Wednesday, 27 June 2007 - 10:20 AM
Pencader 116
340

Spontaneous pattern formation by dip coating of colloidal suspensions on homogeneous surfaces

Moniraj Ghosh1, Fengqiu Fan2, and Kathleen J. Stebe1. (1) johns Hopkins University, Baltimore, MD, (2) Saoirse Corporation, Cambridge, MA

A partially wet, energetically homogeneous plate is withdrawn vertically at velocity U from a suspension of well-wet colloidal particles. Periodic horizontal striped assemblies form at the three phase contact line, with stripe width and spacing depending on the withdrawal velocity U relative to a transition velocity Ut. Thick stripes separated by large spaces form for U < Ut. For U > Ut, thin stripes separated by small spaces form. Stripe spacing is reduced by an order of magnitude and varies weakly with U until a maximum velocity is reached at which the stripes fail to form. A partially wet surface can entrain a meniscus. For U < Ut, the meniscus forms with a pinned contact line. As the plate moves upward, it stretches the meniscus until it becomes too heavy to be retained by the wet, porous network provided by the particles at the contact line. The contact line then jumps backwards to find a new equilibrium location, and the process begins anew. For U>Ut, a film of thickness h is entrained above the meniscus. For h smaller than the particle diameter D, particles aggregate where the entrained film thickens to match up to the wetting meniscus. When an entrained particle becomes exposed to air by evaporation, it becomes the new pinning site. h increases with U; at some velocity, h becomes comparable to D. Particles flow into the film and deposit there in a disordered manner. A diagram summarizing particle deposition is developed as a function of D, U and h.


Web Page: DOI: 10.1021/la062150e