The self-assembly and patterning of thin polymer films on pyroelectric substrates driven by electrohydrodynamic instability† Xiaolei Xi,‡ Dan Zhao,‡ Fei Tong and Tingbing Cao * Received 27th July 2011, Accepted 27th October 2011 DOI: 10.1039/c1sm06434g We use hot PDMS to parallelly transfer and print heat energy onto lithium niobate substrates to induce a local pyroelectric effect. The heterogeneous electrostatic charges built from hot mCP can guide the self-assembly of different thin polymer films and develop them into micropattern. Surface instabilities in thin ( < 100 nm) polymer films have been the subject of increasingly innovative research aimed at better under- standing the physics of their formation and their utility in patterning, organizing, and measuring materials properties on the micro and nanoscale. 1 Subjecting thin polymer films to vertical temperature or electric field gradients produces modulations in the surface tension, buoyancy or charge density. 2,3 Above the threshold of instability, the system generates a great wealth of periodic and hierarchical structures and can be vitrified by reducing the temperature below the glass transition (T g ) state of the corresponding polymers. 4 In a typical instability-driven approach in an electric field, as observed by Russell et al. in 2000, 5 a thin polymer film with an air-gap is confined between two electrodes at a temperature above the T g of the polymer. Upon annealing with a voltage bias between the two electrodes, the origi- nally flat polymer–air interface is destabilized and creates lateral and topographic features on the thin film. Those structures can be frozen by cooling the system back to room temperature quickly. 6 Electrohydrodynamic instability 7 is used to explain the perturba- tion of the initially flat polymer film: the voltage bias between the electrodes induces an electrostatic stress at the interface between the polymer and air due to a mismatch in their dielectric constants, and the electrostatic stress competes with the capillary pressure generated by the curvature of the film form an unstable equilibrium which generates the polymer structures and their striking periodicity. 8 Compared with the instability in polymer films under an external electric field, experimental observations reported by Chou et al. in 1999 9 have puzzled people for many years. A thin layer of a polymer film is confined between the mask and substrate without any external electrical supply and upon annealing, the film still breaks into peri- odic patterns from initially flat surfaces. An internal electric field due to trapped charges has been postulated as the possible driving force for film destabilization, but the hypothesis lacks direct experimental evidence for the existence of those electrostatic charges, and for the location of the charges, whether they are at the polymer–air interface or on the top substrate. 10 In our recent paper, we have intentionally introduced electrostatic charges into fine patterns on a thermally grown silica layer (which acts as an electret, 11 a material which can permanently store elec- trostatic charges) on top of silicon wafer. 12 After spin-coating a uniform layer of polymer onto the topographically flat substrate and annealing it thermally, we found that the thin polymer film can be destabilized and self-organized into micro- and nanostructures that replicate the pattern of charges on the substrate. Theoretical and numerical modeling based on the electrohydrodynamic instability shows excellent agreement with our experimental observations, which provides direct evidence that the internal electric field caused by charges on the substrate is strong enough to destabilize thin poly- meric films and general periodic complex patterns. 13 Our finding introduces a clear source of the electrostatic force and a direct answer for the decade-long puzzle; 10 moreover it demonstrates a new strategy for the bottom-up fabrication of structured functional materials: the polymer film is no longer confined between two electrodes but self- organized on a ‘‘single-electrode’’ setup, which open a way for in situ characterization, chemical modification of the polymer surface or discretionary design of functional substrates for charge embedding. Herein, we chose a ferro- and pyroelectric crystal, Lithium Niobate (LiNbO 3 ), 14,15 as an electret material to generate parallel charge patterns, and thus induced the self-assembly and patterning of the thin polymer film through an electrohydrodynamic process. Compared with general electrets such as silicon dioxide or a poly (methylmethacrylate) (PMMA) film, 16,17 LiNbO 3 shows a high field emission of electrons with modest temperature variations which indicates that the substrate is stimuli responsive to charge embedding or patterning. 18 Previously reported periodically poled lithium niobate (PPLN), was achieved by the fabrication of metallic micro- electrodes onto a LN crystal using photolithography, followed by the selective poling of the crystal domain with a strong electric field, 19 which has been used in the patterning of a micro-lens array 20 and the decoration of nanoparticles directed by their dipolar charges. 21 Recently, Ferraro and Grilli et al. have developed a pyro- electrohydrodynamic shooting approach using an infrared laser or a hot tip from a soldering iron as the thermal stimulus to induce local pyroelectric forces in a LN crystal, which can draw and then dispense nano–pico liquid droplets from the reservoir below the substrate. 22 Department of Chemistry, Renmin University of China, Beijing, 100872, China. E-mail: tcao@chem.ruc.edu.cn † Electronic supplementary information (ESI) available. See DOI: 10.1039/c1sm06434g ‡ These two authors contribute equally to this paper. 298 | Soft Matter , 2012, 8, 298–302 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2012, 8, 298 www.rsc.org/softmatter COMMUNICATION Downloaded by University of California - Riverside on 27/03/2013 16:52:32. Published on 11 November 2011 on http://pubs.rsc.org | doi:10.1039/C1SM06434G View Article Online / Journal Homepage / Table of Contents for this issue