Chin-Tai Chen

Uniform Solute Deposition of Evaporable Droplet in Nanoliter Wells

There have been many microdeposition processes that are based on the evaporation of nanoliter-sized droplets, such as inkjet printing, deoxyribonucleic acid/protein microarrays, or lithography direct writing. However, it is important but still difficult to control the uniformity of the solute deposition from a nanoliter sessile droplet on a plane substrate. This paper proposes a method for uniform solute deposition from evaporable droplet by confining the droplet with rib structures (wells) of specific surface properties. The hydrodynamic process was experimentally investigated and analyzed in detail. Surface wettability on the well surface is verified to be critical for controlling a droplet as a flat film inside a well during evaporation to minimize horizontal solute transfer for uniform solute deposition. Pure water and water/tracing particle mixture (2.57% solid latex, dyed blue) were employed for the test. The results demonstrated that a 97% uniformity is obtained for the solute deposited from a 37-nL droplet in a well with hydrophobic surface (contact angle of 100deg), whereas a 31% uniformity is obtained for a more hydrophilic surface (contact angle of 25deg). The higher hydrophobicity (contact angle above 90deg) on the well surface yields a flatter profile of film during droplet evaporation inside a well and, thus, promotes a more uniform deposition of the solute.

Self-aligned hemispherical formation of microlenses from colloidal droplets on heterogeneous surfaces

This paper proposes a drop-on-demand (DOD) theory and scheme for constructing hemispherical refractive microlenses onto underlying heterogeneous (laterally structured) surfaces that consist of hydrophilic s-domains and hydrophobic p-domains. In theory, the drops would self-align themselves into the s-domains by repelling the p-domains due to surface tension, precisely determining the placement though disobeying the Young–Laplace equation. Using a droplet generator (inkjet printhead), in our experiments, evaporative polyurethane (PU) drops well fitted their footprints (base radii) onto the s-domains with a radius of 100 µm surrounded by the p-domains of Teflon, where the photoresist AZ4620 was used for lifting off the corresponding domains of Teflon. As a result, plano-convex shapes with spherical curvatures were fabricated in an array (spacing L ~ 100 µm) with base (footprint) radius (Rb) ~ 95 µm and curvature radius (Rc) ~ 122 µm. Thus, both the theoretical and experimental results agreed well in a hemispherical shape (deviation Rs + L/2 = 150 µm, in excess of volume), indicates that more complex shapes than spherical ones (such as dumbbell and cross-like) can also be constructed by bridging the drops. Compared to those previous methods using photolithographic techniques, the present method is potentially appropriate for the varying radius and complex placement of array patterns.

Inkjet Printing of Microcomponents: Theory, Design, Characteristics and Applications

Inkjet printing technology has been invented and used in the form of continuous jets for typewriting and recording over 50 years. However, this kind of printing technique becomes truly popular within twenty years for the public at homes and offices greatly thanks to the consumer products of desktop inkjet printers developed and marketed in the middle of the 1980s. Those leading manufacturing companies including Hewlett-Packard (HP), Canon, and Seiko Epson et al. have been celebrated around the world for the evolution of the modern digital printers since then. Compared to the mechanical typewriters in the past, the new inkjet printers perform the printing by dye or pigment-based ink droplets jetted through micro-electro-mechanical actuators, which feature non-impact and digital-control merits, thereby generating less noise and power consumption. Furthermore, as operating in called drop-on-demand (DOD) mode, their droplets with sub-nanoliter volume form the dot-matrix patterns onto medium surfaces, rendering static images for human sight with resolution from low to more than 1200 dot per inch (dpi). To achieve high quality ink printing on various substrates, dispensing the micro droplets precisely from nozzles to underlying medium propose some technical difficulties concerning fluidic issues such as the variations in droplet volume, flight direction, and deposition morphologies. In terms of microfluidic flow, a stream of micro droplets significantly undergo high speed flying in air, impacting on a solid substrate, forming and drying above a substrate surface. Through the whole process, the issues about transition and stability of flow from liquid channels to individual droplets, and vice versa, will be frequently encountered in theory, which need to be delicately dealt with in advance of various applications. For instance, undesirable satellites from main droplets may be generated due to Rayleigh instability; similarly, it is possible to form irregular bulges within a lengthy liquid channel on a nonpenetrable substrate (e.g., glass substrate) instead of porous paper. Moreover, nonuniform evaporation can be caused in nature for volatile sessile droplets placed on flat surfaces, in which the called coffee-ring effect often seen for diluted fluids (Deegan et al., 1998) should be avoided for the requirements of uniform thickness. Besides the conventional dye and pigment inks, almost the materials in solution involve the complexity of evaporable droplets, when applying to the inkjet printing processes that rely heavily on the full understanding of droplet behaviors on various surfaces from wet to dry stages.

Inkjet-Printed Polymeric Microstructures in

We demonstrate an inkjet-based microfluidic technique as droplet vaporization deposition (DVD) that can be applied in the incorporation of hydrophobic substrates with microcavities to generate individual polymeric structures. Using soft-lithography method, the SU-8 and polydimethysiloxane were patterned to form various polygon-shaped cavities (side length ~200-700 μm and depth ~ 25-110 μm) which acted as micromolds (templates) for forming the shapes of the droplets deposited. With aqueous polyurethane droplets generated (single volume ~381 pL), the novel various microstructures in polygonal (tri- to hexagonal) forms were created through the deposition and evaporation processes and characterized with exotic performances in optics such as those of microlenses and micromirrors. Experimental and analytical results illustrated that the curved-surface (sinusoidal) topographies of the structures self-formed and detached from the underlying substrates were dominated by capillary action of fluid during evaporation. The merits over the existing techniques such as photolithography includes the one-step, low-temperature (ambient condition, ~1 atm at 25°C), and cost-effective (no waste materials) processes of solid formations. The successful formation of these structures suggests a new mechanism for self-releasing/detaching from the molds. As a reliable technique for the fabrication of such microstructures, the present DVD method presents an alternative method for various applications particularly including optical microelectromechanical system devices and parts used for assembly.

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We have investigated the morphologies of conductive liquid lines inkjet-printed onto homogeneous substrate. Various liquid lines, featured with closed to open loops at micrometer scale, can be formed by picoliter droplets composed of Ag nanoparticles solution. These lines with two-dimensional geometry in x and y directions are not simultaneously generated from the current printing algorithm and system. In theory, those microdroplets may undergo complex but fundamental phenomena such as slipping (de-pinning), replenishing and shrinking due to interfacial free energy during evaporation. In experiments, with the (printing) time and (interfacial) pressure difference, the printed uniform widths of the liquid lines from 10 picoliter droplets were spontaneously broken to be uneven after drying. Experimental results indicated that capillary flows within the lines were self-induced and oriented from the slipping segments to the pinning segments. As a result, morphological measurements showed relatively uniform thickness of the self-formations along the looped lines: no remarkable coffee-ring effect was found in both the line segments formed.

-Sided Regular Polygonal Cavities

The wireless capsule would have been more comfortable for the patients than the traditional endoscope or enterscope. The wireless capsules, such as Heidelberg capsule, pH capsule or M2A capsule, just would be the diagnostic tool. The present paper would show the prototype system of the functional capsule endoscope that could bring medicine to the nidus and jet the medicine. The prototype system would have the imager, illumination source, microlens, two-way wireless module and medicine-jet subsystem.

Morphologies of conductive looped liquid lines inkjet-printed on substrate surfaces

The paper presents a new processing method for design and fabrication of microlens by microfluidic deposition, in which the lenses can be formed on the media with high precise location in the fabrication process. First we may apply physical vaporization deposition (PVD) or spin coating to fabricate a thin film; secondly, the hydrophilic pattern specified by the mask of lenses can be created by photolithography, where the special lens media is made herein. Finally, the liquid solution of lens material is deposited onto the predetermined location of the lens media by the drop-on-demand fluidic actuator; thus, the microlenses are consequently formed after the complete evaporation of solvent in the solution.

The prototype system of medicine-jet capsule endoscope

We report the experimental results of evaporative cooling effect of a novel piezoelectric droplet actuator designed for a free space utilizing dynamic infrared thermographs as their space-time courses of temperatures recorded. These droplets of pure water were jetted as working liquid of an evaporating cooling system comprising a PZT micro-actuator, controller and liquid delivery for the circulation of air with the cooling space. Cyclic cooling tests demonstrated that these micro droplets well performed the evaporative cooling effectively. The space cooling for such an evaporative cooling system was studied by a high-performance infrared imager that enabled all the temperature changes of the space to be recorded and analyzed per three seconds, yielding uniform water droplets (<; 100 μm) with cooling rate ~ -1.1°C/min.

Design and fabrication of microlens by microfluidic deposition method

Droplet deposition is so ubiquitous for the production of microstructures using versatile liquid-based methods such as inkjet-printing technology. One of key elements for success in the design involves the use of non-homogeneous substrates to effectively control the complex morphologies of solute deposit during evaporation. In this case, there are still few studies investigating what contours of the solid contents form as the solvents evaporate. This paper reports a droplet evaporation deposition (DED) method that improves the controllability for the curvatures of microstructures utilizing the heterogeneous substrates. Experimental and analytical results indicate that the profile curvatures of formations after droplet evaporation are subject to variation in different droplet volumes placed on structured surfaces, thus providing the capability of control over solid contours.

Specific design and implementation of a piezoelectric droplet actuator for evaporative cooling of free space

We reports the experimental results of the novel polygonal microstructures self-assembled from the evaporating picoliter droplets in micro-scale cavities. Utilizing the inkjet-like method, the drop-on-demand (DOD) droplets of colloidal polymers (Polyurethane, PU 15% in volume) can be precisely positioned into the constrained regions on the polymeric (Polydimethysiloxane, PDMS) substrates which are patterned by current (soft-)lithography technique. The novel various microstructures in polygonal (tri-to hexagonal) forms are created and characterized with specifically optical performances applicable to the potential micro devices such as microlenses and mirrors.