Formation of Thin Films from Colloidal Dispersions
The process of drying colloidal dispersions, i.e. evaporating the liquid, to create particulate solids or continuous polymer films is common to a range of important technologies, e.g. forming polymer films from latex dispersions, casting magnetic tapes, depositing highly porous coatings on ink jet papers, forming sol-gel glass, adding anti-reflection coatings to eyeglass lenses, encapsulating vitamins in beads, fabricating photonic crystals from silica sols, spray depositing thin film oxide fuel cells, and manufacturing photographic film. The objective varies but is generally to create a layer of specified thickness and controlled porosity with permeability, strength, optical, and other physical properties appropriate for the application. The particles often are polymer latices or inorganic oxides, e.g. silica, alumina, or zirconia, and the fluid is normally water, but occasionally a low molecular weight organic. Processing of such films raises a number of interesting and difficult issues because of the conflicting constraints for successful film formation and performance properties as implied by the applications.
Our recent work offers a model based on rigorous mechanics for consolidation and cracking of immobilized, i.e. close packed, colloidal spheres that deform viscoelastically due to contact or interfacial forces. Predictions from this model identify, in terms of appropriate dimensionless groups, the conditions under which air-water, polymer-water, or polymer-air interfacial energies suffice to form void-free films as thin films of aqueous polymer latices dry on a rigid substrate. Extension of the model to the opening of cracks yields a minimum capillary pressure for the onset of cracking, when deformation is insufficient or too slow to keep up with evaporation. This prediction derives from equating the elastic energy recovered around an isolated crack with the additional surface energy expended. Equating this critical capillary pressure, which depends on the layer thickness, with an estimate of the maximum capillary pressure sustainable by an air-water interface at the surface of the packing determines a thickness below which cracking should not occur. These predictions are well supported by experimental data for both polymer latices and inorganic oxide colloids.
Several puzzles remain though, as for cracking of linearly elastic solid films. First, many packings do not form cracks until the capillary pressure exceeds significantly the minimum predicted by the energy argument, suggesting that flaws are needed to nucleate cracks. This conjecture is supported indirectly by the slower than predicted development of additional cracks as the pressure is increased further. Second, cracks that follow a moving front across a drying film develop a characteristic spacing and often advance in a stick-slip fashion. This suggests an additional dynamical process, which some attribute to the flow of water driven through the particle packing by gradients in the capillary pressure. Thus we are moving toward a unified understanding of the process to guide future developments.
“Horizontal drying fronts during solvent evaporation from latex films”, AIChE Journal, 44 2088-98 (1998)[with A.F. Routh].
“A process model for latex film formation: limiting regimes for individual driving forces”, Langmuir 15 7762-7773 (1999) [with A.F. Routh].
“Deformation mechanisms during latex film formation: experimental evidence”, Industrial and Engineering Chemistry: Research 40 4302-4308 (2001) [with A.F. Routh].
“Role of capillary stresses in latex film formation”, Langmuir 20 2947-2961 (2004) [with M. Tirumkudulu].
“Cracking in drying latex films”, Langmuir 21 4938-48 (2005) [with M. Tirumkudulu].
Delayed Collapse of Colloidal Gels
Colloidal dispersions with weakly attractive interparticle forces sometimes form gels instead of separating into two equilibrium phases, as expected for molecules. This non-equilibrium and non-ergodic state is of interest in general but also has the propensity in some cases to suddenly collapse when left standing under normal gravity. Others have recognized that the mechanics are controlled by thermally activated creep, which produces a time scale for the collapse that increases roughly exponentially with the strength of attraction. The driving force is presumed to be the gravitational load on the particles that comprise the network. While both are clearly essential, this provides no mechanism for the abrupt collapse, as opposed to a very slow one-dimensional consolidation.
The missing element is the thermodynamic driving force, due to the strong interparticle attractions, that tends to densify the gel but is frustrated by the non-ergodicity produced by those same strong interparticle attractions. Thus knowledge of thermodynamic state of the dispersion relative to an equilibrium phase diagram defining the equilibrium fluid-solid separation and the metastable binodal with the underlying spinodal is essential. Our objective locate collapsing gels in the full phase diagram and construct a mechanistim model relating the collapse to thermodynamic and gravitational driving forces to the instability that drives the collapse.
J.Y. Huh, M.L. Lynch, E.M. Furst, “Microscopic structure and collapse of depletion-induced gels in vesicle-polymer mixtures”, Physical Review E 76(5) 051409 (2007).
V. Gopalakrishnan, K.S. Schweizer, C.F. Zukoski, “Linking single particle rearrangements to delayed collapse times in transient depletion gels” Journal of Physics-Condensed Matter 18(50) 11531-11550 (2006)
R. Buscall, T.H. Choudhury, M.A. Faers, J.W. Goodwin, P.A. Luckham, S.J. Partridge, “Towards rationalizing collapse times for the delayed sedimentation of weakly-aggregated colloidal gels”, Soft Matter 5(7) 1345-1349 (2009).
Electrohydrodynamic Patterning of Thin Polymer Films [with S.Y. Chou]
The Lithographically Induced Self Assembly (LISA) process positions a silicon wafer parallel to and within 100 nm of a second wafer coated with a thin film of polymer. Heating the film above the glass transition temperature of the polymer then allows electric fields to generate flow in the layer. If the upper surface is patterned discontinuously, the film is unconditionally unstable at edges and corners. With a planar mask only disturbances with sufficiently long wavelengths grow. Patterns emerge consisting of ordered arrays of pillars or concentric rings with form and periodicity reflecting the geometry of the template and the balance between surface tension and electrical forces.
This remarkable evolution can now be described through a model incorporating interfacial tension, electric fields affected by the conductivity of the polymer and redistribution of charges on the interfaces, and viscous flow within the film. A linear stability analysis predicts wavelengths in quantitative agreement with those observed and identifies strategies for scaling the process down to the nanometer scale, whereas weakly nonlinear theory and fully nonlinear simulations select hexagonal arrays of pillars under planar masks and elucidate the ability patterns on the mask to guide pillars into long range order.
Current objectives are to scale the patterns down to wavelengths ≤ 100 nm by introducing a conducting phase in the gap and to visualize the evolution of the pillars to learn how to control the growth process.
“Electrostatically induced submicron patterning of thin perfect and leaky dielectric films: a generalized linear stability analysis”, Journal of Chemical Physics 118 3790-3803 (2003) [with L.F. Pease III].
“Limitations on length scales for electrostatically induced submicron pillars and holes”, Langmuir 20 795-804 (2004) [with L.F. Pease III].
“Cylindrically symmetric electrohydrodynamic patterning”, Physical Review E 70 041601 (2004) [with P. Deshpande, L. Chen, S.Y. Chou, and L.F. Pease III].
“Dynamics of the formation of polymeric microstructures induced by electrohydrodynamic instability”, Applied Physics Letters 86 241912 (2005) [with N. Wu].
“Electric-field induced thin polymer film patterns: weakly nonlinear and fully nonlinear evolution” Langmuir 21 12290-12302 (2005) [with N. Wu and L.F. Pease III].
“Electrohydrodynamic instability of dielectric bilayers: kinetics and thermodynamics” Industrial and Engineering Chemistry Research 45 5455-65 (2006) [with N. Wu].
“Charge driven, electrohydrodynamic patterning of thin films”, Journal of Chemical Physics 125 184716 (2006) [with L.F. Please III].
“Toward large-scale alignment of electrohydrodynamic patterning of thin polymer films”, Advanced Functional Materials 16 1992-1999 (2006) [with N. Wu and L.F. Pease III].