Preparation of Evaporation-Resistant Aqueous Microdroplet Arrays as a Model System for the Study of Molecular Order at the Liquid/Air Interface Evelyn Meyer, †,‡ Martin Mueller, † and Hans-Georg Braun* ,†,‡ Leibniz Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany, and Max Bergmann Center of Biomaterials, Budapester Strasse 27, D-01069 Dresden, Germany ABSTRACT Aqueous arrays of microdroplets typically sized between 2 and 10 μm were generated by microfluid contact printing and stabilized with respect to evaporation by incorporation of poly(ethylene oxide). The arrays are used as a model system for the study of structure formation at liquid/air or liquid/liquid interfaces. In particular, we demonstrated the self-organization of fatty acids with photopolymerizable diacetylene units (10,12-pentacosadiynoic acid) at the liquid/air interface of the microdroplets. Topochemical polymerization behavior of this compound and the autofluorescence property of the resulting polyconjugated polymer are appropriate features to prove the molecular order of the amphiphilic molecules at the interface. KEYWORDS: poly(ethylene oxide) • microdroplet arrays • liquid interfaces • polydiacetylenes • microemulsion • soft lithography INTRODUCTION S mall droplets are more and more used as microreac- tors for material syntheses (1, 2), for chemical syn- theses of biofunctional molecules (3, 4) or as microan- alytical devices (5, 6). The droplets may be generated in a microflow system as monodisperse droplets that are freely movable in a two-phase liquid/liquid system (water-in-oil or vice versa) (7). The droplet fluid/fluid interfaces are generally stabilized by surface-active compounds (8) and frequently functionalized with biofunctional molecular units by ap- propriate phospholipids, which self-assemble into mono- or bilayers (9). The interior space can be loaded with soluble reactants or even with single cells (10). For microanalytical investigations positional control of fluid phases becomes relevant and requires the immobilization of microdroplets in well-defined ordered arrays. The droplets should finally include biologically relevant molecular units as well as surface-active compounds. Droplet patterning can be done by various methods with specific advantages and disadvan- tages. Microdroplet patterning of aqueous droplets can be generated by water condensation from the vapor phase onto micropatterned surfaces with predefined areas of preferred wettability (11) or by surface-controlled dewetting on chemi- cally (12) or topographically (13) structured surfaces. Often these water droplets will not contain any additional com- pounds. Droplet arrays patterned by inkjet deposition (14) can contain additional material, but the droplets and their included compounds are distributed over a large contact area because of droplet impact. Another method that has been used to obtain ordered arrays of polymer solutions on a surface is microfluid contact printing (μFCP) (15). μFCP is part of the soft-lithographic patterning techniques (16, 17). Although μFCP is only used as a tool for microdroplet generation in this paper and was described in detail else- where (15), its basic features, because they are relevant for experimental studies described here, will be briefly outlined. A macroscopic droplet of a solution is deposited on the topographically structured surface of a poly(dimethylsilox- ane) (PDMS) stamp. Excessive volume is blown under a N 2 flow and a residual liquid film ruptures because of the stamp topography, and single self-centered microdroplets (Figure 1A) are formed on each protrusion (hexagonal motif; Figure 1D) of the stamp. These droplets are transferred by stamping on a homogeneous surface with low spreading properties (Figure 1B,C). The process of microdroplet formation is strongly dependent on the wettability and the evaporation rate of the fluid on the PDMS stamp. While chloroform solutions with sufficiently low contact angle (θ chloroform/PDMS = 33°) (18) coat the stamp and generate a thin film that decomposes in indi- vidual microdroplets during the evaporation process, aqueous solutions behave completely differently. Hydrophobicity of PDMS (θ water/PDMS = 115°) in combination with its surface topography creates an ultrahydrophobic stamp surface that is nonwettable for water (θ water/PDMS(structured) = 145°) (19). A macroscopic droplet on such a surface shrinks during evapo- ration without forming a thin film that could decompose into isolated microdroplets. Consequently, the generation and * Corresponding author. E-mail: braun@ipfdd.de. Received for review April 9, 2009 and accepted July 15, 2009 † Leibniz Institute of Polymer Research Dresden. ‡ Max Bergmann Center of Biomaterials. DOI: 10.1021/am900249w © 2009 American Chemical Society ARTICLE 1682 VOL. 1 • NO. 8 • 1682–1687 • 2009 www.acsami.org Published on Web 07/30/2009 Downloaded by SLUB DRESDEN on August 28, 2009 | http://pubs.acs.org Publication Date (Web): July 30, 2009 | doi: 10.1021/am900249w