Research Papers

Deposition of Variable Bead Diameter Arrays by Self-Focusing Electrohydrodynamic Jets

[+] Author and Article Information
Nicolas Martinez-Prieto

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: nicolasmartinezprieto2019@u.northwestern.edu

Gabriela Fratta

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: gabrielafratta2014@u.northwestern.edu

Jian Cao

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: jcao@northwestern.edu

Kornel F. Ehmann

Department of Mechanical Engineering,
Northwestern University,
2145 Sheridan Road,
Evanston, IL 60208
e-mail: k-ehmann@northwestern.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO-AND NANO-MANUFACTURING. Manuscript received September 11, 2017; final manuscript received May 29, 2018; published online June 22, 2018. Assoc. Editor: Marriner Merrill.

J. Micro Nano-Manuf 6(3), 031003 (Jun 22, 2018) (11 pages) Paper No: JMNM-17-1050; doi: 10.1115/1.4040450 History: Received September 11, 2017; Revised May 29, 2018

Electrohydrodynamic (EHD) processes were used for direct writing of bead arrays with controllable bead sizes. Experiments were conducted to align layers of bead-on-string structures in an effort to create three-dimensional patterns. The results show that the jet focuses on previously deposited droplets allowing for the selective deposition of material over already deposited patterns. Jet attraction to already deposited solutions on the substrate is attributed to the charge transport at the liquid ink–metal collector interface and the dielectric properties of the water/poly(ethylene oxide) (PEO) solution under an electric field. The deposition process consists of three steps: (1) deposition of a layer of bead-on-string structures, (2) addition of extra volume to the beads by subsequent passes of the jet, and (3) evaporation of the solvent resulting in an array of beads with varying sizes. Patterns with up to 20 passes were experimentally obtained. The beads' height was seen to be independent of the number of passes. The process reported is a simple, fast, and low-cost method for deposition of bead arrays with varying diameters.

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Fig. 1

Experimental setup

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Fig. 2

Process initiation: (a) droplet formation, (b) droplet migration to the tip of the needle after application of the electric field, and (c) jet formation after the droplet falls

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Fig. 3

High magnification image sequence of the deposition process at the collector during the (a) first and (b) third pass

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Fig. 4

High-speed camera snapshots showing: (a) deposition of the bead-on-string structure during the first pass, (b) deposition in the third pass, and (c) 17th pass. Note the increase in droplet volume.

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Fig. 5

Schematic of the deposition process

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Fig. 6

Resulting patterns after (a) 1, (b)–(c) 5 (d) 10, and (e) 20 passes. Note the increase in bead area as the number of passes increases. (c) Shows a five-pass sample where multiple deposition tracks were obtained. Scale bar is 200 μm.

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Fig. 7

SEM image of beads from the 20-pass sample: (a) coffee ring morphology of a bead and (b) the limited thickness of the beads allows the observation of the substrate through the beads at a 2 kV acceleration voltage

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Fig. 8

Effect of number of passes on (a) bead spacing and (b) equivalent bead diameter for three different runs. Runs 2 and 3 for the five-pass sample showed little overlap so the equivalent diameter was not quantified. (c) Relation between equivalent bead diameter and bead spacing.

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Fig. 9

(a) High-speed camera sequence showing the merging of two adjacent beads and (b) microscope image showing typical bead morphology before and after merging

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Fig. 10

Microscopic images of a 10-pass sample. The beads are larger (a) near the edge than (b) near the center.

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Fig. 11

(a) Deposition pattern for circular samples. Deposited patterns with (b) 1, (c) 5, and (d) 10 passes. Higher magnification images showing the shape of the resulting beads with (e) 1, (f) 5, and (g) 10 passes. Scale bar is 200 μm.

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Fig. 12

Effect of number of passes on (a) bead area, (b) bead spacing, and (c) bead height for the circular samples. (d) Normalized bead area by initial jet area.

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Fig. 13

(a) Effect of the number of passes on edge-to-edge spacing and (b) Average edge-to-edge spacing versus the square root of the number of passes. The trendline shows a linear relationship between these two quantities

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Fig. 14

Effect of nozzle diameter in (a) bead size and (b) bead spacing. Effect of stage speed in (c) bead size and (d) bead shape.

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Fig. 15

(a) Image sequence of a droplet falling from the nozzle when the voltage is turned on and turned off after 8 s and (b) Current flowing through the collector during the process

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Fig. 16

Electric potential models for: (a) nozzle-collector, (d) droplet array, and (g) single droplet configurations. (b) Horizontal and (c) vertical electric fields for nozzle-collector configuration. (e) Horizontal and (f) vertical electric fields for droplet array configuration. (h) Horizontal and (i) vertical electric fields for the diameter sweep of single droplet configuration. (j) Plot of horizontal electric field versus normalized distance from the center of the droplet. (k) Horizontal and (l) vertical electric fields for the displacement sweep of a single droplet with a 40 μm diameter.

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Fig. 17

(a) Deposition of PAN/DMF ink on copper. Deposition of PEO/water ink on (b) aluminum and (c) silicon.



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