Structure of Electrospray Printed Deposits for Short Spray Times

[+] Author and Article Information
Nicholas A. Brown, Yaqun Zhu, Ao Li, Mingfei Zhao, Xin Yong

Department of Mechanical Engineering,
State University of New York at Binghamton,
Binghamton, NY 13902

Paul R. Chiarot

Department of Mechanical Engineering,
State University of New York at Binghamton,
Binghamton, NY 13902
e-mail: pchiarot@binghamton.edu

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received June 20, 2017; final manuscript received August 5, 2017; published online September 27, 2017. Assoc. Editor: Yayue Pan.

J. Micro Nano-Manuf 5(4), 040906 (Sep 27, 2017) (6 pages) Paper No: JMNM-17-1040; doi: 10.1115/1.4037695 History: Received June 20, 2017; Revised August 05, 2017

In electrospray printing, a plume of highly charged droplets is created from a conductive ink. Printing occurs by positioning a target substrate (TS) in the path of the emitted material. Here, the ink used is a colloidal dispersion consisting of nanoparticles suspended in a volatile solvent. The selection of a volatile solvent allows for rapid evaporation of the droplets in-flight to produce dry nanoparticles. A net electric charge is imparted on the emitted particles during electrospray. The interaction of this charge with the global electric field and with other charged particles/droplets governs the particles' trajectory and determines the microstructure of the printed deposit. In this study, we characterized the structure of nanoparticle deposits printed using electrospray for deposits with low particle count. During printing, the TS was: (i) held stationary and (ii) translated with various short spray times and substrate velocities, respectively. Examination of both a static and translating TS provided fundamental insights into the printing process. Electrospray printing is capable of exerting much finer control over microstructure compared to other printing techniques. This has significant implications for the manufacturing of thin-films.

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Grahic Jump Location
Fig. 3

Micrographs of typical electrospray printed deposits. (a) Stationary substrate with a spray time of 10 s and a separation distance of 7.5 mm and (b) Translating substrate with a velocity of 1 mm/s and separation distance of 7.5 mm. Both images were converted to binary and highly processed for visualization in this figure. The box along the center of the deposits represents the region of interest (ROI). In these regions, particle counts and locations are calculated for analysis. The height of the ROI is exaggerated for clarity. The scale bars are 750 μm.

Grahic Jump Location
Fig. 1

Electrospray printing of colloidal dispersions. The applied electric potential deforms the liquid meniscus at the end of the capillary tube into a Taylor Cone. A jet is emitted from the apex of the Taylor cone, which breaks up into highly charged droplets. In-flight, the droplets undergo rapid evaporation leaving dry material that is deposited on to the TS.

Grahic Jump Location
Fig. 2

Experimental Setup. A manifold is used to hold a glass capillary tube, which has been pulled to a fine tip. HV is applied to the fluid through the manifold. The nanoparticle ink is supplied to the emitter through the manifold using a syringe pump. A mechanical shutter is used to block material deposition on the TS until the spray is full developed. The separation distance between the emitter and the substrate is controlled in the z-direction using a linear stage. The substrate position is controlled in both the x- and y-direction. A machine vision system (M1) is used to observe the Taylor Cone formation and stability.

Grahic Jump Location
Fig. 4

(a) Particle count for typical electrospray printed deposits on a stationary substrate. The horizontal axis has been normalized to go from edge to edge of the deposit. The vertical axis represents particle counts across the ROI. Above the plot are the composite images of the ROI for each distribution, with spray time increasing from the top to the bottom image. The contrast of the images has been greatly increased for visualization. (b) With increasing spray time, the particle density of the printed deposit increases. The upper left panel is the average particle count for three spray times (3 s, 6 s, and 10 s) across the entire ROI. Each spray time is the average of at least three deposits. The error bars are one standard deviation. The other panels are images taken at the outer ring (i.e., the area of highest particle density) for each spray time. The inset scale bars are 10 μm.

Grahic Jump Location
Fig. 5

(a) Particle count for typical electrospray printed deposits on to a moving substrate. The horizontal axis has been normalized from edge to edge of the printed line. The vertical axis represents particle counts across the ROI. (b) With increasing substrate velocity, the particle count in the ROI decreases. The upper left panel is the average particle count for three different substrate velocities (1 mm/s, 5 mm/s, and 10 mm/s) across the entire ROI with error bars of one standard deviation. The other panels are images taken at the center of the printed line (i.e., area of highest particle density) for each substrate velocity. The inset scale bars are 10 μm.

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Fig. 6

Particles in-flight experience a mutual force from other droplets/particles that are also in-flight. This mutual force is responsible for the radial distribution of particles on the substrate. (a) Particles near the center of the plume have a negligible net mutual force and (b) particles further away from the central axis of the plume experience a net force outward. This is responsible for the high particle density at the center and edge of the deposit.

Grahic Jump Location
Fig. 7

Simulated particle count plot for a moving substrate based on the profile of the 3 s stationary printed deposit. (a) A stationary profile is generated by rotating the 3 s radial profile 2π around the central axis and (b) the stationary profile was translated to replicate the substrate velocity. The edge and center regions of the deposit still exhibit high particle counts (this is not seen in experiments). The depletion region has a relatively higher particle count than the static profile. This is potentially due to the blurring from the edge-enhanced region.



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