding of the stability of thick films, there are several unanswered questions about the specific processes that actually trigger corneal surface nonwettability in normal and dry eyes. Based on measurements of the apolar surface properties obtained with the critical surface tension method of Zisman, it has been thought that the corneal epithelium is hydrophobic and nonwettable by the aqueous tears, but that a hydrophilic mucus coating renders it wettable. 1o-12 However, it has been argued that artifacts in the preparation of the corneal surface, use of apolar diagnostic liquids, and the methods of interpretation (e.g., Zismans method) may have been responsible for these conclusions. 1317 Certainly, applications of modem techniques of measurement and interpretation of surface properties have repeatedly shown normal epithelial cells with associated glycocalyx to be almost as hydrophilic and wettable by aqueous media as by corneal mucus. 14-17 The conclusion is not surprising, because the superficial corneal epithelial cells, like any number of hydrophilic

Bio-Inspired Adhesion and Adhesives: Controlling Adhesion by Micro-Nano Structuring of Soft Surfaces

Although the man made synthetic adhesives have quite high adhesion because of their viscoelasticity or irreversible chemical bonding, they are not reusable and are often prone to particulate contamination and cohesive failure. On the other hand, attachment pads found at the feet of different insects and climbing animals like geckos show high adhesion, self-cleaning and reusability. Decades of research have confirmed that the patterns and structures present at the surface of or buried inside the natural adhesive pads have rendered them these amazing qualities. These observation inspired scientists and researchers to mimic the structures to fabricate soft, synthetic reusable adhesives. This chapter will present a brief review on those efforts with a focus on structure and mechanism of patterned bio-adhesives and synthetic pressure sensitive adhesives.

Self-organized structures in soft confined thin films

We present a mini-review of our recent work on spontaneous, self-organized creation of mesostructures in soft materials like thin films of polymeric liquids and elastic solids. These very small scale, highly confined systems are inherently unstable and thus self-organize into ordered structures which can be exploited for MEMS, sensors, opto-electronic devices and a host of other nanotechnology applications. In particular, mesomechanics requires incorporation of intermolecular interactions and surface tension forces, which are usually inconsequential in classical macroscale mechanics. We point to some experiments and quasi-continuum simulations of self-organized structures in thin soft films which are germane not only to nanotechnology, but also to a spectrum of classical issues such as adhesion/debonding, wetting, coatings, tribology and membranes.