Manipulation and Localized Deposition of Particle Groups with Modulated Electric Fields

Abstract

This paper presents a new micro additive manufacturing process and initial characterization of its capabilities. The process uses modulated electric fields to manipulate and deposit particles from colloidal solution in a contactless way and is named electrophoretically-guided micro additive manufacturing (EPμAM). The inherent flexibility and reconfigurability of the EPμAM process stems from electrode array as an actuator use, which avoids common issues of controlling particle deposition with templates or masks (e.g., fixed template geometry, post-process removal of masks, and unstable particle trapping). The EPμAM hardware testbed is presented alongside with implemented control methodology and developed process characterization workflow. Additionally, a streamlined two-dimensional (2D) finite element model (FEM) of the EPμAM process is used to compute electric field distribution generated by the electrode array and to predict the final deposition location of particles. Simple particle manipulation experiments indicate proof-of-principle capabilities of the process. Experiments where particle concentration and electric current strength were varied demonstrate the stability of the process. Advanced manipulation experiments demonstrate interelectrode deposition and particle group shaping capabilities where high, length-to-width, aspect ratio deposits were obtained. The experimental and FEM results were compared and analyzed; observed process limitations are discussed and followed by a comprehensive list of possible future steps.

Keywords: dielectrophoresis; electrophoretic deposition; electrophoretically-guided micro additive manufacturing (EPμAM); finite element analysis; process characterization; process control; self-assembly.

Figure 1

(a) EPμAM process schematic; (b) electrophoretic deposition process (EDP); and (c) dielectrophoresis phenomenon (DEP).

Figure 2

Schematic of the EPμAM experimental setup. The particle deposition cell is located within a lightproof enclosure above the inverted microscopy setup. The PC workstation is used to control the relay switchboard and to capture digital images from the microscope.

Figure 3

The CAD model and the fabricated microelectrode array. The array is attached to the kinematic mount (Thorlabs Kinematic Prism Mount KM100PM/M) that is used to align and level the array with respect to the bottom of the deposition cell.

Figure 4

PWM duty cycle definition. The developed EPμAM control system allows individual control of each microelectrode site in the electrode array.

Figure 5

Developed workflow for the simple particle group manipulation and EPμAM process characterization experiments. The definition of commonly used terms related to the deposition cell is shown in the upper right corner of the figure. The definition of the control schemes A–D is given in Figure 6.

Figure 6

Microelectrode array actuation strategies. (a) The scheme for creating middle focusing potential ( control scheme A); (b) the scheme for the middle defocusing potential generation (control scheme B); (c) the translation to the 3rd column scheme (control scheme C); (d) the translation to the 4th column scheme (control scheme D); (e) the “horizontal” midline deposition (“squeezing”) scheme (control scheme E); (f) the “horizontal” stretching scheme (control scheme F). Each grid cell corresponds to a single electrode in the microelectrode array, as shown in Figure 3 and in the upper right corner of Figure 5.

Figure 7

Timing results for the main control loop implementation in MATLAB. (a) Cycle times for A, B, C, and D control schemes; (b) cycle times’ boxplots for the A, B, C, and D control schemes.

Figure 8

Generated FEM mesh and geometrical parameters of the 2D model. The electrode sites (small circles) are equidistant with 2.54 mm center-to-center spacing/pitch.

Figure 9

Particle size distribution for the prepared samples of rutile titania. Mean particle size for the prepared samples were 1.62, 1.78, and 1.42 μm, which corresponded to not sonicated, sonicated for 1 min, and sonicated for 2 min, respectively.

Figure 10

Experimental results of rutile titania (TiO₂ (R)) particle group manipulation. (a) Initial particle distribution after settling on the bottom of the mini well; (b) particle group distribution after the particles were focused in the middle of the array with the control scheme A; (c) particle group spread after defocusing particles with control scheme B; (d) particle group arrangement after translation to the 3rd column of the electrode array with control scheme C; (e) grouped particles around the 4th column with control scheme D.

Figure 11

FEM computed electric potential distributions for control schemes A–D. (a) Computed potential distribution for focusing, control scheme A; (b) computed potential distribution for defocusing, control scheme B; (c) computed electric potential distribution for translating particles to the 3rd array column, control scheme C; (d) computed electric potential distribution for translating particles to the 4th array column, control scheme D. The color bar shows the range of the applied electric field from 0 V (blue) to 15 V (red). Readers are referred to the color version of the picture.

Figure 12

Test particle distributions in the built multiphysics 2D FEM model of the EPμAM process. (a) Initial test particle positions for all FEM test runs; (b) test particle positions at the end of the application of focusing, control scheme A; (c) test particle positions at the end of application of the defocusing, control scheme B; (d) test particle positions at the end of translation to the third array column, control scheme C; (e) test particle positions at the end of translation to the fourth array column, control scheme D. Large gray circles represent electrode surface positions.

Figure 13

Example image post-processing results for the beginning of middle defocusing experiments. (a–c) Raw image data A, B, and C cases (panels a, b, and c, respectively); (d–f) computed binary masks for raw image data a-c, respectively; (g–i) binary mask pixel distribution (summed along image y-axis); (j–l) binary mask pixel distribution (summed along image x-axis).

Figure 14

Binary masks trends (top) and their rate of change (bottom). The binary mask area is the sum of all mask pixels. The figure shows data points in the same order as the experiments were performed (middle focus, middle defocus, translate to 3rd, and translate to 4th column).

Figure 15

(a–d) “Horizontal” and (e–h) “vertical” midline depositions snapshots. The bottom right white line length is 1 mm.

Figure 16

Horizontal midline deposit formation. (a–e) Image snapshots during deposit formation with control schemes E and F. The bottom time labels indicate approximate time since the start of the deposition process. The bottom right white line length is 1 mm.

Figure 17

Three-dimensional (3D) scan results of the prepared sample. (a) 3D scan of the obtained deposit height. The color bar in the panel denotes the computed height of the whole sample; (b) 2D intensity image of the scanned sample; (c) 2D height color image of the scanned sample. ROI indicates the region of interest used for quantification of aerial roughness parameters.

Figure 18

Definitions and naming convention for the performed profile scans on the deposited sample. Arrows indicate the scanning direction for the profile height lines shown in the following figures.

Figure 19

Height profile for the V1 line.

Figure 20

Height profile for the V2 line.

Figure 21

Height profile for the H1 line.

Figure 22

Height profile for the H2 line.

Figure 23

Height profile for the H3 line.

Figure 24

Height profile for the H4 line.

Figure 25

Height profile for the H5 line.

Figure 26

Height profile for the H6 line.