The proposed research will generate a fundamental description of the physics involved in the deposition of a monolayer of particles using convective self-assembly. Drawing an evaporating meniscus across a substrate, a process related to the "coffee ring effect" and the Langmuir-Blodgett technique, forms a structure of particles ranging from random and ordered sub-monolayer to well-ordered multilayers. Although many recently developed processes take advantage of this technique, primary questions remain regarding the fundamental physics involved with particle convection and self-assembly. In situ investigation of monolayer deposition will be performed using high speed confocal laser scanning microscopy. Preliminary results suggest many parameters not previously considered affect deposition and various mechanisms generate microsphere pre-alignment in the thin film prior to deposition. Experiments that incorporate processing, suspension, and substrate conditions not previously explored will be used to develop a model to shed light onto the mechanisms that link the controllable macroscopic properties to the microstructure.
Fabrication of colloidal monolayers will be used directly in two specific applications. Performance of InGaN light emitting diodes (LEDs) is inhibited by the index mismatch of the GaN/air interface. The proposed process will be used to fabricate microlens arrays where the microstructure determines the light extraction efficiency. Preliminary results demonstrate increased light output power by 219%. In a parallel effort, design of monolayer arrays labeled with antibodies for whole blood detection of CD4+ lymphocytes will enable enhanced screening for HIV/AIDS. The microstructure of these deposited monolayers will dictate the capture efficiency and proliferation of target cells and enable release of these cells for analysis.
Broader Impacts:
Although colloidal convective deposition is used in many technologies, the fundamental physics is poorly understood. This research will develop a predictive model based on observations obtained from direct 3D particle tracking during deposition for various surface and suspension properties. Through this research, the importance of controlling this microstructure will be demonstrated in two applications that have significant potential impact on their respective industries. First, the microlens arrays fabricated using this technique have the potential to surmount the current state-of-the-art LED photon extraction, allows scale-up for industrial applications, and is low-cost as compared to current techniques of surface patterning via electron beam lithography. In whole blood analysis, this technique can be generalized for a variety detection schemes and will aid development of a process that aims to bring low cost detection to regions lacking proper medical resources. This work will directly provide both graduate and undergraduate educational opportunities in an area at the convergence of several technologically-critical research areas including microfluidics, suspension transport, photonics, and bioengineering.
There is no simpler way to modify a surface’s physical, chemical, and optical characteristics than by depositing particles on its surface. This project explored fundamental aspects and applications of "convective deposition", an evaporation-driven thin film coating process that self-assembles nanoparticles into ordered arrays. The approach differed from previous studies by examining the effect of modifying deposition not by explicitly changing the surface, particle, or fluid properties, but by changing the flow in the thin film and altering the interactions of particles through additions of other species. Intellectual Merit: Our work visualized and described the effect of adjusting the length of the thin film related to the deposition rate in the ability to form highly ordered monolayer coatings of spherical colloids. Using high speed confocal microscopy, two deposition regimes were noted that affected local ordering. To enhance the quality of deposited layers, we studied co-deposition of binary systems micron-sized particles and nanoparticles. Higher ordered arrays over larger areas were obtained and a new instability forming particle stripes was discovered and described. These monolayer coatings were used directly in two proposed applications, fabrication of microlens arrays for LEDs and enhanced cell capture platforms for bioMEMS cell analysis. Microlens arrays demonstrated as much as 270% increase in light extraction efficiency. Ordered arrays of monosized particles particles varying in diameter demonstrated specific capture efficiencies elucidating potential mechanisms of cell capture not previously considered. Broader Impacts: This work impacted the areas of coatings, colloid and surface science, LEDs, and bioMEMS directly in its discoveries. Ongoing work enabled by this project include development of a scalable nanomanufacturing platform for roll-to-roll continuous deposition for commercial applications, optical microlens coatings for dye sensitized solar cells, amorphous thin film silicon solar cells, and OLED coatings. In addition, polymeric membranes for viral separations and ceramic membranes used as battery separation layers have also been enabled by this technology. 8 graduate students, two from underrepresented groups (URGs) and 2 postdoctoral have been involved in the primary study or related applications and at least 10 other graduate students have been trained in using these coatings for various aspects of their research in applications not mentioned above. In addition, this work enabled training of 12 undergraduate students, 5 from URGs also studied these processes directly. Topics related to deposition for optical and bioengineering have been presented in freshman to graduate level classes. Overall, 25 publications, 3 patents, and 12 new applications of this process have been realized based on this work. Results of this research have directly contributed toward obtaining funding for ongoing efforts to scale-up and commercialize this process for coating roll-to-roll and other large area and high throughput substrates. Parties interested in this technology should contact Dr. James Gilchrist directly at gilchrist@lehigh.edu.
