Role of Vapor Mass Transfer in Flow Coating of Colloidal Dispersions in the Evaporative Regime

Dynamic contact line lithography: Template-less complex Meso-patterning with polystyrene and poly(methyl methacrylate)

HYPOTHESIS

Micro/nanopatterning on a 2D surface is apt for cutting-edge miniaturization technology, which directly or indirectly requires high-end expensive lithographic tools. The evaporative deposition at the receding contact-line of a polymer solution, termed as Dynamic Contact Line Lithography (DCLL), can be a potential inexpensive technique for template-less meso-patterning if the deposition patterns from DCLL can be predicted a priori.

EXPERIMENTS

A deposition map (morphological phase diagram) from the myriads of patterns is constructed in terms of contact-line velocity and the polymer concentration. Specifically, two combinations: polystyrene (PS)/cyclohexane and poly (methyl methacrylate) (PMMA)/toluene are used to show the generic nature of the phase diagrams. The surface wettability of Si (water contact angle, CA ~15°) is tuned from CA ~35° to ~98° by patterning with DCLL.

FINDINGS

Directed by the phase diagrams, fabrication of a complex rectangular cross-pattern of PS and PMMA micro-threads with a periodicity of ~65 μm and ~50 μm respectively on a Si surface is demonstrated to establish the robustness and potential of the DCLL and predictive phase diagram.

Thin Coatings of Cerium Oxide Nanoparticles with Anti-Reflective Properties

Cerium oxide, in addition to its catalytic properties, is also known for its optical properties such as ultraviolet (UV) radiation filtering and a relatively high refractive index ( n > 2 ), which makes it an excellent candidate for multifunctional coatings. Here, we focus on the optical properties of thin deposits (≲2 μ m) of densely packed C e O 2 nanoparticles, which we assemble using two evaporation-based techniques: convective self-assembly (CSA, a type of very slow blade-coating) to fabricate large-scale coatings of controllable thickness—from tens of nanometres to a few micrometers—and microfluidic pervaporation which permits us to add some micro-structure to the coatings. Spectroscopic ellipsometry yields the refractive index of the resulting nano-porous coatings, which behave as lossy dielectrics in the UV-visible regime and loss-less dielectrics in the visible to infra-red (IR) regime; in this regime, the fairly high refractive index (≈1.8) permits us to evidence thickness-tunable anti-reflection on highly refractive substrates, such as silicon, and concomitant enhanced transmissions which we checked in the mid-IR region.

Thin wedge evaporation/condensation controlled by the vapor dynamics in the atmosphere

Evaporation or condensation in the vicinity of the immobile (pinned) contact line in an atmosphere of some inert (noncondensable) gas is considered here in a partial wetting configuration. Such a problem is relevant to many situations, in particular to a drop or a liquid film drying in open air. The thermal effects are not important and the mass exchange rate is controlled by the vapor dynamics in the gas. By following previous works, we account for the weak coupling between the diffusion in the gas and flow in the liquid through the Kelvin effect. Such a problem is nonlocal because of the diffusion in the gas. For generality, we consider a geometry of a liquid wedge posed on a flat and homogeneous substrate surrounded by a gas phase with a diffusion boundary layer of uniform thickness [Formula: see text]. Similarly to the moving contact line problem, the phase change leads to the hydrodynamic contact line singularity. The asymptotic analysis of this problem is carried out for the liquid wedge of the length [Formula: see text]. Three asymptotic regions are identified: the microscopic one (in which the singularity is relaxed, in the present case with the Kelvin effect) and two intermediate regions. The Kelvin effect alone turns to be sufficient to relax the singularity. The scaling laws for the interface slope and mass evaporation/condensation flux in each region are discussed. It is found that the difference of the apparent contact angle (i.e., interface slope in the second intermediate region) and the equilibrium contact angle is inversely proportional to the square root of [Formula: see text] and square root of the microscopic length, whatever is the singularity relaxation mechanism.