Patent Grant
8919899
Embodiments of the invention generally relate to microfluidic jetting devices, particularly devices for direct printing of features on a substrate.
General Discussion of Liquid Jetting
It is well-known that a stable liquid jet is formed by forcing a liquid of sufficient viscosity through an orifice—(Edmund, 2006) Macroscopic liquid jets are observed in nature when a pressurized vessel containing a liquid of sufficiently low viscosity is punctured. The flow from the orifice created by the puncture produces a liquid jet with a flow rate that is dependent on the orifice diameter and the pressure within the vessel. Common examples of moderate viscosity jets are honey or oil flowing from an opening. It is also commonly known that jets of medium to low-viscosity liquids, i.e. water, ethylene glycol, and isopropanol, have inherent instabilities and can break up into droplets at varying distances from the jet orifice. Many studies however have been performed to confirm the stable propagation of medium to low-viscosity jets before filament breakup—(Habibi, 2010), (Edmund, 2006). Habibi has performed experimental studies of buckling of stable filaments of silicon oil ejected from an orifice: The silicon oil filaments propagated along distances as great as 60 cm. Edmund has studied the stability of a jet of a viscoelastic liquid formed using hydrodynamic focusing. (Takahashi, 1969) has reported liquid jets of water in air propagating to distances as large as 20 cm before breakup.
The Microfluidic Liquid Jet printing concept was inspired by the observation of stable jets of liquid commonly seen in nature. Examples of microfluidic liquid transport and microfluidic liquid jets are prevalent in nature. Indeed, over 40,000 known species of spiders exist, most of which are classified as web-spinning. Spider silk fibers are spun from pressurized abdominal sacs containing a polymeric solution. The jet dries in-flight to diameters of approximately 2.5 to 4.0 microns, and is used to make intricate patterned webs. The present invention produces a stable co-axial liquid jet by maintaining a constant pressure within a microfluidic flow cell used to form the co-axial liquid distribution.
General Description of Hydrodynamic Focusing
In hydrodynamic focusing, an annular distribution of a core liquid and a sheath liquid is forced through a channel or nozzle, with the core liquid being stretched into a thin filament as the liquids accelerate through the constriction. The width of the core filament is a function of the ratio of the core and sheath flow rates. In hydrodynamic focusing, the diameter of the core liquid is proportional to the fractional volume occupied by the core liquid. As the ratio of the core liquid flow rate to the sheath liquid flow rate is decreased, the volume occupied by the core liquid decreases, and thus reduces the diameter of the core liquid filament. In hydrodynamic focusing applications, the core liquid is stretched into a filament with a width as small as 1 micron.
Hydrodynamic Focusing for Direct Printing Applications
In a Direct Printing technique, a liquid is deposited onto a substrate without the use of masks or lithographic techniques. The present invention uses hydrodynamic focusing to form a thin filament of ink surrounded by a sacrificial sheath liquid. In one application of hydrodynamic focusing to direct printing, two miscible liquids with limited diffusivity are used to obtain focusing of the core liquid. The radius of the core liquid is proportional to the ratio of the core and sheath flow rates, and in application of the present invention, can be varied from approximately 1 micron to 1000 microns.
Microextrusion
In a microextrusion process a viscous liquid is forced through an orifice and into an ambient or vacuum environment to form a filament with a fixed cross sectional area. Examples of extrusion processes are synthetic fiber spinning applications for the textile industry and spinning applications and the production of optical fibers. Microextrusion processes can be used to form filaments of metals, polymers, and ceramics with widths ranging from several hundred nanometers to tens of microns.
Coextrusion
In a coextrusion process two liquids are combined in a flow cell to produce an annular flow of a sheath and core liquid. Examples of coextrusion include the manufacture of microscopic and macroscopic cladded optical fibers and hollow core optical fibers. The coextrusion process can be used to form polymer-coated glass optical fibers. Maccagnan (U.S. Pat. No. 7,323,133 B2) has disclosed a method for coextrusion of ceramic microtubes, defined as tubular structures with characteristic dimensions on the order of microns. Fork (U.S. Pat. No. 8,322,025) describes an apparatus for forming high-aspect ratio gridline structures using a coextrusion process.
Hydrodynamic Focusing
Many apparatuses and process have been described that use hydrodynamic focusing for various applications. A number of inventors have used hydrodynamic focusing to generate a thin liquid filament surrounded by a sheath liquid within a flow cell. Under general conditions, the filament breaks into droplets due to instabilities in the liquid column. In another case, the filament is stretched until particles suspended in the core liquid are aligned and propagate single file through an observation zone for cytometric analysis. Howell (U.S. Pat. No. 8,398,935 B2) discloses a method for the production of an annular flow of low and high viscosity liquids for the production of polymer fibers. Rich (U.S. Pat. No. 8,187,888) discloses an apparatus for a pressure-based liquid delivery system for a flow cytometry application.
Coaxial Jetting
A coaxial jet is formed when the inner filament and outer sheath liquid are jetted from an output orifice to form an annular liquid jet. In 1969 (U.S. Pat. No. 3,416,730), Perry disclosed an apparatus for the production of multiple liquid jets, initially in contact, but without substantial mixing. Hertz (U.S. Pat. No. 4,196,437) discloses a method and apparatus for forming a compound liquid jet that breaks into droplets in an inkjet printing application. The present invention produces a coaxial jet consisting of two liquid streams with limited diffusivity, producing a stable jet with a breakup distance of at least 5 cm.
Summary of the Invention
Introduction
The invention provides for a method and apparatus for direct printing of microscopic to macroscopic features on a substrate in ambient conditions. Of particular interest is the provision of a process and apparatus for stable and repeatable deposition of various liquids onto substrates for additive manufacturing applications, including but not limited to metallization of rigid and flexible substrates, deposition of inorganic and organic samples for sensor applications, and deposition of various inks for green energy applications such as solar cell metallization and fuel cell development. The invention is used to produce a compound annular liquid jet consisting of a sheath and core liquid that propagates for up to 20 centimeters before breakup due to Rayleigh instabilities. The long nozzle to substrate distance allows for controlled evaporation of the sheath liquids using an external heater. A substrate is placed under the exit nozzle of the apparatus, at a height at which the jet is stable and continuous, so that a continuous printed feature is formed as the substrate is translated.
Two-Component Flow and Reduction of the Ink Deposition Rate
The feasibility of a liquid jet printing method depends largely on the ability of the process and apparatus to deliver ink to a substrate at a useful rate. The maximum practical deposition rate is determined by the maximum velocity and acceleration of state of the art motion control system. Stable delivery of an ink from a liquid jet apparatus becomes difficult for nozzle diameters less than approximately 50 microns. Indeed, clogging of small diameter nozzles by trapped bubbles or solid particles can be problematic for jetting systems, rendering such nozzles impractical for prolonged use. The invention circumvents the problem of clogging by using a two-component approach, in which an inner ink flow is forced through a nozzle in a dripping mode, while a sacrificial outer sheath flow is jetted through a larger diameter external nozzle. The relatively large diameter of the exit nozzle greatly reduces or eliminates the occurrence of clogging, and helps to stabilize the deposition process. The inner ink flow is forced through an inner nozzle at flow rates on the order of milliliters per hour, while the sheath liquid is transported through an exit nozzle at flow rates of tens of milliliters per hour.
The Jetting Regime
(Utada, 2008) and (Lei, 2011) have reported that, in co-flowing systems of two immiscible liquids, the dripping to jetting transition is determined by the Weber number of the inner flow or the capillary number of the outer flow. The capillary number represents the relative effect between viscous forces and surface tension forces acting across the interface of two dissimilar liquids. The Weber number represents the relative effect between inertia and surface tension. The processes and apparatuses of the present invention operate in the regime in which the transition from dripping to jetting is determined by the capillary number of the sheath flow. An unperturbed liquid jet will eventually break up into droplets as the liquid column moves towards a configuration that minimizes its surface area. In the present invention, the physical and operational parameters of the system and the fluid properties of the liquids are chosen so that the breakup distance of the jet is greater than 10 cm, and as long as 20 cm.
General Description of the Device
In a generalized case, the device consists of a pressure source and sample chamber or a syringe pump, a deposition head, a liquid orifice, a nozzle heater, a substrate heater, and a process control system. A general embodiment of the device is shown in
Apparatus and Process Parameters
The object of the invention is direct printing of electronic inks and various liquid compositions onto various substrates. In order for the deposition process to be feasible, an upper limit of the sample deposition rate is set by the maximum achievable speed and acceleration of typical motion control systems. The flow is continuous, and the upper limit of the flow rate Qmax is generally in the range of approximately 1 to 5 ml/hr. The flow rate through an orifice is inversely proportionate to the orifice diameter, and for inks with viscosities in the range of 1 to 5 cP, a practical Qmax is obtained with orifice diameters from approximately 10 to 25 microns. An apparatus using an orifice diameter greater than approximately 50 microns will produce flow rates that are too large to be useful for conventional direct printing. The continuous jetting process however becomes problematic due to clogging for orifice diameters less than approximately 50 microns. The present invention uses a two-component flow to reduce the effective diameter of the orifice by channeling the sample liquid (ink) through a sacrificial sheath liquid. The sheath and sample liquids form an annular flow, with the sample liquid in the interior of the flow. The liquids exit the nozzle orifice, and the sheath liquid is evaporated as the pressurized liquid column is jetted into the ambient atmosphere.
The sheath liquid is partially or fully evaporated during the jetting process. Evaporation of any residual sheath liquid is accomplished by heating the substrate to within 1 to 10 degrees of the sheath liquid boiling point. Evaporation of the sheath liquid thus results in the deposition of a trace formed from the core filament.
Rapid Evaporation of the Sheath Liquid—Conversion of Superheat to Latent Heat of Vaporization
In one embodiment of the device, a thermal assist is used to preferentially evaporate the sheath liquid from the compound jet. The sheath liquid is a sacrificial flow that reduces the diameter and the flow rate of the sample liquid as the flows emerge from the exit orifice. Fine line printing is facilitated when the sheath flow is partially or completely evaporated before striking the substrate. Rapid evaporation of the sheath flow is accomplished by using a heater assembly to superheat the pressurized sheath liquid. The sheath liquid is held above atmospheric pressure within the sheath chamber, flow cell, and exit channel, at a temperature above the boiling point of the sheath liquid at atmospheric pressure. As the sheath liquid exits the orifice and encounters the reduced ambient pressure, the energy used to raise the liquid above its atmospheric pressure boiling point is converted to latent heat of fusion, and rapid evaporation of the sheath liquid takes place.
In the preferred embodiment, the inner sample liquid has a boiling point that is substantially higher than that of the sheath liquid, so that the flow exiting the orifice consists of a partially or fully vaporized sheath component and a liquid inner sample component. The combined flow strikes a moving substrate, and produces a continuous line of the sample material.
Limited Diffusion and Viscoelastic Core Liquids
In some inkjet printing applications, viscoelastic inks are used to improve the deposition quality of the printed droplets. (Morrison, 2010) provides a summary of the use of viscoelastic inks formulations in inkjet printing. A viscoelastic ink will undergo an increase in viscosity has the strain rate on the liquid is increased. In inkjet printing, viscoelastic inks are used to minimize the formation of satellite droplets. Higgins (U.S. Pat. No. 8,398,226) discloses and inkjet printing system in which the rheology of an ink changes with the application of a temperature gradient, so that the viscosity of the ink increases upon deposition. Similarly, viscoelastic inks can be used in a two-component jet to improve the print quality of the deposited filament.
Process Control Using a Pressure/Light Intensity Feedback Loop
The maintenance of a stable jet will depend on the ability of the device to produce a stable pressure at the flow cell exit orifice. Referring to
Valving and Shuttering
The present invention provides for valving of the core liquid and shuttering of the sheath liquid. Shuttering of the compound core and sheath liquid jet is also possible. Interruption of the jet to the substrate surface must be accomplished for printing of discreet structures without the use of masks or stencils. In one embodiment a mechanical shutter consisting of a hollow tube connected to a collection chamber and an electromechanical actuator is used to shutter the jet and to collect the core and sheath liquids. In still another embodiment, an electromechanical valve is placed in the core liquid deliver conduit, so that shuttering of the core liquid flow is achieved without interruption of the sheath flow. In the case of a shuttered inner liquid flow, the jet becomes a single component jet composed of a column of the sheath liquid with a diameter equal to that of the compound jet.
Optical Fiber Exit Orifice
In another embodiment, an optical fiber is coupled to the exit channel 13 and forms the nozzle 12 of
Multi-Nozzle Liquid Jet Arrays
The general design of invention is applicable to the manufacture of multi-nozzle arrays. An embodiment of a multi-nozzle liquid jet array is shown in
Laser-Assisted Liquid Jet Deposition
In another embodiment the apparatus of
Direct Printing of UV Curable Inks
In one embodiment of Laser-Assisted Liquid Jet Deposition, the inner liquid is a UV curable ink. Focused or unfocused UV or visible laser radiation is directed onto the jet so that in-flight curing of the ink (core liquid) is accomplished. The laser radiation may also be focused onto the substrate deposition zone to promote real time curing of the deposited ink. A subsequent substrate heating step removes any residual sheath liquid from the substrate surface.
Direct Printing of Films and Discreet Structures
The microfluidic jet device, microjet, is capable of printing continuous lines on a substrate. If the substrate is placed some distance beneath the liquid jet such that the distance is above the point of initiation of Rayleigh instabilities, a continuous line is written as the substrate is moved. The width of the line depends on the device parameters, the fluid parameters, and the substrate speed. The microfluidic jet device is capable of operating at print speeds of approximately 1 to 5000 mm/sec.
Planar Flow Cell Structure with Chevron Groves to Effect Focusing of the Inner Core Liquid
It has been shown that hydrodynamic focusing is achieved in a planar structure when a series of chevron or v-shaped grooves is introduced in the flow cell after the sheath and core liquids are combined. Mott et. al. [U.S. Pat. No. 8,361,413] disclose the design of a planar flow cell, where in the sheath and core flows are introduced side by side, but form an annular configuration as the flows pass the chevrons. Although the sheath and core liquids are introduced in a planar channel where the sheath does not surround the core material, the chevrons force the sheath material to surround the core liquid and cause hydrodynamic focusing to occur. In one embodiment of the invention, the flow cell is made using a planar fabrication technique. The addition of chevron grooves in the planar structure allows the core liquid to be focused with a shaped cross-section. Allowable configurations include, but are not limited to circular and square cross sections. Using a planar fabrication technology to produce flow cells simplifies the manufacture of the flow cells and also enables the manufacture of closely-spaced arrayed flow cells.
3D Printing
The present invention can also be used to build three-dimensional structures using a layer-wise process, wherein simple and complex objects are printed directly from a computer-automated drawing (CAD) file. In the 3D printing process, laser-assisted deposition or a viscoelastic ink is used to deposit a liquid filament with a viscosity sufficient to form a rigid or semi-rigid structure upon which subsequent layers are deposited. In the 3D printing technique, a digital model of an object is intersected with horizontal planes. The horizontal planes form cross sectional representations or slices of the object. Information in each slice is uploaded to a computerized motion control system, so that a solid object can be fabricated using an additive manufacturing process. The Liquid Jet process can be used to fabricate three-dimensional objects from materials including, but not limited to metals, ceramics, and plastics.
3-D Structures for Medical Applications
In yet another embodiment the hydrodynamic focusing flow cell could be used to produce structures for medical applications. The flow cell technology could be used to produce scaffolding for tissue engineering applications. The same flow cell could also be used to print living cells and nutrients for those cells in tissue engineering applications.
Provisional application No. 61/645,225 filed May 10, 2012.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4473830 | Piatt et al. | Sep 1984 | A |
| 4734711 | Piatt et al. | Mar 1988 | A |
| 6572223 | Delametter et al. | Jun 2003 | B2 |
| 20100156970 | Ikushima | Jun 2010 | A1 |
| 20120147073 | Dahrea et al. | Jun 2012 | A1 |
| 20130155138 | Batchelor et al. | Jun 2013 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20140035975 A1 | Feb 2014 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61645225 | May 2012 | US |
