Study of Impingement Types and Printing Quality during Laser Printing

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Study of Impingement Types and Printing Quality during Laser Printing of Viscoelastic Alginate Solutions Zhengyi Zhang,† Ruitong Xiong,† David T. Corr,‡ and Yong Huang*,† †

Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611, United States Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States

‡

ABSTRACT: Laser-induced forward transfer-based laser printing has been being implemented as a promising orificefree direct-write strategy for different printing applications. The printing quality during laser printing is largely affected by the jet and droplet formation process and subsequential impingement. The objective of this study is to investigate the impingement-based printing type and resulting printing quality during the laser printing of viscoelastic alginate solutions, which are representative inks for soft structure printing such as bioprinting. Three printing types are identified: dropletimpingement printing, jet-impingement printing with multiple breakups, and jet-impingement printing with a single breakup. Printing quality, in terms of printed droplet morphology and size, has been investigated as a function of alginate concentration, laser fluence, and direct-writing height based on a time-resolved imaging approach and microarrays of printed droplets. Of these, the best printing quality is achieved with single-breakup jet-impingement printing, followed by multiplebreakup jet-impingement printing, with droplet-impingement printing producing the lowest quality printing. The printing quality can be improved by using high-concentration alginate solutions. The increase of laser fluence may lead to a well-defined primary droplet for low-concentration alginate solutions; however, this can cause the droplet diameter to increase, which may not be desirable. The direct-writing height (i.e., ribbon coating-receiving substrate distance) also influences the print quality. For example, an increase in direct-writing height can cause the printing type to change from the ideal jet-impingement with a single breakup, to the jet-impingement with multiple breakups, and even the least desired droplet-impingement printing, with only slight variations in droplet diameter.

1. INTRODUCTION AND BACKGROUND Laser printing, a laser-induced forward transfer (LIFT)-based technique, is a promising orifice-free direct-write strategy for different printing applications. In particular, it has found increasing applications in printing soft tissue constructs.1,2 During typical laser printing, material to be printed is typically prepared in the form of viscous ink and coated to a laser pulsetransparent quartz support as a thin-film coating, collectively forming a ribbon. Laser pulses, such as ultraviolet, are then guided perpendicularly through the uncoated side of the ribbon and focused on the interface of the quartz support and coated ink. The localized heat generated due to the laser-matter interaction sublimes a small portion of the coating to form a hightemperature, high-pressure vapor bubble.3,4 The consequential bubble expansion results in different types of jets and eventually breaks up into several droplet(s). During a typical laser printing-based fabrication process,5 each structure is made in a pixel-by-pixel, then layer-by-layer fashion. Within a given layer, each continuous feature is made of a number of deposited droplets. While the droplet spacing has been studied for better line printing quality for a given droplet,4,6 how to control the shape and size of individual deposited © XXXX American Chemical Society

droplets is still largely elusive. For laser printing to be a viable precision fabrication technique, its printing performance must first be well understood in terms of the shape and size of printed features, mainly printed/deposited droplets. As numerous studies have been devoted to understanding the jetting dynamics and droplet formation processes during laser printing,7−11 the formation of printed/deposited droplets has been of great interest. It has been found that the size of printed droplets is dependent on operating conditions, such as the laser fluence,3,8,12 laser spot size,13,14 coating film thickness,15 and direct-writing height (the stand-off distance between ribbon coating and receiving substrate),12,16,17 and material properties such as the ink concentration.3,15,18 Generally, the droplet size increases with the applied laser fluence,3 laser spot size,13,14 and coating film thickness.15 While the droplet size is not sensitive to the direct-writing height over a specific range, as reported during glycerol-based solution printing,12,16,17 it is influenced by the direct-writing height during the printing of silver nanoparticle suspensions.13 Since the rheological properties, such as the Received: January 20, 2016

A

DOI: 10.1021/acs.langmuir.6b00220 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Laser printing schematic with an inset illustrating a printing process taken at 80 μs during the printing of 4% alginate solution at a 1500 mJ/cm2 laser fluence.

2. EXPERIMENTAL SETUP

viscosity, surface tension, and/or elasticity, are difficult to decouple in order to study their individual influence on the droplet size, the effects of ink concentration have typically been studied instead. It is reported that there is no monotonic relationship between the printed droplet size and the glycerol3 and alginate15 concentrations, respectively. The objective of this study is to investigate the impingementbased printing type and resulting printing quality during the laser printing of viscoelastic alginate solutions, which are representative inks for soft structure printing. There are two main sequential events during laser printing after initial transient laser-matter interaction and bubble formation: jet/droplet formation and jet/droplet impingement and deposition. To fully realize the potential of laser printing, it is indispensable to systematically study both the jet/droplet formation and deposition dynamics as a series of investigations. The former was investigated with some conclusions related to the jet morphology and breakup mechanism as well as the jetting regime,11 and the latter is studied under well-defined jetting and presented based on the jet/droplet impingement type herein. In this study, the printing type is classified based on the impingement type of transferred droplets, and the printing quality herein is evaluated based on printed droplets by using two metrics: (1) shape fidelity (i.e, whether the printed feature resembles the round shape of laser spot), and (2) size control (i.e., whether the size of the printed feature can be controlled as needed). The effects of material properties (represented by alginate concentration) and operating conditions on the impingement type and resulting printing quality during laser printing have been investigated. In particular, the operating conditions of interest in this study are the laser fluence and direct-writing height. As a widely used constituent of bioinks,19,20 sodium alginate has been chosen to prepare viscoelastic bioinks, and a time-resolved imaging approach has been implemented to examine the impingement-based printing types in this study. This paper begins with a description of the experimental setup, followed by general observations regarding the different impingement-based printing types, and a discussion on the effects of alginate solution concentration, laser fluence, and direct-writing height on the printing type and quality. Then, decision tree and phase diagrams are proposed to evaluate the printing quality during soft structure fabrication. Finally, it is summarized with main conclusions and future work comments.

2.1. Laser Printing Setup and Microarray Deposition. The laser printing apparatus in this study was implemented as matrix-assisted pulsed-laser evaporation direct-write (MAPLE DW) with alginate in a water matrix.3 It had an optical beam deliver system, an argon fluoride (ArF) excimer laser (Coherent ExciStar, 193 nm, 12 ns full-width halfmaximum, 2 Hz repetition rate), a ribbon consisting of an optically transparent quartz disk coated with a thin film of ink on its bottom side, and a poly-L-lysine coated glass slide (Polysciences, Warrington, PA) as the receiving substrate as shown in Figure 1. Computer-controlled XY translational stages (Aerotech, Pittsburgh, PA) were used to control the movement of the ribbon and the receptor substrate, respectively, with respect to the incident laser beam. The direct-writing height, the distance from the bottom of the ribbon coating to the receiving substrate, was adjusted by using a Z motion stage (Aerotech, Pittsburgh, PA), and the diameter of the resulting laser spot was controlled as 150 μm. A FieldMax laser power/energy meter (Coherent, Santa Clara, CA) was used to measure the intensity of applied laser fluence, and the quartz disk had a 15% energy loss. A JetXpert imaging system (ImageXpert Inc., Nashua, NH) was implemented11 to record the jet formation and landing processes based on a time-resolved imaging approach. Figure 1 illustrates a representative image taken during the laser printing of a 4% alginate solution at a laser fluence of 1500 mJ/cm2. The optical micrographs of printed microarrays were taken and processed using ImageJ (National Institute of Health, Bethesda, Maryland) to measure the diameter of printed/deposited droplets. Diameters of printed droplets were measured based on the average value of five representative printed droplets. For those printed droplets with irregular shapes and/or secondary droplets, the equivalent diameter was estimated based on their volume found by assuming a constant contact angle for all droplets. The jet velocity, U, was calculated by a linear fit of the jet head position dependence on the measurement time11,21 before the jet or droplet impinges the receiving substrate. 2.2. Ink Preparation and Printing Conditions. Alginate inks in this study were prepared by dissolving sodium alginate (Sigma-Aldrich, St. Louis, MO) into deionized water to make different alginate solutions with concentrations of 2%, 4%, 6%, and 8% (w/v). The material and rheological properties (density, viscosity, surface tension, and relaxation time) of the inks were characterized accordingly,11 and the kinematic viscosities were determined as 137, 823, 2947, and 7665 mm2/s, respectively. The inks were applied onto the quartz as coating materials using a blade coater (MTI, Richmond, CA) to form a 50 μm thick film for each ribbon. The printing experiments were conducted under different combinations of material properties and operating conditions. In particular, Setup 1 was to investigate the effects of material properties by changing the alginate concentration (2, 4, 6, and 8 w/v%) at laser fluences of 1100, 1300, 1500, and 1700 mJ/cm2, respectively; Setup 2 B


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Figure 2. Schematics of material deposition dynamics and representive images under different alginate concentrations and operating conditions. The direct-writing height was 2.0 mm, and the scale bar is 200 μm. was to study the effects of laser fluence, changing from 500 to 1300 mJ/ cm2 for 2% alginate solution, from 900 to 1700 mJ/cm2 for 4% alginate

solution, from 1100 to 2100 mJ/cm2 for 6% alginate solution, and from 1300 to 2700 mJ/cm2 for 8% alginate solution, all at an interval of 200 C


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3. EXPERIMENTAL RESULTS 3.1. General Observations. As observed, there might be five distinct jetting behaviors during the laser printing of viscoelastic alginate solutions.11 In particular, they can be classified into three regimes: no material transferring, well-defined jetting, and undesirable jetting with a bulgy shape or pluming/splashing as depicted in Figure 2. During no material transferring, the forming jet recoils back into the coating film without transferring any materials. For the jetting with a bulgy shape or pluming/splashing regime, the breakup of initial subjets, the second wind induced-breakup, and/or the atomization of forming jets lead to the formation of droplets randomly scattered over a large area of the receiving substrate. As such, both of these two regimes are considered undesirable during laser printing. It should be noted that the splashing mentioned herein means the disintegration of a forming jet, which is different from the splashing resulting from the jet/droplet impact on the substrate during deposition. In this study, only well-defined jetting, with or without an initial bulgy shape, is considered desirable and evaluated for printing quality. During well-defined jetting, with or without an initial bulgy shape, a forming jet may break up before impinging onto the receiving substrate. Depending on whether the direct-writing height (DWH) is larger than the breakup length (BL) or not, two printing types may result: (a) droplet-impingement printing (Case b1 in Figure 2), during which the direct-writing height is larger than or equal to the breakup length, and a well-defined jet may break up into several droplets before landing on the receiving substrate, and (b) jet-impingement printing (Cases (b2) and (b3) in Figure 2), during which the direct-writing height is smaller than the breakup length, and a well-defined jet reaches the receiving substrate before its breakup. For jet-impingement printing, either single breakup or multiple breakups may finally take place along the after-impingement jet. As such, the jet impingement can be further classified into two impingement types: jet-impingement printing with multiple breakups (Case (b2) in Figure 2) and jet-impingement printing with a single breakup (Case (b3) in Figure 2). During droplet-impingement printing and jet-impingement printing with multiple breakups, the trajectory of droplets resulting from multiple breakups of a long thin jet is easily affected by random perturbations, these divergent droplets may or may not be able to stack on the top of the primary droplet printed. This may lead to the formation of secondary droplets around it as shown in Figure 2(b1) and Figure 2(b2). In particular, it is observed that during droplet-impingement printing, a jet breaks up into several fragments and deposits onto the substrate, resulting in several similarly sized secondary droplets scattering around (Figure 2(b1)). During jet-impingement printing with multiple breakups, a jet keeps thinning and continues feeding into the primary droplet formed on the substrate as observed in a previous study.22 While most of the printed material is deposited as a primary droplet, the thinned ligament may form a beads-on-a-string (BOAS) structure, which breaks up into some divergent fragments, resulting in some small secondary droplets around the primary droplet (Figure 2(b2)). During jet-impingement printing with a single breakup, the jet breaks up into two parts: the top part retracts into the ribbon’s coating film and the bottom part is deposited on the substrate to form a single droplet without any secondary droplets (Figure 2(b3)), to produce the highest quality printing. The printing quality decreases when the jetimpingement printing has multiple breakups, and decreases further when the mode changes to droplet-impingement printing. Despite the reduced printing quality, for some practical printing applications, the presence of secondary droplets during laser printing is tolerated, especially if they merge with a primary droplet. Figure 3 illustrates the printing types under different alginate concentrations and laser fluences at a fixed direct-writing height of 2.0 mm, which was chosen to avoid undesirable contact between the coating film and the gelation solution during typical 3D bioprinting.4 The solid

Figure 3. Printing types as a function of alginate concentration and laser fluence under a 2.0 mm direct-writing height. The delineating solid lines are for illustration only. lines delineating these printing types are drawn here for illustration only. As shown in Figure 3, the printing type may change from no material transferring to droplet-impingement printing to jet-impingement printing to jetting with a bulgy shape and pluming/splashing, if applicable, as the laser fluence increases or the alginate concentration decreases. The best printing quality, achievable under jet-impingement printing with a single breakup, can be realized only at a certain range of laser fluence (medium value) and alginate concentration (high concentration). 3.2. Splashing and Its Threshold. The impingement of droplets onto a solid surface is of importance in many engineering applications including inkjet printing and spray cooling, and has been studied extensively over the past 100 years.23−25 Depending on the kinetic energy level of jets/droplets during impingement, there might be three outcomes: spreading to form a lamella, rebounding off the surface, or splashing. Due to the high elongational viscosity of viscoelastic alginate solutions, rebounding is usually suppressed26 and was not observed in this study. Spreading and splashing commonly occur during the laser printing of viscoelastic solutions. As observed, the typical impact velocity for the transition from spreading to splashing for 2%, 4%, 6%, and 8% alginate concentrations are 41.8, 43.6, 52.9, and 84.9 m/s, respectively. Splashing on the substrate is defined as the disintegration of a droplet into secondary droplets after colliding with a solid surface.27 Generally, jets/droplets with low kinetic energy, either with low impact velocities23 or small droplet sizes,28,29 result in spreading, while jets/droplets with high kinetic energy lead to splashing. For better printing quality, splashing should be avoided, whenever possible, because it may increase the size of printed droplets and cause a loss of droplet shape fidelity. ρRU2

Generally, a dimensionless Weber number We = σ , where ρ is the density, R is the characteristic length, and σ is the surface tension, is used to quantify the occurrence of splashing. A Weber number represents the ratio of the liquid inertia to surface tension effects during impingement, and a critical Weber number (Wec) is introduced herein to determine whether splashing will occur: if We ≥ Wec, splashing may occur; otherwise, it may not. It should be noted that secondary droplets may be attributed to either splashing or divergent subjets. As observed, if the direct-writing height is less than 0.5 mm, only jet-impingement printing with a single breakup happens (if an impingement occurs), and there is no secondary droplet on the substrate. Under the 0.5 mm direct-writing height condition, it is assumed that all observed secondary droplets are due to splashing. By assuming that the direct-writing height does not affect the jet/droplet kinetic energy, Wec for 2%, 4%, 6%, and 8% alginate solutions are identified as 2993, 3331, 5102, and 13657, respectively. These values correspond to laser fluences of 1300, 1700, 1900, and 2500 mJ/cm2, respectively, as observed experimentally. D


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Figure 4. (a) Printed droplet morphology and size as a function of alginate concentration, and time-resolved images of alginate printing process at a 1300 mJ/cm2 laser fluence when using (b) 4% NaAlg and (c) 6% NaAlg. The direct-writing height was fixed at 2.0 mm, and the scale bar is 200 μm.

Figure 5. (a) Printed droplet morphology and size as a function of laser fluence, and time-resolved representative images during 4% alginate solution printing at laser fluence of (b) 1100 mJ/cm2 and (c) 1300 mJ/cm2. The direct-writing height was fixed at 2.0 mm, the concentration percentage is shown to the left of each curve, and the scale bar is 200 μm. Herein, R is taken as the laser spot radius (75 μm). When the concentration increases, Wec increases accordingly, which is attributed

to the effect of increased elasticity27 and increased viscosity of highconcentration alginate solutions. Laser fluences higher than the E


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Figure 6. (a) Printed droplet morphology and size as a function of direct-writing height, time-resolved representative images during printing 4% alginate solution at a 1300 mJ/cm2 laser fluence with direct-writing height of: (b) 0.5 mm, (c) 1.0 mm, (d) 1.5 mm, and (e) 2.0 mm, and (f) optical microscopy image of a microarray printed under optimal conditions (8% alginate solution, 1500 mJ/cm2 laser fluence, and 2.0 mm direct-writing height). The scale bar is 200 μm. cm2 laser fluence, the printing type is jet-impingement printing with a single breakup, which is desirable, for the 6% and 8% alginate solutions. Moreover, the printed droplet diameter decreases when the alginate concentration is increased, under all the conditions investigated. However, it is noted that there is no such a monotonic relationship between the printed droplet size and the glycerol3 and alginate15 concentrations, respectively. In particular, the rate of diameter reduction is higher for high-concentration solutions. It is attributed to more incident laser energy being required to overcome the increased elastic and viscous effects of higher-concentration alginate solutions, rather than transferring a larger volume. Since the formation of secondary droplets is suppressed and the droplet size is reduced, the printing quality improves using high-concentration solutions. It is further observed that the printing type improves from jetimpingement printing with multiple breakups to jet-impingement

aforementioned threshold values for their corresponding solutions are considered undesirable and not evaluated in the study. 3.3. Effects of Alginate Concentration on Printing Quality. The effects of alginate concentration on printing quality in terms of printing type and printed droplet size (or its equivalent size) have been investigated during printing different alginate solutions at different laser fluences. In this study, only representative measurement results (with two or more data points) under well-defined jetting and impingement splashing-free conditions are presented, and the error bars represent ± one standard deviation. As shown in Figure 4a, the printing type can be changed from jet-impingement printing with multiple breakups to droplet-impingement printing (1100 mJ/cm2), or from jet-impingement printing with multiple breakups to jet-impingement printing with a single breakup (1300 and 1500 mJ/cm2), for applicable alginate solutions, with an increase in alginate concentration. At the 1700 mJ/ F


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during inkjet printing.35 By controlling the direct-writing height during inkjetting, the formation of secondary droplets can be suppressed to ensure that only a primary droplet is produced.36 Similarly, the effects of direct-writing height on printing quality have been examined during the laser printing of different alginate solutions at various laser fluences. As shown in Figure 6a, the printing type may change from ideal jetimpingement printing with a single breakup, to jet-impingement printing with multiple breakups, and even to droplet-impingement printing when the direct-writing height increases. Specifically, for the 2% alginate ink, the impingment type can be jet-impingement printing with a single breakup (0.5 mm), jet-impingement printing with multiple breakups (1.0 and 1.5 mm), and droplet-impingement printing (2.0 mm). For the 6% and 8% alginate inks, the impingement type is always jet-impingement printing with a single breakup, and no secondary droplets are observed on the substrate for the full range of direct-writing heights explored (0.5−2.0 mm). Generally, high-concentration alignate inks are preferred to achieve a better printing quality, and the 8% alginate ink was successfully utilized to print 3D tubular and Y-shaped structures.4,38 In order to illustrate the reproducibility of the laser printing process, a microarray (Figure 6f) was printed under a set of optimal conditions (8% alginate solution, 1500 mJ/cm2 laser fluence, and 2.0 mm direct-writing height). There is no appreciable change in the droplet diameter when the direct-writing height is varied from 0.5 mm to 2.0 mm, for all the alginate inks studied. For example, the droplet size decreases merely around 7% when the direct-writing height increases from 0.5 mm to 2.0 mm for all the alginate inks. Similar observations have been reported during the laser printing of different glycerol-based solutions, and there is no noticeable change in their average droplet diameter within a given range of direct-writing height.12,17,22 Generally, as long as the printing type is jet-impingement printing with a single breakup, the direct-writing height does not significantly affect the printing quality. Sometimes a minimum stand-off distance (MSD) is required for certain applications to form a single primary droplet during inkjet printing. For laser printing, such a distance requirement maps to jetimpingement printing with a single breakup, which may lead to the optimal printing quality. However, it should be pointed out that some optimal direct-writing heights may not be easily implemented during practical laser printing. If the ink coating is positioned too close to a liquid surface, the liquid of receiving substrate may contact with the coating due to a surface tension effect. Under such circumstances, a larger direct-writing height is needed to ensure complete separation between the ink coating and substrate, even that means a less favorable impingement type (e.g., loss of jet-impingement printing with a single breakup). Furthermore, if a direct-writing height is too small, there might not be sufficient space for the laser-induced bubble to develop and form a jet, resulting in a failed deposition. The resulting deposition type was also reported as bubble contact printing.37 Figure 6b,c,d,e further shows the time-resolved images during the printing of the 4% alginate solution at a laser fluence of 1300 mJ/cm2, with different direct-writing heights of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm. The printing type is jet-impingement printing for all the directwriting heights investigated. For the direct-writing heights of 0.5 mm and 1.0 mm, no BOAS structure is evident, resulting in jet-impingement printing with a single breakup. For the direct-writing heights of 1.5 mm and 2.0 mm, the BOAS structure is observed along the jet, leading to the jet-impingement printing with multiple breakups. By assuming the perturbation probability is proportional to the ligament length, the shorter the ligament, the less likely that secondary droplets will form. Therefore, the formation of secondary droplets may be suppressed if the direct-writing height is sufficiently small, as seen from Figure 6b,c.

printing with a single breakup as the alginate concentration is increased from 4% to 6%, at a 1300 mJ/cm2 laser fluence as shown in Figure 4b,c. The secondary droplets are effectively eliminated as the concentration increases, and a single round primary droplet can be achieved during the laser printing of 6% alginate solution. During jet-impingement printing with multiple breakups, the growth of perturbations is arrested, resulting in the BOAS structure as shown in Figure 4b. The capillary force further breaks the interconnected filaments to form several secondary droplets after breakup. During jet-impingement printing with a single breakup, no BOAS structure is formed along the ligament, and only a single breakup takes place at the middle of the ligament, as shown in Figure 4c. The temporal evolution of a viscoelastic ligament and the formation of BOAS structures depend on the relative magnitude of the viscous, inertial, elastic, and capillary stresses.30 Moreover, excessive viscous effects30 and/or elastic effects30,31 have been found to decrease the number of secondary droplets, and even suppress the formation of BOAS structures. For alginate solutions, both the storage and loss moduli increase with the alginate concentration, indicating the increase of elastic and viscous effects. Therefore, when the alginate concentration is increased, the number of secondary droplets decreases, and the formation of BOAS structures is suppressed along the forming viscoelastic alginate ligament during laser printing. This results in a change from multiple breakups (Figure 4b) to a single breakup (Figure 4c). During jetting, both the elastic energy stored in a jet and the viscous dissipation energy increase with the alginate concentration. As a result, the remaining kinetic energy of a jet decreases, and its velocity lowers. As observed, it takes longer (∼210 μs) for the 6% alginate jet to reach the receiving substrate than that of the 4% alginate jet (∼150 μs). Moreover, the increased elastic and viscous effects resist the capillary effect and delay the ligament necking,32 resulting in a longer breakup time for the 6% alginate solution (∼550 μs) than that of the 4% alginate solution (∼400 μs) after impinging on the substrate. 3.4. Effects of Printing Conditions on Printing Quality. 3.4.1. Effects of Laser Fluence. The effects of laser fluence on printing quality have been investigated during the printing of different alginate solutions. As shown in Figure 5a, the printing type may change from droplet-impingement printing to jet-impingement printing with multiple breakups (2% and 4%), or keep ideal jet-impingement printing with a single breakup (6% and 8%), at applicable laser fluences when the laser fluence increases. The droplet size increases almost linearly when the laser fluence increases as observed during the laser printing of polymer solutions,33 cell suspensions,34 and glycerol-based solutions.3 For highconcentration alginate solutions such as the 6% and 8% solutions, it is easy to choose a lower laser fluence in order to have a better printing resolution. However, for the 2% and 4% solutions, the printing type may be droplet-impingement printing at lower laser fluences. As measured, the typical equivalent droplet size during droplet-impingement printing is 187.8 μm (900 mJ/cm2) for the 2% solution and 193.4 μm (1100 mJ/ cm2) for the 4% solution, while the typical equivalent droplet size during jet-impingement printing with multiple breakups is 219.8 μm (1100 mJ/ cm2) and 241.1 μm (1300 mJ/cm2), respectively. Since jet-impingement printing with multiple breakups may lead to a well-defined primary droplet, it is preferable when compared to droplet-impingement printing. As such, a higher laser fluence is preferred for the printing of the 2% and 4% alginate solutions even though the resulting equivalent droplet size may be larger. Overall, the selection of laser fluence should be a compromised decision. Time-resolved images of 4% alginate solution printing under a directwriting height of 2.0 mm are further investigated to appreciate the effects of laser fluence on the impingement type, as shown in Figure 5b,c. An increase in laser fluence results in increases in both jet velocity and breakup length. For example, at a laser fluence of 1300 mJ/cm2, the breakup length is larger than the direct-writing height, and the impingement type changes from droplet-impingement printing observed at 1100 mJ/cm2 (Figure 5b), to jet-impingement printing with multiple breakups (Figure 5c). As a result, secondary droplets still exist but change from uniformly sized to smaller scattered droplets. 3.4.2. Effects of Direct-Writing Height. It has been recognized that the direct-writing height, also commonly referred to as the stand-off distance, plays an important role in determining the printing quality

4. DECISION TREE AND PHASE DIAGRAMS FOR PRINTING QUALITY EVALUATION For a given direct-writing height, the alginate concentration and laser fluence affect the jet velocity, jet breakup length, and printing type, and their effects can be better represented using three different threshold We numbers: Wet1 as the material transferring threshold, Wet2 as the threshold for jetting with a G


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Figure 7. Decision-tree for printing type analysis.

bulgy shape or pluming/splashing, and Wec for the threshold for splashing on the substrate. When We < Wet1, the pressure of laser-induced bubble is not high enough to overcome the coating’s surface tension and/or ambient pressure, such that the ink material cannot be ejected as a jet, and no material is transferred. When We > Wet2, the laser-induced bubble has a pressure that is too high, and it bursts, resulting in jetting with a bulgy shape or pluming/splashing. When Wec < We < Wet2, the splashing on the substrate after droplet or jet impingement is inevitable. All these three printing scenarios should be avoided during laser printing, and the desirable printing type is only expected when Wet1 < We < Wec, as illustrated in Figure 7. It should be noted that the threshold numbers are specific to a given alginate concentration. For example, for the 6% alginate solution, Wet1 = 102, Wet2 = 9449, and Wec = 5102; for the 8% alginate solution, Wet1 = 160, Wet2 = 21272, and Wec = 13657. The jetting process during laser printing can be mainly characterized using three main time scales: inertio-capillary or

Rayleigh time scale tc = ρR3/σ , longest polymer relaxation time scale λ, and visco-capillary time scale tv = η0R/σ, where η0 is the zero-shear viscosity.11 For the 2−8% sodium alginate solutions studied herein, the Ohnesorge number η t Oh = tv = ρσ0 R is larger than 1, which means that the thinning c

and breakup process is dominated by the viscous force. As such, the printing process for these viscoelastic alginate solutions may be better described by Oh and the elasto-capillary number

(Ec =

λ tv

=

λσ η0R

),

and both are material property-based

dimensionless numbers. Moreover, these can each be derived based on the relative significance of the aforementioned three time scales. Therefore, the printing types can be further mapped out in a 3D phase diagram, represented in a (We, Oh, Ec) space, under a given direct-writing height of 2.0 mm, as shown in Figure 8a. Four shaded planes separate the well-defined jetting and H


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Figure 8. Printing type: (a) as a function of We, Oh, and Ec numbers and (b) as a function of Oh and Ec numbers at a given We number.

5. CONCLUSIONS AND FUTURE WORK Printing quality during the laser printing of viscoelastic alginate solutions has been investigated as a function of three key variables: alginate concentration, laser fluence, and direct-writing height using a time-resolved imaging approach and microarrays of printed droplets. The printing type during laser printing is classified based on the impingement type of transferred droplets. Under well-defined jetting and impingement splashing-free conditions, three printing types are identified: droplet-impingement printing, jet-impingement printing with multiple breakups, and jet-impingement printing with a single breakup. Overall, for better printing quality, it is preferable to have jetimpingement printing with a single breakup, followed by jetimpingement printing with multiple breakups, then dropletimpingement printing. The printing quality can be improved by using high-concentration alginate solutions, since the formation of secondary droplets is suppressed and the droplet size is reduced. An increase of laser fluence may lead to a well-defined

impingement splashing-free working domain into three different printing type-based zones, and they are for illustration only. For given Oh and Ec numbers (e.g., the same alginate concentration), the jet breakup length increases as the laser fluence increases, which indicates an increasing We number.11 Therefore, as the We number increases, the jet breakup length may be larger than the direct-writing height, and the printing type may change from droplet-impingement printing to jet-impingement printing. For a given We number, the jet breakup length increases, and the BOAS formation is suppressed, when the alginate concentration increases, indicating an increasing Oh number or decreasing Ec number.11 As a result, the printing type may change from droplet-impingement printing, to jet-impingement printing with multiple breakups, to jet-impingement printing with a single breakup. Figure 8b further depicts the transition of printing types at two given We numbers (300 and 1000) when the alginate concentration increases. I


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primary droplet for low-concentration alginate solutions, however, the droplet diameter may increase, which may be not desirable. Overall, the laser fluence selection should be a compromised decision. When the direct-writing height increases, the printing type may change from ideal jet-impingement printing with a single breakup to jet-impingement printing with multiple breakups to droplet-impingement printing. However, there is no significant change in the droplet diameter by varying the direct-writing height from 0.5 mm to 2.0 mm for all the alginate inks studied. The resulting knowledge may help implement laser printing for the fabrication of soft structures4,38 from various viscoelastic fluids and suspensions. Building from these studies, future work could explore (1) investigating the effects of donor layer thickness and laser spot diameter on the impingement-based printing type as well as printing quality, (2) simulating and validating of the material deposition process and its outputs, such as the formation of BOAS structures and droplet size during the laser printing of viscoelastic fluids, (3) studying the droplet formation process during the printing of particle-laden viscoelastic suspensions, such as cells suspended in hydrogel solutions, to determine the influence of particulates on printing dynamics, fidelity, and quality, and (4) assessing how those optimal printing conditions identified herein influence the printing performance (i.e., fidelity, geometric control) when fabricating 3D soft structures.

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AUTHOR INFORMATION

Corresponding Author

*Address: University of Florida, Gainesville, FL 32611, USA. Phone: 001-352-392-5520; Fax: 001- 352-392-7303; E-mail: yongh@ufl.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was partially supported by the U.S. National Science Foundation (CMMI 1314830 and 1537956), and the discussion with Dr. Changxue Xu of Texas Tech is highly appreciated.

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REFERENCES

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Study of Impingement Types and Printing Quality during Laser Printing

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