Contact Instability in Adhesion and Debonding of Thin Elastic Films

Based on experiments and 3-D simulations, we show that a soft elastic film during adhesion and debonding from a rigid flat surface undergoes morphological transitions to pillars, labyrinths and cavities, all of which have the same lateral pattern length scale, close to \lambda/H ~ 3 for thick films, H > 1 micrometer. The linear stability analysis and experiments show a new thin film regime where \lambda/H \approx 3+ 2 (\gamma/3 \mu H)^(1/4) (\gamma is surface tension, \mu is shear modulus) because of significant surface energy penalty (for example, \lambda/H = 6 for H = 200 nm; \mu = 1MPa).Comment: Accepted for publication in Phys. Rev. Let

Microparticle assembly and contact line dynamics

This thesis addresses three topics. First, microparticle assembly on solid surfaces from an evaporative suspension is studied. It is well known that microparticles collect near three phase contact lines owing to evaporative fluxes. In a dip coating configuration, if the evaporative flux and plate withdrawal velocity U are matched, large colloidal crystals form. Here, I investigate the consequences of varying the plate withdrawal rate, and find that periodic striped patterns emerge which depend strongly on U. The stripes form when three phase contact lines “jump”, or recede rapidly, upon detaching from well-wet particle aggregates on less wet substrates. Stripe width, spacing and height change abruptly at a transition velocity which can be related to a Landau-Levich transition in the flow. The second part of my thesis is a numerical simulation of drop spreading and retraction as a function of drop scale. The drop moves over a thin liquid film, and drop motion is initiated by an impulsive change in surface wettability. Owing to the presence of the film, these simulations require no closure condition at the 'apparent' contact line. Rather, relationships emerge between the contact line velocity and the dynamic contact angle. For nanoscopic drops, molecular effects dominate the drop motion. For drops an order of magnitude larger than the thin film, regimes emerge in which drops move according to Tanner's law, a relationship derived for macroscopic drops. Drop retraction is considerably more rapid than spreading owing to rapid dewetting events near the contact line. This thesis concludes with a discussion of a technique for creating multifunctional surfaces presenting discrete patches of several proteins. The technique relies on microcontact printing (µCP) to define active regions, and the use of a microfluidics device to deliver proteins to those regions. The surfaces are used to capture cells from a suspension, to sort cells from a mixed suspension, and to study the shear-dependent cell adhesion. Potential applications include biosensors, fundamental studies of cell adhesion, and of stem cell differentiation