Fabrication of a 3D Multi-Depth Reservoir Micromodel in Borosilicate Glass Using Femtosecond Laser Material Processing

Abstract

Micromodels are ideal candidates for microfluidic transport investigations, and they have been used for many applications, including oil recovery and carbon dioxide storage. Conventional fabrication methods (e.g., photolithography and chemical etching) are beset with many issues, such as multiple wet processing steps and isotropic etching profiles, making them unsuitable to fabricate complex, multi-depth features. Here, we report a simpler approach, femtosecond laser material processing (FLMP), to fabricate a 3D reservoir micromodel featuring 4 different depths-35, 70, 140, and 280 µm, over a large surface area (20 mm × 15 mm) in a borosilicate glass substrate. The dependence of etch depth on major processing parameters of FLMP, i.e., average laser fluence (LFav), and computer numerically controlled (CNC) processing speed (PSCNC), was studied. A linear etch depth dependence on LFav was determined while a three-phase exponential decay dependence was obtained for PSCNC. The accuracy of the method was investigated by using the etch depth dependence on PSCNC relation as a model to predict input parameters required to machine the micromodel. This study shows the capability and robustness of FLMP to machine 3D multi-depth features that will be essential for the development, control, and fabrication of complex microfluidic geometries.

Keywords: 3D multi-depth channels; femtosecond laser material processing; femtosecond laser micromachining; laser machining; micro/nanotechnology fabrication; micromodels; porous media.

Figure 1

Schematic illustration of the femtosecond laser material processing (FLMP) setup. The path of the laser beam is fixed while the CNC stage allows XYZθ motions.

Figure 2

Line profile scan across 4 mm-length line feature machined into borosilicate glass using FLMP at ${\mathrm{PS}}_{\mathrm{CNC}}$ of 0.25 mm/s and 0.617 mJ power. The profile was recorded at 2 µm/s, 10 Hz, and 2 mg applied force. The full-width at half-maximum (FWHM) value of 12.3 µm of the Gaussian-like etched profile was determined as the spot size of the focused laser beam diameter.

Figure 3

Line profile scan of FLMP etch area (1500 µm × 1500 µm) machined into borosilicate glass substrate at ${\mathrm{PS}}_{\mathrm{CNC}}$ of 0.1 mm/s and ${\mathrm{LF}}_{\mathrm{av}}$ of 329.06 J/cm². The profile was recorded at 5 µm/s, 10 Hz, and 2 mg applied force. The inclined etch surfaces make a contact angle (θ) of ~8° with the vertical plane.

Figure 4

A plot showing the linear dependence of etch depth on average laser fluence (${\mathrm{LF}}_{\mathrm{av}}$) while keeping all processing parameters constant, such as ${\mathrm{PS}}_{\mathrm{CNC}}$ at 0.25 mm/s. Legend—experimental data points: black squares, data fitting: red trace.

Figure 5

A plot showing an inverse (green trace), logarithmic (blue trace) and a three-phase exponential decay (red trace) dependence of etch depth on CNC processing speed (${\mathrm{PS}}_{\mathrm{CNC}}$). All processing parameters were kept constant, including average laser fluence (${\mathrm{LF}}_{\mathrm{av}}$) at 329.06 J/cm². Approximate portions of the plot that shows fast, medium, and slow exponential decays are represented by ${\mathrm{FA}}_1$, ${\mathrm{MA}}_2$, and ${\mathrm{SA}}_3$, with pre-exponential decay factors of 136.9648, 94.0098, and 45.0741 µm, respectively. Legend—experimental data points: black squares, inverse data fitting: green trace, logarithmic data fitting: blue trace, exponential data fitting: red trace.

Figure 6

CAD schematic illustration of the 3D multi-depth reservoir (R) micromodel. (a) a 2D top view showing a description of all components of the micromodel and their dimensions in µm units, and a zoom-in portion that shows the pore body bounded by 3 solid hexagon grains, and the uniform widths of the pore space (m) and pore throat (n) which gives an aspect ratio ($\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$n$}\right.$) = 1. (b) a 3D top view design showing the borosilicate glass substrate (grey) and the etch area (green) with multiple depths. (c) a front view of (b), grey arrow direction, showing the 4 depths of the reservoir micromodel –35, 70, 140, and 280 µm relative to the surface of the borosilicate substrate. The notations used here are R1 matrix, R1 outer sink, and R1 inner sink that represents the main reservoir matrix, the large outer circular sink, and the small inner circular sink of reservoir 1, etc. Emphasis was placed on the reservoir matrices and sinks, therefore portions (grey area) of the inlet channel was not etched as shown in (b).

Figure 7

Images of several sections of the FLMP fabricated 3D multi-depth reservoir micromodel: (a) the red line illustrates the line profile scan path which goes through three pore spaces and two hexagonal pore bodies for each reservoir (R), (b) a zoom-in section of reservoir 3 where the image was focused at the etched surface, and (c–e) shows inlet portions of reservoirs 1, 2, and 3, respectively. Images (b,e) looks blurrier than (c,d) due to deeper depth as the microscope was focused on the bases of the channels. The scale bars are 250 µm.

Figure 8

A plot showing the surface profilometer line scans across the matrices of the 3D multi-depth reservoir micromodel shown in Figure 7a. The black, red, and blue traces correspond to the profiles of reservoirs R1, R2, and R3, respectively.

Figure 9

Images of the 3D multi-depth reservoir micromodel (a,b) taken with an optical microscope and showing the (a) outer and (b) inner circular sinks of R3 in focus. The red line across (a) indicates a 2D line profile scan path while (c) is a 3D image of reservoir R2 sink recorded with the surface profilometer at 2 mg force, 10 µm/s speed, 10 Hz, and 3 µm scan interval. The arrows in (a,b) point to the location of machined debris remaining after sonication.

Figure 10

A plot of line profile scans across the circular sinks of the 3D multi-depth reservoir micromodel. The black, red, and blue traces correspond to the profiles of reservoirs R1, R2, and R3, respectively. The green trace was a repeated scan for reservoir R3 from the newly etched outer sink surface (layer 1) due to instrument limit which is observed as a smooth horizontal etch surface (indicated by blue arrow).