Frugal Imaging Technique of Capillary Flow Through Three-Dimensional Polymeric Printing Powders

Podsumowanie

The proposed technique will provide a novel, efficient, frugal, and non-invasive approach for imaging fluidic flow through a packed powder bed, yielding high spatial and temporal resolution.

Streszczenie

The development of novel imaging techniques of molecular and colloidal transport, including nanoparticles, is an area of active investigation in microfluidic and millifluidic studies. With the advent of three-dimensional (3D) printing, a new domain of materials has emerged, thereby increasing the demand for novel polymers. Specifically, polymeric powders, with average particle sizes on the order of a micron, are experiencing a growing interest from academic and industrial communities. Controlling material tunability at the mesoscopic to microscopic length scales creates opportunities to develop innovative materials, such as gradient materials. Recently, a need for micron-sized polymeric powders has been growing, as clear applications for the material are developing. Three-dimensional printing provides a high-throughput process with a direct link to new applications, driving investigations into the physio-chemical and transport interactions on a mesoscale. The protocol that is discussed in this article provides a non-invasive technique to image fluid flow in packed powder beds, providing high temporal and spatial resolution while leveraging mobile technology that is readily available from mobile devices, such as smartphones. By utilizing a common mobile device, the imaging costs that would normally be associated with an optical microscope are eliminated, resulting in a frugal-science approach. The proposed protocol has successfully characterized a variety of combinations of fluids and powders, creating a diagnostic platform for quickly imaging and identifying an optimal combination of fluid and powder.

Wprowadzenie

Inkjet-based binder jetting into powder media represents an important technology in additive manufacturing (3D printing). The binder jetting process begins with the deposition of functional fluids into powder media using a scanning inkjet printing process. Specifically, an inkjet print head translates over the powder surface, depositing the liquid binding agent onto a powder surface, and thereby forming a solid part in a layer-by-layer fashion¹. Inkjet-based binder jetting technologies generally include sand, metal powders, and polymeric powders. However, to expand the materials' space in binder jetting, a fundamental approach to investigating fluid-powder and powder-powder interactions, tribology, powder packing density, and particle aggregation is required. Specifically, for fluid-powder interactions, a critical need exists for the ability to image fluid flow through powder beds in real-time. This promises to be a powerful tool for researchers to include as a characterization technique and potentially as a screening method for different combinations of fluids and powders^2,3,4, as well as more complex systems, such as concrete 3D-printing systems that utilize particle-bed methods.

The development of novel imaging techniques of molecular and colloidal transport, including nanoparticles, is an active area of investigation in microfluidic and millifluidic studies. Probing intermolecular interactions by imaging techniques can be challenging, as little work has been done to probe these types of interactions under the conditions of unsaturated and unsteady fluid flow. Many of the studies that are reported in the literature have focused on a saturated, pre-wetted, porous media, such as glass bead^{5,6,7,8,9,10,11,12} and soils^{13,14,15,16,17,18}. This technique provides a non-invasive approach, resulting in high temporal and spatial resolution^2,3,4,19. Furthermore, the developed technique provides a novel method for characterizing and quantifying nano-scale and micron-scale particle transport in a variety of porous media, focusing on polymeric powders.

The proposed technique utilizes a mobile device to record unsaturated, unsteady fluidic transport through porous polymeric media with particle dimensions that are representative of the powders used in 3D printing systems that utilize fluidic powder-bed fusion technologies. This technique is advantageous as the flow cells are cost-effective, reusable, small, and easily handled, illustrating the dominant aspects of frugal science. The ability to implement these simple experiments into a field study is very straightforward, eliminating the complications, cost, and time that are required in optical microscopy. Given the ease of creating the setup, the access to quick results, and the minimal number of sample requirements, this technique is an optimal platform for diagnostic screening.

Protokół

1. Preparing the microfluidic flow cell

NOTE: For this protocol, a commercial microfluidic flow cell will be utilized. By using a commercial product that is designed for light penetration from an optical microscope, any challenges regarding brightfield illumination of the media will be minimized.

1. Start preparing the microfluidic flow cell by covering the outlet with parafilm to seal one end of the channel so that the empty flow cell may be packed with polymeric powder. Before starting the experiment, confirm that the microfluidic channel is clean and dry.
   1. Tape the metric paper ruler directly beneath the flow channel.
   2. Weigh the microfluidic flow cell with the parafilm and the ruler attached. The mass of the flow cell is the unpacked flow cell mass (m_u).

2. Packing the powder into the channel

1. When packing the powder, use a plastic pipet to transfer the powder. Note that particles may adhere to the outside of the pipet tip, which is a result of tribocharging.
   1. While introducing the powder into the channel, tap the flow cell at least five times to compact the powder. Continue packing until the powder reaches the beginning of the opening of the flow channel.
      NOTE: Tapping compacts the powder within the channel with the goal of providing a reproducible diagnostic tool. For certain applications, this effort can be a higher, lower, or equivalent level of powder compaction than the compaction that is observed in the application of interest. If there are issues with the reproducibility of tapping or with packing powder within the application, consider performing ASTM D7481-18²⁰.
   2. Remove the powder present on the outer surface of the flow cell with a wipe soaked in alcohol.
      NOTE: Some types of particles can be hydrophobic, so water may not remove the particles well.
2. Once the powders are packed, visually inspect the flow cell for loosely packed powder. If the powder within the flow cell appears loosely packed (Figure 1), tap the flow cell five more times. If the powder packing appears consistent and compact, weigh the flow cell to measure the mass of the polymeric powder (m_p - m_u; see Equation 1).
   1. Calculate the bulk packing density (ρ) using the difference between the unpacked (m_u) and packed flow cell mass (m_p) and dividing it by the volume of the flow cell. The volume of the flow cell is then known [length (l): 50 mm, width (w): 5 mm, channel depth (h): 0.8 mm].
           Eq 1
   2. Confirm that the packing density is in the typical range of 0.45 g/mL to 0.55 g/mL for polymeric powders^2,3,4,21. Leave the flow cells in the fume hood until steps 3 and 4 are completed.
      CAUTION: Particles with a diameter of less than 10 µm can penetrate into the lungs and potentially enter the bloodstream, which can cause health problems related to the pulmonary and cardiovascular systems. The polymeric powders that were used in this experiment have a particle diameter of approximately 50 μm. Therefore, inhalation of the particles has less potential to cause health problems, but smaller particles are present even in narrow particle size distributions. For the safest environment, preparation of the flow cells should be done in a fume hood.

3. Preparing the solvent

1. Prepare a 75 wt% solution of ethanol in water. Note that the solvent will be referred to as the fluid in the rest of this manuscript.
   ​CAUTION: Make sure that the beaker used to prepare the solution is free from any surfactants, as surfactants will affect the results.

4. Preparing the white light table

1. To prevent flooding the detector (camera) with too much light, cover the light table with an opaque material, such as a 3D printed cover in black polylactic acid (PLA) filament (Supplementary Figure 1). Ensure that the material has an opening that is the size of the microchannel (5 mm x 55 mm) to allow light to illuminate the powder.
   NOTE: Too much light means the screen or monitor of the camera will appear white and the microchannel will not be visible. Therefore, the detector will be unable to focus the lens on the microchannel.
2. To ensure that the camera on the mobile device can capture the contrast between the wet and dry powder, use the light table at a low to medium light intensity.
   NOTE: High light intensity is at 100%. The other two settings are relative to the high light intensity; the setting for low light intensity is at ~30%, and medium light intensity is at ~65%.
3. Align the camera on the mobile device directly above the light table. Confirm that the camera is perpendicular to the top of the light table (Figure 2).
4. Orient the camera on the mobile device so that the long axis of the mobile device aligns with the longest axis of the flow cell.

5. Starting the experiment

1. Place the flow cell on the light table and focus the camera on the mobile device on the flow channel.
   NOTE: For optimal results, a darker (reduced overhead lighting) recording space will typically provide better image resolution. If a dark space is not available, minimizing changes in overhead lighting (lights being turned on, turned off, or being dimmed) during recording should improve graphic signals and minimize undesired noise in the experiment.
2. After focusing the camera on the mobile device, select the record button. Add 125 μL of fluid to the open inlet of the microchannel using a pipet.
3. Record the flow for 2 min or until all the powder is wetted visibly.

6. Analyzing the data

1. Transfer the video file from the mobile device to the computer for easy access. Note that videos over 2 min may not load in the software at this time, as the file size can be excessively large.
2. Download Tracker, a free software from the Physlets website²². This software can track position, velocity, and acceleration in the following video files: .mov, .avi, .mp4, .flv, .wmv, etc. For the following steps, please refer to Supplemental File.
   NOTE: For Mac users, install the latest version of the software for the software to function properly. Additionally, Mac users may require a video engine (Xuggle), animated GIF files (.gif), or image sequences that consist of one or more digital images (.jpg, .png, or pasted from the clipboard).
3. Once the software is installed, open the Tracker software. From the File menu, select Open File to load the transferred video file from step 6.1 on the desktop of the computer.
4. Click the Clip Settings icon, which looks like the film strip, to define the Starting Frame and the Step Size.
   NOTE: Placing the mouse over an icon will identify the icon.
   1. Define the Starting Frame. The Starting Frame is defined as the frame in which the first contrast (the contrast between the wet and dry powder) is observed.
   2. Set the Step Size. Step Size refers to the frame step size, which the software would analyze. From prior experiments, the optimum Step Size is 10.
5. Click on the Calibration Tool, the icon with the blue ruler, to the right of the Clip Settings button. From New, select Calibration Stick.
6. To zoom in on the ruler in the video, right-click on the area to magnify and Select Zoom in from the list. Once appropriately magnified, define the beginning and end of 1 mm on the ruler taped to the microchannel, and Type 1 mm to define the distance.
7. Click on the Coordinate Axis Tool, which is the purple icon, to the right of the Calibration Tool. Set the Origin for the x- and y-axis, using the starting frame while doing this step.
8. To define the initial point of analysis, create a Point Mass. Click on Create, then select Point Mass. Use Shift + Control to change the size of the rectangle. The initial point is where the inlet and the channel connect.
   NOTE: The rectangle indicates the domain, defined by the user, that the software will scan to find the contrasting wet and dry powder. The boundary allows the user to define the region where the initial point will be observed.
   1. Click on Search Next a couple of times to verify that the software is analyzing the correct area. If the software is functioning properly, click on Search and wait for the software to finish analyzing the video. If the software cannot automatically find a matching image intensity from the previous frame to the current frame, the software will stop and wait for the user to redefine the search area.
      NOTE: For reproducibility and the ability to compare different experimental results, choose the fastest or the slowest point of the fluidic flow front (region of contrast between the wetted and dry powder) for every sample.
   2. If an analysis error is observed on the live plotted data on the right-hand side of the Tracker screen, click on the Data Point once on the step prior to the erred data point. On the main screen, modify the red rectangular search area location to search the region of interest and repeat step 6.8.1.
      NOTE: If an error exists, right-click on the inaccurate data point and deselect the point for further analysis.
9. Once the analysis is complete, copy and paste the results into a spreadsheet. The results saved in the spreadsheet comprise the distance and time data.
10. Plot the copied data in the spreadsheet as the distance of fluid transport through the powder bed as a function of time.

Wyniki

In the section on analyzing data, the data for the time-lapsed images in Figure 3 illustrate the 75 wt% ethanol solution infiltrating the polycarbonate (PC) powder. Fluorescein was added to the solution to enhance the image quality for this publication. In the time-lapse images, the time-resolved process begins when the fluid is added to the inlet. Time, t, starts as soon as the fluid begins to penetrate the channel. The series of images demonstrates the progression of the fluid and...

Dyskusje

The protocol that is provided is highly dependent on the material characteristics of the particles that are chosen. Material properties impacting flow include particle size distribution^{2,3,4,5,11,21}, particle surface roughness¹¹, chemical properties at the particle surface^2...

Materiały

Name	Company	Catalog Number	Comments
µ-Slide I Luer	ibidi	80191	Microfluidic flow cell
Beaker	Southern Labware	BG1000-800	Glassware
CALIBRE 301-58 LT Natural Polycarbonate Resin	TRINSEO LLC	CALIBRETM 301-58 LT	Natural polycarbonate resin
Ethanol	Sigma Aldrich	1.00983	Solvent
Fume Hood	Kewaunee	Supreme Air LV Fume Hoods	Used with 92 FPM at 18" opening
iPhone 7 plus	Apple		Camera
Opaque 3D printed material			The CAD drawing is provided in the supplemental file
ORGASOL  2002 ES 6 NAT 3	ARKEMA	A12135	Polyamide powder
Pipet	VWR	10754-268	Disposable Transfer Pipet
Pipette	Globe Scientific Inc.	3301-200	Pipette that can hold 125 µL of fluid
Polystyrene	Advanced Laser Materials, LLC.	PS200	Polystyrene for sintering
Tracker			Video analysis and modeling tool
VariQuest 100 White Light Model 3-3700	FOTODYNE	3-3700	White light
Water			Distilled water