Using Convective Flow Splitting for the Direct Printing of Fine Copper Lines
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T. CUK*, S.M. TROIAN**, C.M. HONG and S. WAGNER*** *ELE Department, Princeton University, Princeton, NJ 08544, taniacuk~diprinccton.edu **CHE Department, Princeton University, Princeton, NJ 08544 ***ELE Department, Princeton University, Princeton, NJ 08544 ABSTRACT
We have developed a technique for the printing of copper lines using solutions of a metal organic precursor, copper hexanoate. A 500-jim written liquid line is observed to split into two 100-pjm wide lines. We observe further splitting into four parallel lines in experiments with written lines of copper hexanoate solution in chloroform. Surface profiles indicate that the thickness, width and number of lines formed are strongly dependent on the solution viscosity and volume per unit length deposited. From particle tracking visualization and surface profiling, we have found that evaporative cooling produces Marangoni convection patterns that accrete the solute along two key boundaries of flow. INTRODUCTION
Recently, many studies have been made on the self-organized ring formation in drying drops'5. One can observe this ring formation in the everyday occurrence of a coffee-stain-a stain that is much darker at the edges than in the center of the drop. Applications include highresolution printing, nanoparticle arrays for light emitting and light processing devices, and colloidal selfFigure 1: Optical micrograph of organization for photonic crystals. This study two copper lines, each 100 pm in specifically demonstrates the use of solute segregation in width, formed by capillary writing the printing of fine metallic lines, a 544 1um wide liquid ribbon of Past studies targeted the "pinning" of the "contact
line" as the means for solute accretion in drying drops.
0.10 g CuHex/ ml CHCI3 onto a glass
slide.
As the suspension evaporates, the "contact line"-the triple phase junction of the drying drop-moves inward; its motion can be halted due to either chemical or geometrical surface roughness. In its halted state, the
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100
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contact line is "pinned"; when the contact line resumes its motion, it "depins"6' 7. The pinning of the contact line can cause ring formation in two ways-capillary motion from center to edge' and meniscus pinning of the particle array5 . In this study, we utilize a new geometry-that of a ribbon-to form multiple pairs of parallel lines, rather than a drop to form rings. We also demonstrate the importance of thermocapillary forces, in addition to contact line pinning, in directing solute accretion. EXPERIMENT Ribbon Deposition
Liquid ribbons of copper hexanoate, Cu 2(OH 2)2(O2CR)4 with R= (CH 2)4CH3, in chloroform, were written with glass capillaries of inner diameter approximately 100/num. The precursor copper hexanoate can be reduced to pure metallic form by annealing. We have 267 Mat. Res. Soc. Symp. Proc. Vol. 624 © 2000 Materials Research Society
observed that the copper hexanoate (CuHex) Ribbon Profiles: Concentration accretes into pairs of 80000 parallel lines narrower in lJ/ width than the original 70000liquid ribbon. In
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