Experimental investigations and analytical modeling of multi-pass CO2 laser processing on PMMA

Accepted Manuscript Title: Experimental investigations and analytical modeling of multi-pass CO2 laser processing on PMMA Author: Shashi Prakash Subrata Kumar PII: DOI: Reference:

S0141-6359(16)30225-2 http://dx.doi.org/doi:10.1016/j.precisioneng.2017.02.010 PRE 6536

To appear in:

Precision Engineering

Received date: Revised date: Accepted date:

19-9-2016 17-1-2017 13-2-2017

Please cite this article as: Shashi Prakash, Subrata Kumar, Experimental laser investigations and analytical modeling of multi-pass CO2 processing on PMMA, (2017), http://dx.doi.org/10.1016/j.precisioneng.2017.02.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Highlights

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1. This research investigates the multi-pass CO2 laser microchanneling process on PMMA.

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2. The effects of multi-pass processing on microchannel parameters like microchannel width, depth, heat affected zone, surface roughness and surface profiles were assessed and explained.

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3. Multi-pass processing results in cleaner microchannels with better surface smoothness without the need of any extra attachment or post-processing. 4. An energy based analytical model has been proposed for determining the micro channel profiles and depth in multipass processing.

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5. The proposed model results are in close agreement with original dimensions.

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Experimental investigations and analytical modeling of multi-pass CO2 laser processing on PMMA

Mechanical Engineering Department, Indian Institute of Technology Patna, India Corresponding Author: Dr. Subrata Kumar, Assistant Professor, Mechanical Engineering Department, Indian Institute of Technology Patna, India, email: [email protected], Ph:+91-612-302-8039

Abstract

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Shashi Prakash, Subrata Kumar∗

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In this research work, microchannels have been fabricated utilizing multi-pass CO2 laser processing on Poly-methyl meth-acrylate (PMMA) substrates. CO2 laser engraving machines are cost effective and less time consuming compared to other tools and methods of fabricating microchannels on PMMA. However, the basic problem of low surface finish of the microchannel walls still restricts thus fabricated product from many potential applications. In this work, experimental and theoretical investigations of multi-pass CO2 laser processing on PMMA have been conducted. A number of experiments were performed to establish the relationship between laser power and scanning speed with microchannel parameters like width, depth, heat affected zone, surface roughness and surface profiles. Experiments were conducted at four different power settings with 50 mm/s of constant scanning speed and seven numbers of passes in each setting. Changes in thermo-physical properties of PMMA were observed for as-received PMMA sample and PMMA sample residing in heat affected zone (HAZ) for first pass and secondary passes respectively. Effect of different numbers of passes on microchannel width, depth, HAZ and surface roughness were explored for different power setting. Microchannel profiles resulting from different numbers of passes have been compared. Energy dispersive X-ray analysis was performed to determine elemental composition after each pass. Many advantages of multi-pass processing over single-pass processing were recorded including high aspect ratio, low heat affected zone, smoother microchannel walls and reduced tapering of microchannels. An energy balance based simple analytical model was developed and validated with experimental results for predicting microchannel profiles on PMMA substrate in multi-pass processing. Multi-pass processing was found to be time and cost effective method for producing smooth microchannels on PMMA.

PMMA, laser, microchannel, multi-pass

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Introduction

The applications of microchannel based microfluidic devices for biomedical and chemical analysis based applications have gained a significant momentum in past decade. Polymers, as a base substrate of these microfluidic devices offer many advantages over other traditionally used materials like silicon, quartz and glass [1]. Fabrication of microchannels on these traditional substrates is technically demanding and time consuming apart from their higher input cost and requirement of clean room facilities [2]. Polymers, on the other hand offer easy to fabricate and cost effective solution for many of the microfluidic devices. Microfluidic devices with

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polymeric substrate materials are used in various applications like flow cells, capillary electrophoresis, miniaturized PCR (polymer chain reaction), clinical dentistry and diagnostics, cell handling and micro-reactors and containers [3, 4]. The dimension range and shape of these microchannels differ considerably depending upon type of applications. However, most of these dimensions lie in the order of 10-500 µm [5]. Poly methyl methacrylate (PMMA) has attracted considerable attention as a base material for many microfluidic devices due to many favorable properties. PMMA (commonly pronounce as acrylic) is a non porous solid which possesses excellent optical transparency, chemical inertness in neutral aqueous solutions and resistance to hydrolysis [6], which makes it suitable for many microfluidic applications. Ease of fabrication and lower input cost further improves its suitability for such devices. Microchannels on a polymer substrate can be fabricated using various techniques. Many rapid prototyping methods for microchannel fabrications were discussed by Sollier et al. [2]. Chen et al. [7] utilized micromilling process for fabricating microchannels on PMMA. Apart from it, etching [8], lithography [9], injection molding [10], imprinting and embossing [11] etc. have also been used by various researchers. Lasers as a fabricating tool offer distinct advantages over other methods in terms of cost, simplicity, simple processing and lower time consumption [12]. In fact, lasers acts as precise “thermal tools” for rapid fabrication of polymer based microfluidic devices [13]. PMMA has been found to possess high absorbance in infrared zone [14]. Due to this, CO2 lasers are widely used for PMMA processing. Apart from CO2 laser, ultraviolet lasers [15], excimer lasers [16] and ultrashort lasers [17] have also been used for micro-structural fabrication on it. However, femtosecond lasers and excimer lasers suffer from low etching rate, high input cost and higher maintenance requirements [18]. On the other hand, with the advent of low cost commercial CO2 laser, the microchannel fabrication process has become lot more easier and cheaper. CO2 laser has been found to be especially suitable not only for machining PMMA [19, 20] but also for altering its surface properties like wettability [21]. CO2 lasers mostly ablate the surface via thermal processes [22]. Microfabrication on PMMA by CO2 laser has been studied and well presented by number of authors over the years. Klank et al. [20] has studied the theoretical aspects of CO2 laser microfabrication on PMMA. Similarly, Snakenborg et al. [23] has utilized the one dimensional heat transfer equation to model the laser photothermal ablation process. A simple analytical model for single-pass microchanneling process was given by Prakash and Kumar [24]. A single-pass CO2 laser machining suffers with common defects such as bulging around the rims, splashing, resolidification and high surface roughness due to photothermal ablation mechanism and high thermal gradient in a localized cutting zone [18]. Minimizing the photothermal energy deposition may reduce such defects. In order to minimize the heat related defects, few techniques have been suggested by some researchers. Huang et al. [25] used preheating of PMMA surface at 70 - 90 o C during the microfluidic chamber fabrication process and approximately 100.86 nm of surface roughness was achieved. Microfluidic circular chambers were fabricated with 3 mm diameter and 2 mm height. Cheng et al. [26] did thermal annealing of the microchannels after the laser fabrication process to obtain high quality surfaces. Hong et al. [6] and Prakash & Kumar [27] used defocused processing for obtaining high quality wall surface. Multi-pass processing instead of singlepass fabrication is one such technique advocated by few authors. Multi-pass processing results in better surface integrity and topography apart from lowered heat related defects. Li et al. [28] presented a two-pass processing for minimizing bulging in CO2 laser microfabrication on PMMA. Samant and Dahotre [29] also established that multi-pass processing results in minimum thermal damage and cracking. The basic reason for reduced thermal damage may be attributed to lower thermal energy deposition in each pass compared to single-pass processing. In spite of significant advantages of multi-pass processing for microchannel fabrication on PMMA, no detailed experimental study of multi-pass CO2 laser microchanneling on PMMA is available in existed literature to the best of author’s knowledge. Also, to the best of author’s knowledge, no model was found to be existed for predicting microchannel dimensions when subjected to multi-pass CO2 laser processing. In the previous study [24], an analytical model for single-pass and two-pass was developed. However, the same model may not be extended for more number of passes because of many complex parameters. In this research work, a detailed experimental study on multi-pass CO2 laser microchanneling has been presented. The effects of laser parameters on microchannels have been elaborated. A physics based analytical Pagemodel 3 of 29 model has also been presented for predicting the microchannel profiles for multi-pass processing. The 2

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Figure 1: Reflectivity at different angles of incidence

Material properties and material removal mechanism

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takes into account for the change in beam focus in each pass as well as change in thermo-physical properties due to multiple passes of laser scanning.

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CO2 laser ablation is a kind of photothermal ablation in which the material removal takes place by the process of heating, melting and vaporization. CO2 laser ablation depends heavily on chosen material properties. PMMA is an amorphous thermoplastic whose properties vary from grade to grade. Even belonging to same grade does not guarantee the same material property and it may differ depending upon manufacturing procedures. Therefore, it becomes very important to determine material properties for better insight into the photothermal ablation process. In CO2 laser photothermal ablation process, laser beam is tightly focused over the work-piece surface confined in a very tiny spot. This focused laser beam interacts with material for removal of material. In this process, spectroscopic properties (absorptivity, reflectivity and transmissivity) of PMMA for CO2 laser beam wavelength play a very crucial role. Spectroscopic tests were performed to determine these material properties. Therefore, for complete understanding of material removal process of PMMA, it is necessary to determine the thermo-physical properties of it and observe the heating, melting and vaporization process quantitatively. DSC (differential calorimetric analysis) and TGA (thermo-gravitymetric analysis) tests were employed to determine PMMA’s thermo-physical properties and decomposition behaviour.

Spectroscopic and thermo-physical properties

In this work, 3 mm thick commercially available transparent PMMA was used. At CO2 laser beam wavelength i.e. at 10.6 µm, the reflectivity of flat PMMA surface was found to be 4.67% while the transmissivity was 0.31%. Therefore, the absorptivity of PMMA can be calculated to be 95.02% [24]. Further, PMMA’s reflectivity and absorptivity at different angles of incidence for randomly polarized beam was found to be varying from 4.31% at 0 degree of incidence to 100% at 90 degree of incidence (figure 1). On a CO2 laser system, PMMA undergoes various physical changes before being completely vaporized. All these changes take place in a very small interaction time (often in milliseconds). Phase transition, melting, decomposition and vaporization history of PMMA were recorded utilizing simultaneous TGA/DSC tests (Perkin Elmer STA 6000, USA). In multi-pass processing, laser head moves over a flat as-received surface in first pass and on ablated surface in consequent passes. An example of an ablated surface is given in figure Page 4 of 29

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Figure 2: Ablated structure after first pass

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2. Heat affected zone (HAZ) can be visualized and measured surrounding the microgrooves in these optical micrographs taken on Olympus optical microscope (Japan) as shown in figure 2. Typical dimensions of these HAZ on the top of the surface may lie around 55 - 150 µm for microchanneling applications. The part of material residing in HAZ may possess different thermo-physical properties compared to as-received sample surface. Since the laser ablation after first pass takes place in this HAZ region, it becomes important to determine thermo-physical properties of material residing in this zone too. For evaluating thermo-physical properties of PMMA, two as-received PMMA samples were taken. One of the as-received surfaces (sample 1) was tested in as it is condition on TGA/DSC equipment. Another sample (sample 2) was first heated beyond its glass transition temperature (before the start of decomposition) and then brought back to room temperature for resembling to material residing in HAZ region. The results were found to be consistent in all three tests. After this, sample 2 was taken to regular TGA/DSC test. For both the samples, three number of tests were performed and average of their values were selected for plotting the TGA/DSC curve and material property determination. Figure 3 shows the TGA (figure 3 (a)) and DSC (figure 3 (b)) plots for both kinds of samples. TGA plot (figure 3 (a)) does not reveal any significant difference between both the samples. Both the samples undergo glass transition, melting, decomposition and vaporization at the same temperature. However, DSC plot (figure 3 (b)) for both the samples differ from each other. Enthalpy of vaporization of the material residing in HAZ was found to be 4560 kJ/kg which was significantly higher than as-received PMMA sample (2231.78 kJ/kg). All other thermo-physical properties of the material remain same as given in reference [24] .

Material removal mechanism in multi-pass processing

Material removal mechanism of as-received PMMA is commonly known and already mentioned by many authors [5, 20, 30]. TGA results show that in both the cases i.e. as-received sample and sample of material residing in HAZ, the material undergoes a glass transition at around 110o C. Further, melting of both the samples start at 160o C and decomposition initiates at 230o C lasting up-to 393o C. PMMA (C5 O2 H8 )n decomposes into monomers, carbon dioxides, carbon monoxide and water. These byproducts are perfectly volatile in nature and results in formation of visible white plumes in surroundings. DSC results signify the involvement of larger amount of latent heat in vaporization process of sample 2. Possible reasons for increase in latent heat lie in complex cross-linking inside the material. Once, the material is heated up beyond its glass transition temperature, the degree of freedom for molecular chains also changes. The difficulty in movement of these chains may enhance the requirement of energy for further phase transitions. However, the exact cause and nature of this phenomenon is still unknown and may require further research. Page 5 of 29

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Figure 3: (a) TGA plot, and (b) DSC plot for PMMA samples, Hvp denotes enthalpy for as-received sample while, HvHAZ denotes enthalpy for sample 2.

Experimental details

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The experiments were conducted on a 3 mm thick, commercially available cast acrylic sheet using commercial CO2 laser (VLS 3.60, USA) system. Each PMMA specimen was created in size of 30 mm width and 30 mm length using the CO2 laser. Microchannels were carved on each specimen with different beam power and pass settings. The beam diameter at focus was found to be 237µm. All the experiments were conducted at a constant 900 PPI (pulse per inch) and 50 mm/s of scanning speed while beam power and number of passes were varied at different levels. A range of experiments were performed to determine the effects of these input parameters on output parameters viz. microchannel depth, microchannel width, surface roughness and heat affected zone (HAZ). Power was varied at four levels (1W, 1.5 W, 2W and 3W) and experiments were performed for total of seven numbers of passes for each power setting. As experimentally observed, sufficient time gap was maintained between each passes so as to ensure normal room temperature at the surface in each subsequent passes. Total twenty eight (28) number of experiments were performed according to full factorial technique. Experiments were performed following the standard randomization practice of design of experiments (DOE). Apart from these designed experiments, few other experiments were also conducted in which 500 µm depth microchannels were etched in 1 to 7 numbers of passes. In these experiments power was kept constant while speed was varied so as to result in a 500 µm depth microchannels Details of such experiments have been given in following sections. The output dimensions of microchannel were measured by Olympus 3-D microscope (STM6) using different objective lenses. Average surface roughness values (Ra) were measured using Zygo Zegage surface profilometer and Mitutoyo SJ-400 stylus based surface roughness tester. The application of surface profilometer was limited to low depth microchannels. For deep microchannels, stylus based surface roughness was found to be more suitable than optical surface profilometer. The tip diameter of stylus used was 2 µm. The procedure adopted for measuring the surface roughness of microchannel wall is given in figure 4. Each sample was measured at three different locations across microchannel length. The average value has been used to depict the surface roughness variation in this work. Details of input parameter values with corresponding output parameter values have been given in table 1. In the table, parameters have been sorted according to laser power and number of passes subsequently. These parameters have been depicted in graphical forms in following sections. Page 6 of 29

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Table 1: Details of input and output parameters Surface RoughExp. Power No. of Width Depth ness, Ra, No. (W) pass (µm) (µm) (µm) 14.2±2 15 1 1 171 54 6.7±1 11 1 2 180 87 3.97±0.6 12 1 3 185 120 1.86±0.1 6 1 4 189 150 0.6±0.1 25 1 5 189 185 0.44±0.1 20 1 6 192 210 0.3±0.1 17 1 7 192 240 13.76±1.9 19 1.5 1 206 78 5.98±1 2 1.5 2 209 140 3.3±0.6 16 1.5 3 206 200 1.1±0.3 8 1.5 4 206 240 0.54±0.1 7 1.5 5 204 275 0.36±0.2 23 1.5 6 210 320 0.18±0.1 10 1.5 7 204 360 11.5±1.7 21 2 1 216 100 5.5±0.8 18 2 2 216 185 2.87±0.7 3 2 3 223 265 1±0.3 26 2 4 223 330 0.45±0.2 5 2 5 223 400 0.24±0.1 28 2 6 218 475 0.14±0.1 13 2 7 219 520 6.37±1.1 22 3 1 236 148 4.36±1.3 4 3 2 234 265 1.7±0.7 14 3 3 240 385 0.86±0.2 24 3 4 244 490 0.2±0.1 27 3 5 238 600 0.17±0.1 9 3 6 244 710 0.11±0.1 1 3 7 244 800

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4.1

Experimental result analysis Microchannel width

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Figure 4: Surface roughness measurement using stylus probe (a) laser fabrication of microchannel on a thin PMMA sheet, (b) breaking the sheet along the channel and (c) surface roughness measurement using Mitutoyo surface roughness tester

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Microchannel width generally increases with increase in power due to larger power deposition on the workpiece surface. Figure 5 shows the width variation at different power settings in different number of passes. It was found that at each power setting, width is larger in corresponding pass at larger power setting. At any particular power setting, width was found to be varying in different passes. For example at 1 Watt of power, microchannel width was found to be varying from 171 µm to 192 µm (11.11% change) during all the seven passes. However, at higher power settings, the variation in width in different passes was found to be smaller than lower power settings. Only 3% increase in width was observed at 3 Watt setting in all the seven passes. Width was found to be increasing or decreasing in subsequent passes at any particular power setting. The increase of width in different passes was observed to be extremely small. In many cases, width was found to be remaining constant in subsequent passes (experiment number 6&25, 20&17, 2&16 etc.). Width was even found to be decreasing by a very small magnitude in subsequent processes. In experiment number 7, (1.5 W, 5th pass), width was found to be 2 µm smaller than previous 4th pass experiment (experiment number 8). The decrease in width may be attributed to extremely small amount of swelling of the top surface due to formation of softened zone (HAZ) around the microchannels. Thus a multi-pass processing may be employed to increase the depth without significant increase in the width, thus producing high aspect ratio microchannels which are desirable in many microfluidic applications. Further experiments were conducted to compare the width of microchannels of equal depth but fabricated in different number of passes. Microchannel of depth of 500 µm was fabricated by utilizing one to seven numbers of passes respectively (figure 6). Width of the channel fabricated in one-pass was found to be 48 µm more than width of seventh-pass, thus producing narrower width. Therefore, it can be concluded that any microchannel of a fixed depth, fabricated in larger number of passes possesses smaller width compared to one fabricated in lesser number of passes for CO2 laser microchanneling process on PMMA.

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Figure 5: Width variation with Power in different passes

Figure 6: Width of microchannel of depth 500 µm fabricated in different numbers of passes, corresponding power (P in Watt) and scanning speed (U in mm/s) given below them was utilized in fabricating the microchannels with different number of passes respectively. Page 9 of 29

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Figure 7: Depth variation with power in different passes

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Figure 8: Microchannel depth in different passes at 1.5 W and 50 mm/s, (given scale is of 200 µm)

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Microchannel depth is most important deciding factor for most of the microfluidic applications. Many microfluidic devices need high aspect ratio [31], while some rely on low aspect ratio. High aspect ratio microchannels result in high surface contact area and fluid throughput which are specially useful for many chemical devices [32]. Depth of microchannel increases with increase in power and passes. The variation of microchannel depth has been shown in figure 7. It should be noted that depth increase at same power deposition in each pass does not multiply directly. Instead, depth increase becomes smaller after first pass due to increased enthalpy and inclined walls of microchannels allowing larger energy requirement and less energy to be absorbed respectively. Figure 8 shows the microchannel depth at 1.5 W and 50 mm/s in different numbers of passes. In the first pass, the depth goes to 78 µm and thereafter, increase in depth in each pass varies between 62 µm to 35 µm. However, no particular trend was observed for this kind of difference in different passes. In fact, the maximum difference in depth in secondary passes was found to be 85% (experiment number 18) and minimum 44.87 % (experiment number 7) of the original depth obtained in first pass (experiment number 21 and 1 respectively). In other passes, the depths in subsequent passes were found to be varying between these two limits.

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Surface roughness and morphology

The average surface roughness, (Ra) was found to be improving significantly in each pass at each power setting. Figure 9 shows the surface profiles generated from Zygo surface profilometer after 1-pass, 3-pass, 5-pass and 7-pass respectively. The variation of surface roughness with different numbers of passesPage has 10 been of 29 9

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Figure 9: Microchannel surface profile generated from surface profilometer (a) after 1-pass, (b) after 3-pass, (c) after 5-pass and (d) after 7-pass

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depicted in figure 10. It can be observed that surface roughness in the first pass remains excessively high. This is particularly high at low power settings. The surface roughness decreases drastically in fourth pass at all the power settings. The decrease in surface roughness continues with each pass however, no significant difference was observed in sixth and seventh pass. The surface roughness can be brought to the level of approximately 0.1- 0.2 µm using multi-pass processing in almost all the power settings. Further figure 11 shows the surface roughness variation for a 500 µm depth microchannel fabricated in different numbers of passes. Microchannel fabricated with seven numbers of passes was found to be of best quality i.e. 0.18 µm. PMMA particles of a as-received surface are generally mono-spaced and consist of micro-spheres of approximately equal sizes [33]. Figure 12 shows the SEM image of center of a microchannel. The image shows non-uniformly dispersed PMMA particles. The PMMA micro-sphere sizes also vary because of unequal heat distribution at different locations inside the microchannel. Figure 13 shows the SEM micro-graphs of the microchannel walls resulting due to different numbers of passes. It can be seen that in each subsequent passes, the channel walls becomes smoother than previous passes. Of all the seven passes, the microchannel resulting from seven numbers of passes was found to be smoothest one. The difference in elemental weight of carbon and oxygen inside the microchannel in each pass was studied using energy dispersive X-ray (EDX) analysis (figure 14). Table 2 shows the weight percentage and atomic weight percentage of C and O elements of PMMA. Raw MMA monomer consists of five C and two O atoms. Thus the weight percentage of C/O in as-received PMMA is 1.88. It was found that C/O ratio increases in each pass. The increase in weight percentage of C (carbon) signifies more amount of charring/melting taking place after each-pass thus increasing the re-solidifying carbon content inside the microchannel. Measurement error in EDX was found to be less than 2% while the amount of carbon increased in HAZ region after each pass. Page 11 of 29

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Figure 10: Surface roughness (Ra) variation with power in different number of passes

Figure 11: Surface roughness (Ra) for 500 µm depth microchannel fabricated in different number of passes

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Figure 12: Scanning electron microscopic image of center of a microchannel at the inside lower end

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Table 2: Energy dispersive X-ray composition analysis consisting of C and O elements PMMA EDX result C O Weight % 64.17 35.83 as-received Atomic wt. % 70.46 29.54 Weight % 66.39 33.61 1-Pass Atomic wt. % 72.46 27.54 Weight % 67.19 32.81 2-pass Atomic wt. % 73.17 26.83 Weight % 68.04 31.96 3-Pass Atomic wt. % 73.93 26.07 Weight % 68.80 31.20 4-Pass Atomic wt. % 74.60 25.40 Weight % 69.04 30.96 5-Pass Atomic wt. % 74.81 25.19 Weight % 69.46 30.54 6-Pass Atomic wt. % 75.18 24.82 Weight % 69.59 30.41 7-Pass Atomic wt. % 75.30 24.70

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Figure 14: EDX spectra of PMMA

Heat affected zone (HAZ)

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HAZ plays a very crucial role in bonding process in microfluidic devices. HAZ causes swelling of top surface producing bulging/bumps on the channel edges and ends measuring from 1µm to 10 µm. Figure 15 shows the SEM (scanning electron microscopy) image of bulges on the top surface and at the channel ends. Primary reason for formation of these bulges is accumulation of heat into a localized zone due to low thermal conductivity of the material. Some part of this heat, which is neither sufficient to vaporize or melting of the material nor being able to be conducted into the material immediately, manifests itself in formation of bumps/bulge. The excessive bumps may produce hindrance in leak proof bonding of the upper surface of the microchannels. HAZ also creates problem in optical viewing in many applications. Thus, HAZ should be minimized to most possible extent. In CO2 laser processing, being a thermal ablated process, it’s impossible to eliminate heat affected zone completely. However, this can be minimized using multi-pass processing. Figure 16 shows the HAZ variation with power in different passes. HAZ increases with increase in power. HAZ either remains constant or increases in secondary passes but never decreases. In most of the cases HAZ remains constant in subsequent processes. Even if the HAZ increases, the extent of increase is extremely small (of order of 1-2 µm). Experiments were carried out to determine HAZ of a microchannel of depth 500 µm fabricated utilizing different numbers of passes. Figure 17 demonstrates the HAZ difference in a 500 µm depth microchannel fabricated in 1-pass and 7-pass. It can be noticed visually that HAZ has been considerably reduced in 7pass processing compared to 1-pass processing. Figure 18 shows the HAZ variation in different numbers of passes for 500 µm depth microchannel. It can be observed that in multi-pass processing, HAZ may decrease to approximately half in 7-passes compared to HAZ resulting from one-pass processing. Also, an abrupt decrease in HAZ was found when a two-pass processing was carried out (106 µm to 70 µm) instead of singlepass processing. HAZ decrease in subsequent passes become smaller.

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Figure 15: Bulging on the top surface and channel ends

Figure 16: HAZ variation with power in different passes Page 16 of 29

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Figure 17: HAZ in a 500 µm depth microchannel resulting from 1-pass and 7-pass

Figure 18: HAZ for a 500 µm depth microchannel fabricated in different number of passes

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Microchannel profile

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Figure 19: (a) Microchannel profile comparison in different passes, (b) lower end comparison of microchannels resulting from different numbers of passes (c) channel profile comparison by imposing the channel profiles resulting from all seven passes (color coding for different number of passes remain same in (b) and (c))

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Microchannel profiles were observed for different power settings and different numbers of passes. Figure 19 shows the microchannel profile at 2 W and 50 mm/s for all the seven number of passes. The lower ends of the microchannels are compared in figure 19 (b). It was found that in each pass the lower end of the microchannel narrows down. The profiles of 500 µm depth microchannel fabricated in one-pass and seven-pass can also be visualized in figure 17. Here also, narrowing down of the microchannel walls can be visualized significantly. Narrowing of microchannel was observed to be the primary indication of multi-pass processing. The primary reason behind this phenomenon is the falling of laser beam on inclined wall in secondary passes instead of flat surface of the first-pass. The wall inclination in each pass increases thus making the channel wall less tapered. With the increase of wall inclination, the amount of beam reflected by the surface increases and absorptivity of the surface decreases. This makes the lesser amount of material to vaporize from side walls thus narrowing down of the walls. It can also be visualized that wall narrowing starts as the wall inclination becomes higher because of rapid increase in reflectivity of the material at higher inclination angles. At 2 W and 50 mm/s, the angle of inclination increases from 53o in first pass to 83o in seventh pass. With the increase of wall inclination angle, surface tapering phenomenon also decreases. Figure 19 (c) was drawn by imposing the microchannel profile resulting from all seven passes at 2 W and 50 mm/s. Channel walls of secondary passes were found to be overlapping each other. This indicates that the amount of material removal from the side walls is extremely small in subsequent passes. Majority of the material removal takes place from the lower end of the microchannel. It was also observed that during secondary passes, small amount of swelling takes place on side walls thus minimizing the width of the channel by a very small magnitude over the entire cross-section.

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Analytical modeling for multi-pass processing

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PMMA is a thermoplastic amorphous material. The thermal conductivity of PMMA is very low which ultimately results in high thermal diffusion time. In case of CO2 laser photothermal ablation of PMMA, material removal takes place due to heating and vaporization. Only a small amount of melting takes place in this process. The energy gets stored locally as soon as it is deposited on the surface. Developed analytical model is based on law of conservation of energy. Apart from very small interaction time, thermal conductivity of PMMA is also very small and as a result of both, energy loss due to conduction is extremely low. By assuming that the heat conduction loss is negligible, it can be conveniently assumed that all the laser beam energy absorbed by the PMMA substrate is equal to the energy utilized in vaporizing certain mass of material. The convection and radiation losses from the surface were also ignored in this modeling because of their small contributory part. The CO2 laser beam was assumed to be Gaussian in nature and circular in shape. The model was developed following the earlier approach by Prakash and Kumar [24]. However, the earlier developed model was restricted for one-pass and two-pass only and was not capable of extending to multi-pass processing. The earlier developed model also not accounted the change in focus and material properties in second pass. Further, the wall inclination angle was also not considered for secondary passes. The present model has been developed for more number of passes of laser scanning. Further, modifications have also been proposed including change in focus in each pass, change in absorptivity with wall inclination and change in the value of enthalpy of vaporization as further described. For the first-pass of laser scanning, it can be assumed that the laser beam of initial radius w1 at the focal plane and power P1 travels in the direction of x-axis with speed U1 on PMMA substrate (figure 20).

Figure 20: Multipass laser microchanneling process schematic, laser moving over an already etched surface The PMMA sheet was kept at the focal plane of focusing lens. The depth after the first pass is given by [24]: z1 = z1max e

−2y 2 2 w1

(1)

where z1 is the microchannel depth at any y−location of the microchannel after the first pass and z1max is the Page 19 of 29

18

maximum microchannel depth lying on the center line, i.e. at y = 0 and is given by: s α 2 P1 z1max = ρ × Hvp πw12 U1

(2)

w = w1

2 #1/2  λ(z + δ ) f 1 + M2 πw12

us

"

cr

ip t

where α is the absorptivity at zero angle of incidence (perpendicular to the striking surface), ρ is material density and Hvp is enthalpy of vaporization of as-received PMMA. In the second pass, laser moves over an already ablated surface. This causes defocusing of incident laser beam over the next surface to be ablated. The defocusing of the beam increases as the depth increases. This causes change in energy density of incident laser beam. Therefore, change in beam radius after each pass should also be taken into account. The beam radius after each pass can be determined using equation 3 as below [34]:

(3)

Ac ce p

te

d

M

an

where w is the beam radius at depth z from the surface, w1 is initial beam radius at focal plane, M2 is beam quality parameter, λ is the laser beam wavelength and δf is distance between focal plane and work-piece. Further, the values of thermo-physical properties of the newly evolved laser ablated surface also differ from as-received surface. In secondary passes, the surface which is going to be ablated may consist of HAZ and as-received part. Ablation from side walls are considerably smaller than ablation from lower ends. In fact the ablation from the side walls fully occur within HAZ part while at the lowest end it consist of a mixed zone comprising of immediate HAZ part followed by as-received part. Determination of exact distribution of these two zones around the microchannel profile is tough to be determined analytically. Let Hvm represents enthalpy of vaporization of the mixed zone whose value was approximately considered to be the average of enthalpy value of as-received zone and HAZ part. Further, the ablated zone on the side walls are very small and therefore for simplifying the model,Hvm was considered as enthalpy of overall ablating surfaces in secondary passes. (The consequences of this assumption will be discussed in section 7.) Let w2 represents beam radius at the surface ablated in first pass. Let P2 and U2 represent power and scanning speed in second pass. Since CO2 laser ablation takes place in direction of incident beam, in each pass a longitudinal shift of the ablated profile can be assumed. Using equation 1 and 2, the equation for maximum depth after second-pass can be written as;

z2max = z1max +

α ρ × Hvm

s

2 P2 πw22 U2

In the second pass, some part of the beam falls on inclined plane which is symmetrical on both sides of channel, rather than a flat surface as in first-pass. This causes a change in angle of incidence of laser beam on the surface. Since absorptivity of the beam also depends upon angle of incidence, it is necessary to introduce a variable absorptivity factor (α(φ)), where φ is angle of incidence. It can be assumed that, there is no transmissivity of CO2 laser beam from the inclined surfaces, since the surface has become rougher in secondary passes. Variable absorptivity factor (α(φ)) can be written as : α(φ) = 1 − <(φ)

(4)

The values of variable reflectivity (<(φ)) can be determined from the data obtained from angle of incidence test (figure 1). Angle of incidence at any point on the plane is equal to slope of that point and can be Page 20 of 29 determined by differentiating the surface profile with respect to y-axis (figure 21). 19

ip t cr us an

M

Figure 21: Angle of incidence

d

Hence, similar to the first pass equation 1, the equation for depth profile for second pass can also be written as: 2

te

z2max × α(φ) −2y 2 e w2 z2 = α

(5)

Ac ce p

Similarly, for third pass, it can be written as

z3

and

z3max × α(φ) −2y 2 = e w3 α

z3max = z2max

α + ρ × Hvm

s

2

2 P3 πw32 U3

Hence, for the nth pass, the equation for microchannel profile and maximum depth can be written as;

zn =

2 zn, max × α(φ) −2y e wn2 α

(6)

and zn max = z(n−1),max + 20

α ρ × Hvm

s

2 Pn πwn2 Un

(7)

Page 21 of 29

Equation 6 and 7 can be used to determine microchannel profile and maximum depth after n number of passes. In case of same output power (P) and scanning speed (U) in all the passes and considering that beam radius (w) change in each pass remains negligible (considering in micron range), the maximum depth in nth pass can be mathematically described by a simple equation 8: r   α 2 P 1 n−1 zn, max = + (8) ρ πw2 U Hvp Hvm

α Cn = ρ

r

2 π



1 n−1 + Hvp Hvm



us

where

P Cn Uw

cr

zn, max =

ip t

Equation 8 can also be written as:

(9)

(10)

r

2 [1 + (n − 1)ξ] π

(11)

M

α Cn = ρ Hvp

an

C is a constant whose value depends upon material properties and number of passes. vp = ξ, then equation 10 can be written as Let us assume that HHvm

Experimental verification of analytical model

Ac ce p

6

te

d

It can be seen that the values of constant C linearly varies with different numbers of passes. Hence, maximum depth can simply be prediction by using equation 9 and 11. Both the equations can also be used to determine power and scanning speed for achieving a definite depth in pre-decided number of passes. The value of constant C will vary according to different grades of PMMA.

The verification of the predicted depth profile was conducted to assess developed model’s applicability. The predicted channel profiles for each microchannel were plotted using equation 6 and 7 in order to compare it with actual profiles using same dimensional scale (figure 22). The value of Hvm was taken as an average value of enthalpy of as-received zone and HAZ part (= Hvp +H2 vHAZ ). Microchannel profiles were plotted with the help of a small program written in MATLAB. Plotted microchannel profile has been superimposed on the optical micrograph of actual microchannels for visual comparison. In figure 22, the microchannels shown, were etched at 3 Watt power and 50 mm/sec. for all the seven passes respectively. Predicted channel profiles were found to be close to actual microchannel profiles (figure 23. Percentage error in depth prediction was calculated as shown in table 3.Maximum error was found to be 10.342% while minimum error was recorded to be 0.464% in prediction of maximum microchannel depth.

7

Discussion

PMMA absorbs 95% of total CO2 laser beam energy [24]. Due to this high absorptivity, the CO2 laser beam machining of PMMA incurs very small loss of beam energy. When compared to single-pass processing, multipass laser processing involves less energy deposition in each pass. In order to deposit less energy, √ either the power is lowered or the scanning speed is increased or both. The thermal diffusion length lt u 4αth ti , (αth is thermal diffusivity and ti is interaction time) determines the heat propagation in short interaction time ti . Hence, the increased scanning speed results in lower diffusion length. Total thermal spread also depends

Page 22 of 29

21

ip t cr us an M d te Ac ce p Figure 22: Predicted and actual channel profile for microchannels fabricated at 3 W and 50 mm/s in (a) 1-pass, (b) 2-pass, (c) 3-pass, (d) 4-pass, (e) 5-pass, (f) 6-pass, (g) 7-pass

Page 23 of 29

22

ip t

cr

% error

0.67 2.164 3.430 6.221 5.149 9.376 10.342 3.151 4.777 6.936 0.464 6.025 7.548 10.175 7.277 3.932 6.383 3.543 2.913 3.543 1.487 8.727 0.563 3.439 3.672 3.200 3.618 1.627

us

an

M

d

Ac ce p

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

te

S. No.

Table 3: Percentage error calculation Exp. Power No. of Actual Predicted No. (W) pass depth depth (µm) (µm) 15 1 1 54 53.64 11 1 2 87 88.88 12 1 3 120 124.12 6 1 4 150 159.33 25 1 5 185 194.53 20 1 6 210 229.69 17 1 7 240 264.82 19 1.5 1 78 80.46 2 1.5 2 140 133.31 16 1.5 3 200 186.13 8 1.5 4 240 238.89 7 1.5 5 275 291.57 23 1.5 6 320 344.16 10 1.5 7 360 396.63 21 2 1 100 107.28 18 2 2 185 177.73 3 2 3 265 248.08 26 2 4 330 318.31 5 2 5 400 388.35 28 2 6 475 458.17 13 2 7 520 527.73 22 3 1 148 160.92 4 3 2 265 266.49 14 3 3 385 371.76 24 3 4 490 476.58 27 3 5 600 580.80 9 3 6 710 684.31 1 3 7 800 786.99

Page 24 of 29

23

ip t cr us an

M

Figure 23: Comparison of actual and predicted maximum depth in different experiments

Ac ce p

te

d

upon energy deposition by laser beam apart from thermal diffusion length. As a conclusive result of multipass processing, the extent of total thermal spread length is lowered either due to low beam power or high scanning speed in each different pass. The thermal spread during the laser processing may lead to significant changes in various material properties. The region over which these property changes take place is also called heat affected zone (HAZ). A small thermal spread results in smaller HAZ. Reduction of heat affected zone also minimizes the various thermal defects during multi-pass processing. Multi-pass processing hardly affects the microchannel width and heat affected zone in different passes for the same power and scanning speed setting in each pass. This also indicates that laser beam energy do not spread in direction of width or HAZ substantially. Understanding of surface roughness phenomenon is a complex issue in multipass processing. Figure 24 was drawn to show the relative beam intensity variation with different power at a fixed scanning speed. Different relative fluence values have been described in this figure (not exactly scaled). HAZ fluence is the beam intensity at which HAZ formation starts. Similarly, melting and evaporating fluence describe the amount of energy required to start melting and pure vaporization process. Above evaporating fluence, PMMA quickly evaporates. Required amount of energy is equivalent to area under the respective fluence lines. Zone between evaporation and melting fluence determines amount of melting taking place for a particular beam intensity value. Vaporization starts just above the melting fluence i.e. evaporation start line. Pure melting occurs between melting fluence and evaporation start line. Between evaporation start line and evaporation fluence, there exist a mushy zone. Exact material behaviour in this zone is quite unpredictable as PMMA melts and evaporate in this zone unpredictably. It also involves large energy band. Considering figure 10, surface roughness was found to be larger in low power settings (low beam intensities) compared to high power settings in initial passes. For low power, there is significant melting zone. At the same time, the unpredictable zone is also comparably large which results in non uniform material ablation causing resolidifying zone formation as shown in SEM image of microchannels in first pass (figure 13). For large beam intensities, larger part of energy remains available for pure vaporization. However, as soon as the amount of vaporized material increases, the increased amount of plume through narrow zones may cause significant absorption of input beam power and allowing only a part of it to go through. Also,Page the 25 large of 29 24

ip t cr us

an

Figure 24: Beam intensity variation with power

Ac ce p

te

d

M

amount of vapor plume exerts significant pressure on the melting walls causing non-uniform erosion resulting in rough surface formation. Though exact quantification of these phenomenon are extremely difficult due to unpredictable behaviour of PMMA, few empirical observations can be made on the basis of above facts. Surface roughness at 1 W power was observed to be much higher than 3 W because of larger melting part as well as larger availability of energy beyond vaporization fluence at 3 W in initial passes. Further, the depth of 148 µm due to 3 W power do not cause significant plume formation to exert uneven forces on microchannel walls. However, as soon as the power reaches certain threshold value so as to cause significant plume formation, the surface roughness may increase drastically with increasing power. Due to larger power involvement in single-pass processing, the dense vapor plume exerts eroding forces on melt walls causing surface to solidify irregularly. When compared to single-pass, multi-pass processing produces very small amount of vapor plume in each-pass which allows melt layer to remain on the microchannel walls. This low density vapor plume is not capable of eroding the surface but exerts the pressure force on the melt surface to settle down on the walls resulting in improved surface finish of the microchannels. Since, in multi-pass processing, heat energy in each pass is supplied at a lower rate compared to single-pass processing, significant melting phenomenon takes place on the material surface. This melting phenomenon becomes more significant in secondary passes due to increase of value of enthalpy of vaporization. The larger enthalpy of vaporization necessitates larger amount of required energy to start vaporization process. Hence a larger energy band remains available for melting phenomenon to take place. With this melting behaviour, thermocapillarity causes a net morphology change resulting in smoother channel walls. In each subsequent passes, the energy absorption of side walls decreases due to larger reflectivity and less absorptivity because of increased angle of inclination of walls. Most of the microfluidic devices require the surface roughness to be below 1 µm. In all the power settings, approximately five passes were required to reach the level of below 1 µm. Larger the number of passes better is the surface finish of microchannel walls. Using equation 8, it can be stated that maximum depth of microchannel is directly proportional to output power (P) and inversely proportional to scanning speed (U) and beam radius (w). These three parameters belong to laser beam properties. Other than these, maximum depth depends upon thermophysical and spectroscopic properties viz. density, enthalpy and absorptivity. Different grades of PMMA may possess different thermo-physical and spectroscopic properties depending upon manufacturer and method of preparation. Therefore, a pre-determination of these properties is essential for applying this equation. In this work, equation 7 has been used for verification of the theoretical results. For determination of material removal on side walls, the enthalpy value of mixed zone was used but in actual conditions it consist of Page only 26 HAZ of 29 25

Conclusion

an

8

us

cr

ip t

part. This causes an error in exact prediction of microchannel walls. Due to this reason, side wall predicted profile was larger than actual profile since lower value of enthalpy was used in its determination resulting in larger ablation (figure 22). Further, due to refractive index of the material, the beam propagation direction varies slightly than assumed but does not affects significantly. However, for maintaining the simplicity of the model, change in refractive index phenomenon was not considered. The mismatch in predicted wall profile and actual wall profile may also be attributed to this. However, no significant error in maximum depth prediction occurs either due to enthalpy assumption or refractive index changes since on the lower ends of microchannels consist of mix parts of HAZ region and as-received zone and beam remains almost perpendicular to the striking surface. It can be seen that at the lower ends, the predicted profile matches well with actual microchannel profiles. The error in sidewall predictions are quite small and can be weighed low on account of simplicity of the proposed model. The equation 9 can also be used for depth prediction without producing much error. In fact equation 9 is a simplified version of equation 7 which does not includes change in beam diameter term. It should be noted that developed model is only applicable in micron ranges. For deeper depth, several other phenomenon like plasma formation, vapor absorption, multiple wall reflection etc. may become pre-dominant which have not been considered into this model.

Ac ce p

te

d

M

Multi-pass processing was found to improve the surface smoothness of microchannel walls. Multi-pass processing do not result in significant change in width or HAZ in multiple passes. A multi-pass microchanneling process can produce high aspect ratio microchannels with significantly reduced width when compared to microchannels fabricated in single pass. HAZ can also be minimized to almost half using multi-pass processing. Microchannel depths, etched in secondary passes were found to be smaller compared to first-pass. The maximum achievable depth in secondary passes was found to be varying between 85% to 44.87% of the depth achieved in first-pass. For the same energy deposition setting in each pass, microchannels were found to become narrower in each pass. Wall inclination angle was also observed to be increasing in each pass. This in turn results in lowering of surface wall tapering since the walls become straighter in multi-pass processing. Enthalpy increase in secondary passes results in requirement of more energy to vaporize unit mass of PMMA compared to first-pass. Since laser processing results in photothermal ablation, defects related to heat are indispensable. Multi-pass processing results in smoother wall structure as well as clog free fabrication of microchannels compared to single-pass processing. The developed models can predict microchannel depth and profile for many number of passes with close approximation. Overall, it can be concluded that multi-pass processing presents a significant improvement over single-pass processing on the cost of time and money. However, these can be optimized for most favorable solution.

Acknowledgment

Authors acknowledge the financial support of Department of science and technology (DST), government of India for providing INSPIRE fellowship to one of the author.

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