Patterned Colloidal Coating Using Adhesive Emulsions
Started by Lauren Hartle, Fall 2011.
Prof. David Weitz and Laurence Ramos demonstrate the ability to pattern a hydrophilic surface with hydrophobic features, allowing them to selectively coat a surface with colloids. The advantages of colloidal coatings are numerous: colloidal particles of precisely controlled size can be produced using many materials. These patterned surfaces offer an alternative to field-induced self-assembly of 2-D and 3-D structures and an opportunity to study the resulting growth processes.
Emulsions exhibit special adhesive properties that are useful in patterned surfaces. For example, surfactant molecules will coat a hydrophobic substrate, producing a monolayer of surfactant molecules that resembles the surface of the emulsion particles. Under the right experimental conditions (for example, salt concentration and temperature) the monolayer coating the hydrophobic surface will attract the emulsion particles. This paper demonstrates the phenomenon with micron scale hydrophobic patches on a hydrophilic surface. The emulsion is liquid-crystal droplets in water.
Substrate: A periodic lattice (100 microns square) of photoresist squares and blank "holes" is produced on a glass coverslip via standard masking and developing methods. The exposed glass is made hydrophobic by evaporating a silane solution off the surface. When the photoresist is removed, a regular lattice of alternating hydrophobic and hydrophilic squares results. These steps are shown in Figure 1.
Emulsion: The liquid crystal emulsions consist of a few volume percent cyanobiphenyls in an aqueous solution of sodium dodecyl sulfate and sodium chloride. Sodium chloride concentration is manipulated to tune the attraction of the surfaces in the emulsion. This mixture is filtered to set the average particle size at 1um. During observation, the mixture is placed atop the patterned substrate, and sealed with an o ring and a coverslip. A typical optical image is shown in Figure 2.
Liquid crystal drops coalesced into larger, homogeneously-sized drops and adhered to the hydrophobic regions of 2-D hydrophobic/hydrophillic lattice (Figure 3 D/E/F). Patterning prevented the coalescence of polydisperse drops, as observed with purely hydrophobic or hydrophilic surfaces (Figure 3 B/C), and bulk mixtures (Figure 3 A).
The strengths of the technique are its relative indifference to the coating material contained within the emulsion, and precise pattern control it permits. The technique is promising for materials and phases that cannot be easily patterned at such small length scales. One could take advantage of the optical effects produced by the materials of interest, and the specific geometry of the substrate pattern. Furthermore, such substrates could yield a high-throughput test sample for studying crystallization, aggregation, and any number of processes under different surface conditions. (more complex substrate processing would be necessary to expand the library of surface conditions available on a single sample). The tradeoff for the technique's versatility is its sensitivity to temperature and salt concentration. For the method to be feasible, it is essential that 1) The desired material and phase exist within the range of parameters necessary for substrate-droplet adhesion, and 2) these parameters be reasonably and consistently achievable.
Soft Matter Connection
This paper is a great example of the power of surface interactions--by tuning surface interactions, researchers are able to use standard patterning methods to pattern, grow, and study materials that were previously unfeasible.