Patent Publication Number: US-2023146881-A1

Title: Electrospray printer

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/276,171, filed on Nov. 5, 2021, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates to electrospray printing and electrospray printing systems. 
     BACKGROUND 
     Various additive manufacturing or printing processes can be used to print a print material onto a target surface. One additive printing process is chemical vapor deposition (CVD). In CVD, a print material, such as a polymer print material, is formed on the target surface from precursor gases, which react or decompose to produce the desired thin film deposit. Unfortunately, CVD may require expensive instrumentation, can emit toxic gaseous by-products during reaction, and/or may be an expensive printing process. 
     Another additive printing process is inkjet printing. Inkjet printing can print a variety of print materials, such as conductive, insulating, and organic print materials. Although inkjet printing is typically less expensive than CVD, inkjet printing has several limitations. The limitations include disruptive evaporative effects after a volatile ink is delivered to the target surface and/or difficulty in creating conformal films. 
     SUMMARY 
     The present disclosure relates to a printhead, an electrospray printing system, and a method of operating an electrospray printing system that produce conformal films on any type of target substrate. The electrospray printing system can be used to print a variety of print materials. Example print materials include, but are not limited to, semi-conducting materials, conducting materials, polymers, or insulating materials. The solution may be a homogeneous solution or a colloidal solution. Generally, if a print material can be included in a solution, an electrospray printing system disclosed herein can print the print material onto a target substrate. 
     The electrospray printing technology is material versatile and can be used to print on metal target substrates, semiconductor target substrates, and other types of target substrates. The design of the printhead uses electrostatic focusing to create different thicknesses and/or patterns with high material density. The print material is delivered to the target substrate in a dry state or in a semi-dry state, avoiding issues associated with solvent evaporation after printing. As an additive manufacturing technique, material can be delivered to a specific region of the target substrate to create fine features or large-area films. The electrospray printing does not require a direct line-of-sight to the target surface and can create highly conformal coatings. The thickness of the printed film is governed by several parameters, including, but not limited to, the electrical properties of the target substrate. Therefore, different thicknesses can be printed on multi-material substrates in a single pass. The electrospray printer is designed to be user friendly and may incorporate computer-controlled positioning of the target and/or the printhead, automated control of the printhead, and/or real-time monitoring of the electrospray printing process. 
     In one aspect, a printhead for an electrospray printing system includes a housing, a mixing receptacle in the housing, and an emitter in the housing. The mixing receptacle is operable to receive at a first inlet a solution and receive at a second inlet an electrical potential that is applied to the solution to produce an electrically charged solution. The solution includes a print material suspended in a solvent. The emitter is operable to receive the electrically charged solution at a third inlet and emit the electrically charged solution at an outlet. The electrical potential produces an electric field that directs electrically charged droplets towards an orifice of the printhead. Each of the electrically charged droplets include the print material suspended in the solvent. The housing may further include at least one air flow inlet operable to provide a flow of air into the printhead. 
     In another aspect, an electrospray printing system includes a printhead, a power supply, and a device operable to supply a solution to the printhead. The printhead includes a housing, a mixing receptacle in the housing, and an emitter in the housing. The mixing receptacle is operable to receive at a first inlet a solution and receive at a second inlet an electrical potential that is applied to the solution to produce an electrically charged solution. The solution includes a print material suspended in a solvent. The emitter is operable to output the electrically charged solution toward an orifice of the printhead. The power supply is operably connected to the mixing receptacle. After the emitter outputs the electrically charged solution, a plume of electrically charged droplets form in the printhead. The electrically charged droplets include the print material suspended in the solvent. An electric field within the printhead is operable to guide a spray of electrically charged particles to a target substrate. In certain embodiments, the electrically charged particles are electrically charged dry particles or electrically charged semi-dry particles. 
     In yet another aspect, a method includes receiving, at a printhead, a solution comprising a print material suspended in a solvent, and receiving, at the printhead, an electrical potential that is applied to the solution to produce an electrically charged solution. The electrically charged solution is emitted into the printhead that transitions into, within the printhead, a plume of electrically charged droplets that comprises the print material suspended in the solvent. A spray of electrically charged particles is emitted from the printhead onto a target substrate, the electrically charged particles comprising of the print material. An electric field that is created by the electrical potential guides the plume of electrically charged droplets within the printhead and guides the spray of electrically charged particles onto the target substrate. 
     In another aspect, any of the foregoing aspects individually or together, and/or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various features and elements as disclosed herein may be combined with one or more other disclosed features and elements unless indicated to the contrary herein. 
     Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure. 
         FIG.  1    illustrates an electrospray printing system in accordance with the prior art; 
         FIG.  2    illustrates an example electrospray printing system in accordance with embodiments of the disclosure; 
         FIG.  3    illustrates a cross-sectional view of an example implementation of the printhead shown in the dashed box in  FIG.  2    in accordance with embodiments of the disclosure; 
         FIG.  4    illustrates line-of-sight and non-line-of-sight electrospray printing of a component in accordance with embodiments of the disclosure; 
         FIG.  5    illustrates an example of a conformal dry film printed by an electrospray printing system in accordance with embodiments of the disclosure; 
         FIG.  6    illustrates an example of a conformal semi-dry film printed by an electrospray printing system in accordance with embodiments of the disclosure; 
         FIGS.  7 A- 7 C  illustrate print material deposition and pinholing in accordance with embodiments of the disclosure; 
         FIG.  8    illustrates a graph of film thickness over spray time in accordance with embodiments of the disclosure; 
         FIG.  9    illustrates an optical property of a semi-dry film that is printed by an electrospray printing system in accordance with embodiments of the disclosure; 
         FIG.  10    illustrates an optical property of a dry film that is printed by an electrospray printing system in accordance with embodiments of the disclosure; 
         FIG.  11    illustrates an example graph of example plots of a percentage of transmission versus wavelengths of light in accordance with embodiments of the disclosure; 
         FIG.  12 A  illustrates an example graph of a dielectric strength of a silicon substrate; 
         FIG.  12 B  illustrates an example graph of a dielectric strength of a film that is electrospray-printed onto a silicon substrate in accordance with embodiments of the disclosure; 
         FIG.  13 A  illustrates a water droplet on an electrospray printed particulate film in accordance with embodiments of the disclosure; 
         FIG.  13 B  illustrates a water droplet on an electrospray printed densified film in accordance with embodiments of the disclosure; 
         FIG.  14    illustrates a flowchart of a method of electrospray printing in accordance with embodiments of the disclosure; and 
         FIG.  15    illustrates a block diagram of a computing device that may be used in the electrospray printing system shown in  FIG.  2    in accordance with embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Embodiments are described herein with reference to schematic illustrations of embodiments of the disclosure. As such, the actual dimensions of the layers and elements can be different, and variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are expected. For example, a region illustrated or described as square or rectangular can have rounded or curved features, and regions shown as straight lines may have some irregularity. Thus, the regions illustrated in the figures are schematic and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the disclosure. Additionally, sizes of structures or regions may be exaggerated relative to other structures or regions for illustrative purposes and, thus, are provided to illustrate the general structures of the present subject matter and may or may not be drawn to scale. Common elements between figures may be shown herein with common element numbers and may not be subsequently re-described. 
       FIG.  1    illustrates an electrospray printing system  100  in accordance with the prior art. A power supply  102  is operably connected to a solution reservoir  104 . A device  105  holds the solution  106  that flows into the solution reservoir  104 , where the power supply  102  is used to charge particles in the solution  106  within the solution reservoir  104 . The solution  106  is composed of a solute material (hereinafter “print material”) suspended in a volatile solvent (hereinafter “solvent”). 
     The electrical potential produced by the power supply  102  creates an electric field (represented by the dashed lines  108 ) between the solution reservoir  104  and a target substrate  110 . The electric field is used to atomize the solution  106  into a plume  112  of electrically charged droplets  114 . The electrically charged droplets  114  are sprayed onto the target substrate  110  to create a film of the print material on the target substrate  110 . 
     As shown in  FIG.  1   , the unfocused electric field results in a broad deposition of the print material onto the target substrate  110 . In some instances, the broad deposition of the print material results in longer spray times when a thicker or more dense film of the print material is desired. 
     Embodiments disclosed herein provide a printhead, an electrospray printing system, and a method of operating an electrospray printing system that produce conformal films. The technology is material versatile and can be used to print on target substrates that are, or include, metals, semiconductors, and other types of target substrates. The design of the printhead uses electrostatic focusing to deliver a print material to a target substrate in a dry state or in a semi-dry state. Both the dry state and the semi-dry state avoid issues associated with solvent evaporation after printing. As an additive manufacturing technique, the print material can be delivered to a specific region of the target substrate to create fine features or large-area films. The electrospray printing does not require a direct line-of-sight to a surface of the target substrate. A thickness of the printed film is governed by several parameters, including, but not limited to, the electrical properties of the target substrate and/or the dryness of the print material at the time of deposition. Therefore, different thicknesses can be printed on multi-material substrates in a single pass. The electrospray printer is designed to be user friendly and may incorporate computer-controlled positioning of the target substrate and/or the printhead, automated control of the printhead and/or the electrospray printing system, and/or real-time monitoring of the electrospray printing process. 
       FIG.  2    illustrates an example electrospray printing system  200  in accordance with embodiments of the disclosure. A cross-sectional view of a print system  202  is shown in  FIG.  2   . The print system  202  includes a printhead  204  held in suspension over a print bed  206 . The print bed  206  is made of, or enclosed with, a conductive material. A non-limiting nonexclusive example of the conductive material is a metal, such as aluminum. 
     In certain embodiments, the print bed  206  is operably attached to a first stage  208  and the printhead  204  is operably attached to a second stage  210 . The first stage  208  may be operable to translate the print bed  206  in a first direction (e.g., an x-direction) and in a second direction (e.g., a y-direction), while the second stage  210  is operable to translate the printhead  204  in a third direction (e.g., a z-direction). In other embodiments, the first stage  208  and/or the second stage  210  can be operable to move differently. For example, the first stage  208  may be operable to translate the print bed  206  in the first direction, the second direction, and the third direction. 
     The print system  202  may further include an imaging system  212  that is positioned to provide images of an outlet of a solution receptacle within the printhead  204  (e.g., the outlet  326  of the solution reservoir  314  in  FIG.  3   ). In some embodiments, the printhead  204  is made of a transparent dielectric material, or the printhead  204  includes a transparent window that enables the imaging system  212  to capture images of the outlet. The imaging system  212  is used to monitor the output from the outlet to ensure a stable spray (e.g., monitor a Taylor cone  324  shown in  FIG.  3   ). The stability of the spray may be based on the electrical potential (e.g., the voltage) applied to the solution and/or the flow rate of air into the printhead  204 . If captured images of the spray show an unstable spray, the applied electrical potential and/or the flow rate of the air can be increased or decreased to improve the stability of the spray. In non-limiting nonexclusive examples, the imaging system  212  may be implemented as a camera operable to capture still images, a video camera operable to capture video, or a combination of the camera and the video camera. 
     The printhead  204 , the print bed  206 , the first stage  208 , the second stage  210 , and the imaging system  212  are housed within an enclosure  214 . A frame of the enclosure  214  may be made of any suitable material, such as aluminum. Panels can be positioned between the openings in the frame. The panels may be transparent, semi-transparent, or opaque. In a non-limiting nonexclusive example, plastic panels are positioned between the openings in the frame. Thus, the enclosure  214  fully encloses the environment within the enclosure  214 . 
     The enclosure  214  may include a fume extractor  216  that extends from the interior of the enclosure  214  to the exterior of the enclosure  214 . In certain embodiments, the fume extractor  216  is a system that uses a fan to pull fumes and particulates (e.g., volatile chemicals or particles) that are inside the enclosure  214  into a filtration system (not shown) that is outside of the enclosure  214 . 
     A power supply  218 , a device  220 , and an airflow controller  222  are be positioned outside of the enclosure  214 , although other embodiments are not limited to this arrangement. One or more of the power supply  218 , the device  220 , and the airflow controller  222  may be positioned within the enclosure  214 . In certain embodiments, the power supply  218 , the device  220 , and the airflow controller  222  are controlled by a computing device  224 . In certain embodiments, the computing device  224  enables the power supply  218 , the device  220 , and/or the airflow controller  222  to be controlled remotely. For example, a second computing device (e.g., other computing devices  1422  in  FIG.  14   ) may be operably connected to the computing device  224  to enable a user to remotely control the power supply  218 , the device  220 , and/or the airflow controller  222 . An example implementation of the computing device  224  is shown in  FIG.  15   . 
     A positive terminal  226  of the power supply  218  extends through an opening  228  in the enclosure  214  to be inserted into the printhead  204 . The positive terminal  226  is used to apply an electrical potential (e.g., a voltage) to the solution within the printhead  204 . In a non-limiting nonexclusive example, the electrical potential ranges between four (4) and five (5) kilovolts (kV). A negative terminal  230  (e.g., ground) extends through the opening  228  in the enclosure  214  to be operably connected to the print bed  206 . An example implementation of the printhead  204  is shown in  FIG.  3   . 
     The device  220  is operable to supply the solution to the printhead  204 . One or more outlet tubes (collectively referred to as outlet tube  232 ) of the device  220  extend through an opening  234  in the enclosure  214  and are inserted into the printhead  204  to provide the solution to the printhead  204 . In a non-limiting nonexclusive example, the device  220  is implemented as a syringe pump  236 . 
     The airflow controller  222  is operable to supply one or more air flows to the printhead  204 . One or more outlet tubes (collectively referred to as outlet tube  238 ) of the airflow controller  222  extend through an opening  240  in the enclosure  214  and are inserted into the printhead  204 . An inlet  242  of the airflow controller  222  receives air, such as compressed air, and supplies the air flow(s) to the printhead  204 . 
       FIG.  3    illustrates a cross-sectional view of an example implementation of the printhead  204  shown in the dashed box  300  in  FIG.  2    in accordance with embodiments of the disclosure. A first outlet tube  238 A of the airflow controller  222 , a second outlet tube  238 B of the airflow controller  222 , the outlet tube  232  of the device  220 , and the positive terminal  226  of the power supply  218  are inserted or attached to openings  304 ,  306 ,  308 ,  310 , respectively, in a top surface  312  of a housing  313  of the example printhead  204 . In certain embodiments, the housing  313  is made of a dielectric material. Other embodiments are not limited to this implementation. 
     The outlet tube  232  of the device  220  is received by a solution reservoir  314 . In a non-limiting nonexclusive example, the solution reservoir  314  is a microfluidic T-junction that includes a mixing receptacle  316  and an emitter  318  operably connected to the mixing receptacle  316 . The outlet tube  232  of the device  220  supplies the solution  320  to the solution reservoir  314  (e.g., the mixing receptacle  316 ). The positive terminal  226  of the power supply  218  is operably connected to the solution reservoir  314  (e.g., the mixing receptacle  316 ) to apply the electrical potential (e.g., the voltage) to the solution  320  that is within the solution reservoir  314 . 
     The electrically charged solution  322  is output from the solution reservoir  314  (e.g., the emitter  318 ) into the printhead  204 . In certain embodiments, a Taylor cone  324  forms within the printhead  204  outside an outlet  326  of the solution reservoir  314  (e.g., the emitter  318 ). In a non-limiting nonexclusive example, a size of the outlet  326  (e.g., a diameter of the outlet  326 ) is approximately one hundred and fifty (150) micrometers. 
     Within the printhead  204 , the Taylor cone  324  transitions to a plume  328  of electrically charged droplets  330 . A focused electric field (represented by dashed lines  332 ) produced by the electrical potential that is applied to the solution  320  causes the plume  328  of electrically charged droplets  330  to form into a narrow plume of electrically charged droplets  330 . The focused electric field further guides the electrically charged droplets  330  toward an orifice  215  of the printhead  204 . In a non-limiting nonexclusive example, a size of the orifice  215  (e.g., a diameter of the orifice  215 ) is in the range of tens of millimeters (e.g., approximately thirty (30) millimeters). 
     The first outlet tube  238 A and the second outlet tube  238 B of the airflow controller  222  are each operable to provide a flow of air into the printhead  204  to assist in evaporation of the electrically charged droplets  330  within the printhead  204 . In some embodiments, the flow of air also assists in focusing the electrically charged droplets within the printhead  204 . In a non-limiting nonexclusive example, the flow of air (air flows  334 ) is co-linear with the emitter  318  and is in the range of five (5) to ten (10) liters per minute. Rapid solvent evaporation from the electrically charged droplets  330  occurs at least within the printhead  204  in part due to the air flows  334  output from the first outlet tube  238 A and the second outlet tube  238 B. In some embodiments, the rapid solvent evaporation within the printhead  204  produces a spray of electrically charged dry particles within the printhead  204 . A film that is formed with electrically charged dry particles is discussed in more detail in conjunction with  FIG.  5   . In other embodiments, slower solvent evaporation within the printhead  204  produces a spray of electrically charged semi-dry particles within the printhead  204 . A film that is produced with electrically charged semi-dry particles is discussed in more detail in conjunction with  FIG.  6   . 
     Electrical charge  336  can build up on the interior surface  338  of the housing  313  of the printhead  204 . The electrical charge  336  assists in focusing the electric field (represented by the dashed lines  332 ) and the formation of the plume  328  (the narrow plume) within the printhead  204 . The focused electric field directs the spray of the electrically charged dry or semi-dry particles  339  through the orifice  215  of the printhead  204  and onto a print surface  341  of the target substrate  302 . The rate of the delivery of the electrically charged dry or semi-dry particles  339  can be controlled or adjusted by adjusting the flow rate of the solution  320  into the printhead  204  (e.g., into the mixing receptacle  316 ) and/or by adjusting a solution mass loading (e.g., the amount of print material in the solution  320 ). 
     In certain embodiments, one or more conductive electrodes (collectively referred to as conductive electrode  340 ) are included within the printhead  204 . The conductive electrode  340  is operably connected to ground to cause the conductive electrode  340  to be a grounded conductive electrode. The conductive electrode  340  is adjacent to at least one interior surface  338  of the housing  313 . In certain embodiments, the conductive electrode  340  is adjacent all of the interior surfaces  338  of the housing  313 . The interior surfaces  338  may be the vertical interior surfaces, the angled interior surfaces, and/or the top surface  312 . In the illustrated embodiment, the conductive electrode  340  is shaped into a ring that is positioned along the interior surfaces  338  of the housing  313  below the solution reservoir  314  (e.g., below the emitter  318 ). The conductive electrode  340  facilitates in the focusing of the focused electric field, the formation of the plume  328  of the electrically charged droplets  330 , and/or in reducing the amount of the electrical charge  336  that can build up on the interior surfaces  338  of the housing  313 . In embodiments that omit the conductive electrode  340 , a higher electrical potential may be needed to form the plume  328  of the electrically charged droplets  330 . For example, an electrical potential greater than ten (10) kV may be required to form the electrospray. 
       FIG.  4    illustrates line-of-sight and non-line-of-sight electrospray printing of a component  400  in accordance with embodiments of the disclosure. The housing  313  and the conductive electrode  340  are omitted for clarity. The component  400  can be suspended within the printhead (e.g., the printhead  204  in  FIG.  2    and  FIG.  3   ), or the component  400  may be positioned outside of the printhead just below or adjacent to the orifice of the printhead (e.g., the orifice  215  in  FIG.  2    and  FIG.  3   ). The component  400  is shown as an electrical wire in  FIG.  4   , although other embodiments are not limited to this type of component. The component  400  can have any shape, size, and/or orientation. Additionally, the component  400  may be made of any type of material. 
     The positive terminal  226  of the power supply  218  is operably connected to the solution reservoir  314  (e.g., the mixing receptacle  316 ), and the negative terminal  230  of the power supply  218  (e.g., ground) is operably connected to the component  400 . The plume  328  of electrically charged droplets  330  is output from the solution reservoir  314  (e.g., the emitter  318 ). Although not shown in  FIG.  4   , a Taylor cone may form at the outlet  326  of the solution reservoir  314  (e.g., the emitter  318 ). The focused electric field (represented by dashed lines  332 ) directs electrically charged particles (e.g., electrically charged dry particles or electrically charged semi-dry particles) onto the surface of the component  400 . The focused electric field guides the electrically charged particles to a portion of the surface (e.g., area  402 ) that is in the direct line-of-sight of the solution reservoir  314  (e.g., the emitter  318 ), as well as to portions of the surface that are not in the direct line-of-sight of the solution reservoir  314  (e.g., areas  404 ,  406 ). The focused electric field drives the electrically charged particles to the backside and around the geometry of the component  400 . Over time, a conformal coating or film is produced over the entire surface of the component  400  without having to move the solution reservoir  314  (e.g., without moving the printhead  204 ) and/or without moving the component  400  (e.g., the target substrate). 
       FIG.  5    illustrates an example of a conformal dry film  500  that can be printed by an electrospray printing system in accordance with embodiments of the disclosure. The film  500  is comprised of electrically charged dry particles  502  and is referred to herein as a dry film or a particulate film. Some or all of the individual electrically charged dry particles  502  of the print material are discernible when examined under a microscope. In certain embodiments, the particulate film  500  is produced when a higher amount of solvent evaporates prior to delivery of the print material onto the target substrate  302 . In the example embodiment, the print material is a polymer material (e.g., a polyimide), although other embodiments are not limited to this type of print material. 
     As shown in the graphic representation  504 , the electrically charged droplets  330  initially include the print material  506  suspended in the solvent  508 . The solvent  508  continues to evaporate as the electrically charged droplets  330  are directed to the target substrate  302  (e.g., while the electrically charged droplets  330  are in the printhead). This is shown by the electrically charged droplets  330 ′, which include the print material  506  and less solvent  508 ′. Prior to deposition of the print material  506  onto the target substrate  302  (e.g., while the electrically charged droplets  330  are still in the printhead), the solvent evaporates completely to produce the electrically charged dry particles  502 . The electrically charged dry particles  502  are deposited onto the target substrate  302  to form the particulate film  500 . 
       FIG.  6    illustrates an example of a conformal semi-dry film  600  that can be printed by an electrospray printing system in accordance with embodiments of the disclosure. The film  600  is comprised of electrically charged semi-dry particles and is referred to herein as a semi-dry film or a densified film. With the densified film  600 , all (or nearly all) of the individual particles of the print material  506  are not discernible when examined under a microscope. As shown in  FIG.  6   , the composition and the surface of the densified film  600  are more continuous across the densified film  600  compared to the particulate film  500  shown in FIG.  5 . Additionally, in certain embodiments, a thickness T1 of the densified film  600  is less than a thickness T2 of the particulate film  500  shown in  FIG.  5   . 
     In certain embodiments, the densified film  600  is produced when a lower amount of solvent evaporates prior to delivery of the print material onto the target substrate  302 . Although the densified film  600  is semi-dry, no post-processing is needed to “dry” or cause the remaining solvent to evaporate. The amount of solvent in the densified film  600  is very low at the time of deposition onto the target substrate  302  such that the remaining solvent evaporates rapidly from the densified film  600 . 
     As shown in the graphic representation  602 , the electrically charged droplets  330  initially include the print material  506  suspended in the solvent  508 . The solvent  508  evaporates as the electrically charged droplets  330  are directed to the target substrate  302  (e.g., while the electrically charged droplets  330  are in the printhead). This is shown by the electrically charged droplets  330 ′, which include the print material  506  and a first reduced amount of solvent  508 ′. When the print material  506  is deposited onto the target substrate  302 , the solvent has not completely evaporated, as depicted by the electrically charged droplets  330 ″ that comprise the print material  506  and a second reduced amount of solvent  508 ″. The electrically charged droplets  330 ″ are deposited onto the target substrate  302  to form the densified film  600 . The remaining solvent evaporates from the densified film  600  (evaporation represented by lines  604 ). As discussed earlier, the amount of solvent that remains in the densified film  600  is low and no post-processing of the densified film  600  is needed to “dry” or cause the remaining solvent to evaporate after deposition onto the target substrate  302 . 
       FIGS.  7 A- 7 C  illustrate print material deposition and pinholing in accordance with embodiments of the disclosure. Initially, electrically charged particles  700  are deposited on the target substrate  702 , as shown in  FIG.  7 A . The electrically charged particles  700  can be dry particles or semi-dry particles. Although the target substrate  702  is shown as having flat or level surfaces, other target substrates may have surfaces with various topologies and/or geometries (e.g., non-uniform geometries). For example, the target substrate  702  may include one or more components (e.g., component  400  in  FIG.  4   ) on at least one surface of the target substrate  702 , or the target substrate  702  is one or more components (e.g., component  400  in  FIG.  4   ). 
     As the electrically charged particles  700  continue to be deposited on the target substrate  702 , the charge in the existing electrically charged particles  700  (e.g., the top layer of the electrically charged particles  700 ) accumulates. As new electrically charged particles  704  are emitted toward the previously printed electrically charged particles  700 , the electric force of the accumulated charge repulses the new electrically charged particles  704 . This repulsion causes the new electrically charged particles  704  to move and be deposited in charge-void regions  706  on the target substrate  702  that have little to no previously printed electrically charged particles  700  (movement represented by arrows  708 ). This charge-induced movement of the new electrically charged particles  704  due to repulsion is known as pinholing and is shown in  FIG.  7 B . Pinholing causes the electrically charged particles  700 ,  704  to be printed as a film  710  on the target substrate  702  ( FIG.  7 C ). The growth of the film  710  is a more uniform growth without having to move the printhead and/or the target substrate  702  in order to cover the charge-void regions  706 . 
       FIG.  8    illustrates a graph of film thickness over spray time in accordance with embodiments of the present disclosure. The vertical axis represents a film thickness and the horizontal axis a spray time. Initially, at time T1, the print material  800  is sprayed onto the target substrate  702 . Again, although the target substrate  702  is shown as having level surfaces, the target substrate  702  can include one or more components (e.g., component  400  in  FIG.  4   ) on at least one surface of the target substrate  702  or the target substrate  702  is one or more components (e.g., component  400  in  FIG.  4   ). 
     As the spray time increases, the thickness of the film  800 ′ increases linearly (or substantially linearly). Additionally, as described earlier, the charge in the film  800 ′ accumulates as the thickness of the film  800 ′ increases. The linear or substantially linear growth is shown in the time period between time T1 and time T2. 
     At or near time T3, the accumulated charge produces a repulsive force that forms a charge-barrier  802  over the film  800 ″. The charge-barrier  802  functions as a self-regulating force to the thickness of the film  800 ″. For example, between time T2 and time T3, the thickness of the film  800 ″ approaches a given thickness value V1. Thus, over time, the growth or thickness of the film  800 ″ becomes asymptotic. This asymptotic behavior can be useful in that the thickness of the film  800 ″ may be controlled without the need to carefully regulate the spray time. In certain embodiments, the thickness of the film  800 ″ is determined by other factors, such as the electrical potential that is applied to the solution and/or a conductivity of the charged solution. 
     The films, including the densified films and the particulate films, can be printed based on one or more characteristics that are associated with the films. One characteristic is an optical property of the film. Films can be printed with different optical properties due to the microstructures of the films. The optical property includes a transparency of the films.  FIG.  9    illustrates an optical property of a semi-dry film  900  that is printed by an electrospray printing system in accordance with embodiments of the disclosure. In the illustrated embodiment, the print material is a polymer (e.g., a polyimide). The densified film  900  is printed onto a glass substrate  902 , where the glass substrate  902  is positioned on a segment of a ruler  904 . As depicted, the densified film  900  is a highly transparent film. In a non-limiting nonexclusive example, a densified film that is formed with a polyimide can have an index of refraction that is comparable to the index of refraction of KAPTON® films. 
       FIG.  10    illustrates an optical property of a dry film  1000  that is printed by an electrospray printing system in accordance with embodiments of the disclosure. In the illustrated embodiment, the print material is a polymer (e.g., a polyimide). The film  1000  is printed onto the glass substrate  902  that is disposed on a segment of the ruler  1002 . In  FIG.  10   , the particulate film  1000  is an opaque film. In certain embodiments, the particulate film  1000  absorbs and/or refracts most of the wavelengths in the electromagnetic spectrum. A particulate film can become increasingly opaque with longer spray times. 
     A microstructure of a film can be tuned based on solvent evaporation such that the film can be printed with an optical property that includes or is between high transparency and high opacity. An electrospray-printed film may be printed to have a selected optical property by, for example, adjusting the flow rate of the air into the printhead and/or by changing a flow rate of the solution into the printhead (e.g., into the mixing receptacle  316  in  FIG.  3   ).  FIG.  11    illustrates an example graph  1100  of example plots of a percentage of transmission versus a range of wavelengths in accordance with embodiments of the disclosure. The vertical axis represents a transmission percentage and the horizontal axis represents a range of wavelengths (in nanometers), where the wavelengths range from two hundred (200) nm to eight hundred (800) nm. As a point of reference, the transmission percentage of a glass substrate (e.g., a fluorine doped tin oxide (FTO) substrate) is represented by the plot  1102 . The percentage of transmission in the plot  1102  increases starting around three hundred (300) nm and remains high up the wavelength of eight hundred (800) nm. 
     The percentages of transmission of multiple particulate films are based on different spray times and are represented by solid lines in the graph  1100 . In the illustrated graph  1100 , the plot  1104  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of one (1) minute. The percentage of transmission in the plot  1104  begins to increase around three hundred (300) nm, peaks at a percentage of transmission of approximately sixty (60) percent at or around a wavelength of four hundred (400) nm, and decreases between the wavelengths of four hundred (400) nm to eight hundred (800) nm. 
     The plot  1106  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of five (5) minutes. The percentage of transmission in the plot  1106  begins to increase around three hundred (300) nm, peaks at a percentage of transmission of approximately twenty (20) percent at or around a wavelength of four hundred (400) nm, decreases between the wavelengths of four hundred (400) nm to around six hundred and fifty (650) nm, and increases between the wavelengths of around six hundred and fifty (650) nm to eight hundred (800) nm (e.g., to a percentage of transmission of approximately ten (10) percent). 
     The plot  1108  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of ten (10) minutes. The percentage of transmission in the plot  1108  begins to increase around three hundred (300) nm, peaks at a percentage of transmission of approximately ten (10) percent at or around a wavelength of four hundred (400) nm, decreases between the wavelengths of four hundred (400) nm to around six hundred and fifty (650) nm, and increases between the wavelengths of around six hundred and fifty (650) nm to eight hundred (800) nm (e.g., to a percentage of transmission of approximately five (5) percent). 
     The plot  1110  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of twenty (20) minutes. The percentage of transmission in the plot  1110  begins to increase around three hundred and fifty (350) nm, peaks at a percentage of transmission of approximately five (5) percent at or around a wavelength of four hundred (400) nm, decreases between the wavelengths of four hundred (400) nm to around six hundred (600) nm, and remains at approximately zero between the wavelengths of around six hundred (600) nm to eight hundred (800) nm. 
     The plot  1112  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of forty (40) minutes. The plot  1114  represents the percentage of transmission across the range of wavelengths for a particulate film with a spray time of sixty (60) minutes. The percentages of transmission in the plots  1112 ,  1114  are substantially at a percentage of transmission of zero (0) across the range of wavelengths. The plots  1104 ,  1106 ,  1108 ,  1110 ,  1112 ,  1114  illustrate the increasing opacity of the particulate films as the spray times increase. 
     The percentages of transmission of multiple densified films are based on different spray times and are represented by dashed lines in the graph  1100 . In the illustrated graph, the plot  1116  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of one (1) minute. The percentage of transmission in the plot  1116  begins to increase around three hundred (300) nm and approaches a given percentage of transmission of approximately sixty (60) percent by the wavelength of eight hundred (800) nm. 
     The plot  1118  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of five (5) minutes. The percentage of transmission in the plot  1118  begins to increase around three hundred (300) nm and approaches a given percentage of transmission of approximately fifty (50) percent by the wavelength of eight hundred (800) nm. 
     The plot  1120  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of ten (10) minutes. The percentage of transmission in the plot  1120  begins to increase around three hundred (300) nm and approaches a given percentage of transmission of approximately forty-five (45) percent by the wavelength of eight hundred (800) nm. 
     The plot  1122  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of twenty (20) minutes. The percentage of transmission in the plot  1122  begins to increase around three hundred (300) nm and approaches the given percentage of transmission of approximately fifty (50) percent by the wavelength of eight hundred (800) nm. 
     The plot  1124  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of forty (40) minutes. The percentage of transmission in the plot  1124  begins to increase around three hundred (300) nm and approaches the given percentage of transmission of approximately forty-five (45) percent by the wavelength of eight hundred (800) nm. 
     The plot  1126  represents the percentage of transmission across the range of wavelengths for a densified film with a spray time of sixty (60) minutes. The percentage of transmission in the plot  1126  begins to increase around three hundred (300) nm and approaches the given percentage of transmission of approximately fifty (50) percent by the wavelength of eight hundred (800) nm. The plots  1116 ,  1118 ,  1120 ,  1122 ,  1124 ,  1126  illustrate the increasing transparency of the densified films as the spray times increase. Except for the plot  1116 , the percentages of transmission in the plots  1118 ,  1120 ,  1122 ,  1124 ,  1126  all reach around forty-five (45) to fifty (50) percent by the end of the wavelength range. In the plot  1116 , the percentage of transmission reaches around sixty (60) percent by the end of the wavelength range. 
     The electrospray-printed films can have additional characteristics and may be printed based on one or more selected characteristics. One additional characteristic of an electrospray-printed film is a dielectric strength of the electrospray-printed film.  FIG.  12 A  illustrates an example graph  1200  of a dielectric strength of a silicon substrate, and  FIG.  12 B  illustrates an example graph  1202  of a dielectric strength of a film that is electrospray-printed onto a silicon substrate in accordance with embodiments of the disclosure. The horizontal axis represents time (in seconds), the right vertical axis represents current (in milliamps (mA)), and the left vertical axis represents volts. The silicon substrate is clamped between two metal electrodes and an increasing voltage is applied to one metal electrode. Similarly, the film-coated silicon substrate is clamped between the two metal electrodes and an increasing voltage is applied to one metal electrode that is in contact with the film. The dielectric strengths of the silicon substrate and the film coating on the silicon substrate are determined by how much time passes before a current breaks through and propagates in the silicon substrate and the voltage that were applied at the time of breakthrough. 
     As shown in  FIG.  12 A , a current breaks through very quickly (e.g., after one (1) to two (2) seconds) after application of approximately one hundred (100) volts to the silicon substrate. However, as shown in  FIG.  12 B , a current does not break through immediately after the application of the volts to the film. Instead, the current breaks through at or near twelve (12) seconds and at approximately one thousand, one hundred, and sixty (1160) volts, and the current propagates through the film to the silicon substrate. The twelve second time delay and the voltage at breakthrough in  FIG.  12 B  indicate the film has a higher dielectric strength compared to the silicon substrate. 
     Another characteristic is a hydrophobicity of an electrospray-printed film. In certain embodiments, hydrophobicity also depends on whether the film is a dry film or a semi-dry film. For example, an underlying silicon substrate is moderately hydrophilic, with a water contact angle of approximately fifty (50) to sixty (60) degrees. In contrast, electrospray-printed particulate (dry) films, such as a dry polyimide film, are also moderately hydrophobic but with an average water contact angle of one hundred and ten (110) degrees.  FIG.  13 A  illustrates a water droplet  1300  on an electrospray printed particulate film  1302  in accordance with embodiments of the disclosure. Moderate levels of hydrophobicity can reduce moisture penetration into and through the films. Additionally, moderate levels of hydrophobicity can provide good resistance to corrosion. 
     In some instances, due to the smoother surfaces, densified (semi-dry) films can be more hydrophilic compared to particulate films.  FIG.  13 B  illustrates a water droplet  1304  on an electrospray printed densified film  1306  in accordance with embodiments of the disclosure. As shown, a densified film may have an average water contact angle of eighty (80) degrees. 
       FIG.  14    illustrates a flowchart of a method of electrospray printing in accordance with embodiments of the disclosure. Initially, as shown in block  1400 , a flow of the solution is received by the printhead (e.g., by the mixing receptacle  316  in  FIG.  3   ). One or more flows of air are received by the printhead at block  1402 . At block  1404 , a charge or an electrical potential is applied to the solution to produce an electrically charged solution. In the example embodiment shown in  FIG.  3   , the electrical potential is applied to the solution in the mixing receptacle  316  to produce the electrically charged solution. 
     The electrically charged solution is emitted into the printhead to produce the narrow plume of electrically charged droplets (block  1406 ). The solvent in the electrically charged droplets begins to evaporate thereafter to produce a spray of electrically charged dry particles or electrically charged semi-dry particles. As described earlier, the one or more flows of air assist in the evaporation process. 
     Next, as shown in block  1408 , the electrically charged particles (e.g., dry particles or semi-dry particles) are then deposited onto a target substrate. The target substrate may have level or uniform surfaces or at least one non-uniform surface. For example, the target substrate can have one or more components on at least one surface, or the target substrate may be one or more components (e.g., component  400  in  FIG.  4   ). A focused electric field that is created by the electrical potential guides the narrow plume of electrically charged droplets within the printhead and guides the spray of electrically charged particles onto the target substrate. 
     A determination is made at block  1410  as to whether the electrospray-printed film has achieved one or more characteristics of a film. The one or more characteristics include, but are not limited to, a selected thickness, an optical property, a dielectric strength, and/or a level of hydrophobicity. If a determination is made that the printed film has not achieved the one or more characteristics, the method returns to block  1408  where the printing of the film continues. When a determination is made at block  1410  that the printed film has achieved the one or more characteristics, the method passes to block  1412  where the printing process ends. 
     As should be appreciated,  FIG.  14    is described for purposes of illustrating an example method and is not intended to limit the disclosure to a particular sequence of blocks. One or more blocks may be added or omitted. For example, block  1410  can be omitted in some embodiments. Additionally or alternatively, some of the blocks may be performed in parallel rather than in sequence. For example, blocks  1400  and  1402 , or blocks  1400 ,  1402 , and  1404 , can be performed in parallel in certain embodiments. 
       FIG.  15    is a block diagram of a computing device  1500  that can be used as the computing device  224  shown in  FIG.  2    in accordance with embodiments of the disclosure. Optionally, the computing device  1500  can be a well-known computing system including, but not limited to, personal computers, servers, handheld or laptop devices, multiprocessor systems, microprocessor-based systems, network personal computers (PCs), minicomputers, mainframe computers, embedded systems, and/or distributed computing environments including a plurality of any of the above systems or devices. Distributed computing environments enable remote computing devices, which are connected to a communication network or other data transmission medium, to perform various tasks. In the distributed computing environment, the program modules, applications, and other data may be stored on local and/or remote computer storage media. Some or all of the components in the computing device  1500  may further be implemented in other types of devices, such as any device that is operable to include one or more sensor modules and at least one input surface. 
     The example computing device  1500  includes a processing device  1502  and a memory  1504  (e.g., a storage device). Any suitable processing device  1502  can be used. For example, the processing device  1502  may be a microprocessor, an application specific integrated circuit, a field programmable gate array, or combinations thereof. 
     Depending on the configuration and type of the computing device  1500 , the memory  1504  may comprise, but is not limited to, volatile storage (e.g., random access memory), non-volatile storage (e.g., read-only memory), flash memory, or any combination of such memories. The memory  1504  may include a number of program applications and data files, such as an operating system  1506  and one or more program applications  1508 . The one or more program applications  1508  may include a graphical user interface program and at least one control applications. In some instances, at least one program application is operable to cause one or more operations described herein to be performed. For example, a program application can be used to control, or cause to be controlled, one or more operations of the airflow controller  222 , the device  220 , and/or the power supply  218  shown in  FIG.  2   . The one or more program applications  1508  can comprise processor-executable instructions, that when executed by the processing device  1502 , cause operations to be performed. 
     The operating system  1506 , for example, may be suitable for controlling the operation of the computing device  1500 . Furthermore, embodiments of the disclosure may be practiced in conjunction with a graphics library, other operating systems, or any other application program and is not limited to any particular application or system. 
     The computing device  1500  may have additional features or functionality. For example, the computing device  1500  can include a display  1510  and one or more input devices  1512  that allow a user to enter information into the computing device  1500 . The input device(s)  1512  can include one or more buttons, a keyboard, a trackpad, a mouse, a pen, a sound or voice input device, or an audio input (e.g., a microphone jack). The display  1510  may also function as an input device (e.g., a touch-sensitive display that accepts touch and/or force inputs). 
     The computing device  1500  may also include additional storage devices such as a removable storage device  1514  and a non-removable storage device  1516 . The removable storage device  1514  and the non-removable storage device  1516  are operable to store processor-executable instructions, that when executed by the processing device  1502 , may cause operations to be performed. The memory  1504 , the removable storage device  1514 , and/or the non-removable storage device  1516  may include RAM, ROM, electrically erasable read-only memory (EEPROM), flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic storage devices, or any other article of manufacture which can be used to store information, and which can be accessed by the computing device  1500 . In one embodiment, the memory  1504 , the removable storage device  1514 , and the non-removable storage device  1516  do not include a carrier wave or other propagated or modulated data signal. 
     The computing device  1500  may include one or more output devices  1518  such as a display (e.g., display  1510 ), an audio transducer (e.g., a speaker), a visual indicator (e.g., a light emitting diode), a vibration transducer for providing the user with tactile feedback (e.g., haptic feedback), an audio output (e.g., a headphone jack), or a video output (e.g., a HDMI port) for sending signals to or receiving signals from an external device. The aforementioned devices are examples and others may be used. 
     The computing device  1500  may also include one or more wired or wireless communication devices  1520  allowing communications with other computing devices  1522 . Examples of suitable communication devices  1520  include, but are not limited to, a radio frequency (RF) transmitter, receiver, and/or transceiver circuitry, a universal serial bus (USB), and/or parallel and/or serial ports. 
     As should be appreciated,  FIG.  15    is described for purposes of illustrating example components and is not intended to limit the disclosure to a particular combination of hardware or software components. In other embodiments, the computing device  1500  can include additional or fewer components than the components shown in  FIG.  15   . For example, a computing device may omit the removable storage device  1514  and/or the non-removable storage device  1516 . 
     It is contemplated that any of the foregoing aspects, and/or various separate aspects and features as described herein, may be combined for additional advantage. Any of the various embodiments as disclosed herein may be combined with one or more other disclosed embodiments unless indicated to the contrary herein. 
     Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.