Double transfer printing of small volumes of liquids

Stretching breakup of a conical liquid bridge with a moving contact line

The stretching breakup of a conical liquid bridge is the core process of micro-dispensing. To precisely control the droplet loading and improve the dispensing resolution, a detailed study of bridge breakup with a moving contact line is required. A conical liquid bridge is established by an electric field and stretching breakup is investigated here. The effect of contact line state is investigated by examining the pressure at the symmetry axis. Compared to the pinned case, the moving contact line causes a shift of the pressure maximum from the bridge neck to top, and it facilitates the evacuation of the bridge top. For the moving case, factors affecting the contact line motion are then considered. The results show that the increase of the stretching velocity U and the decrease of the initial top radius R _top accelerate the contact line motion. And the amount of contact line movement is basically constant. To analyze the influence of the moving contact line on bridge breakup, neck evolution is tracked under different U. An increase of U decreases the breakup time and increases the breakup position. Based on the breakup position and the remnant radius, the influences of U and R _top on remnant volume V _d are examined. It is found that V _d decreases with an increase of U and increases with an increase of R _top. Accordingly, different sizes of remnant volume can be obtained by adjusting U and R _top. This is helpful for the optimization of liquid loading for transfer printing.

Rupture of Liquid Bridges on Porous Tips: Competing Mechanisms of Spontaneous Imbibition and Stretching

Liquid bridges are commonly encountered in nature and the liquid transfer induced by their rupture is widely used in various industrial applications. In this work, with the focus on the porous tip, we studied the impacts of capillary effects on the liquid transfer induced by the rupture through numerical simulations. To depict the capillary effects of a porous tip, a time scale ratio, R_T, is proposed to compare the competing mechanisms of spontaneous imbibition and external drag. In terms of R_T, we then develop a theoretical model for estimating the liquid retention ratio considering the geometry, porosity, and wettability of tips. The mechanism presented in this work provides a possible approach to control the liquid transfer with better accuracy in microfluidics or microfabrications.

Rupture of a Liquid Bridge between a Cone and a Plane

In this work, a systematic experimental study of the rupture of an axially symmetric liquid bridge between a cone and a plane was performed, with focus on the volume distribution after break up. A model based on the Young-Laplace equation is presented, and its solutions are compared to experimental data. Cones and conical cavities with different aperture angles were used in our experiments. We found that this aperture influences the potential pinning of the contact line, the meniscus shape, and therefore the liquid transfer. For half aperture angles α < 70°, where no pinning was observed, the liquid bridge slips off from the cone and almost no transfer to the cone is observed. However, at α > 70°, contact line pinning on the cone induces a net liquid transfer to the cone at rupture. In the case of conical cavities, a maximum of liquid transfer is observed for at α = 110°. The distance at which the rupture of the liquid bridge occurs is also discussed. The model can fairly predict the transfer ratio and the rupture height of the liquid bridge.

Electrostatic Assist of Liquid Transfer between Flat Surfaces

Transfer of liquid from one surface to another plays a vital role in printing processes. During liquid transfer, a liquid bridge is formed and subjected to substantial extension but incomplete liquid transfer can produce defects that are detrimental to the operation of printed electronic devices. One strategy for minimizing these defects is to apply an electric field, a technique known as electrostatic assist (ESA). However, the physical mechanisms underlying ESA remain a mystery. To better understand these mechanisms, slender-jet models are developed for both perfect dielectric and leaky dielectric axisymmetric Newtonian liquid bridges with moving contact lines. Nonlinear partial differential equations describing the evolution of the bridge radius and interfacial charge are derived and then solved using finite element methods. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface over a wide range of capillary numbers. The electric field modifies the pressure differences inside the liquid bridge and, as a consequence, drives liquid toward the more wettable surface. For leaky dielectrics, charge can accumulate at the liquid-air interface. Application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the surface charge. Flow visualization experiments reveal that when an electric field is applied, more liquid is transferred to the more wettable surface because of a modified bridge shape that causes depinning of the contact line. The measured values of the amount of liquid transferred are in good agreement with predictions of the perfect dielectric model.

A Platform for Electric Field Aided and Wire-Guided Droplet Manipulation

Small droplets can be manipulated based on controlling the adhesion work to a hydrophobic wire. The wire can be used to pick up, transport, and lay down droplets with volumes between picoliters to microliters avoiding the sliding of droplets over chip surfaces. Handling of droplets on surfaces with large steps such as engraved wells or channels is possible.

Stretching liquid bridges with moving contact lines: comparison of liquid-transfer predictions and experiments

Transfer of liquid from one surface to another plays a key role in printing processes. During liquid transfer, a liquid bridge is formed and then undergoes significant extensional motion while its contact lines are free to move on the bounding solid surfaces. In this work, we develop one-dimensional (1D) slender-jet and two-dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. For very low capillary numbers (Ca) (quasi-static stretching), predictions from both models of the amount of liquid transferred agree well with the experimental data, provided that the difference in receding contact angles (|Δθ_r|) between the two surfaces is sufficiently large. Notably, the amount of liquid transferred is primarily governed by the overall bridge shape and is not significantly influenced by contact-line motion toward the end of bridge stretching. For O(1) values of Ca, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of Ca (and very low values of Ca when |Δθ_r| is small enough), predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface heterogeneities that are not included in the models. In this regime, there can be significant differences between the predictions of the 1D and 2D models, which is due to the tendency of the contact line to slip more in the 1D model. The models are also used to understand the influence of initial bridge shape on liquid transfer and to rationalize related experimental observations. The results from these fundamental studies will aid the optimization of gravure and other printing processes for manufacturing of printed electronic devices.

Surface-scribed transparency-based microplates

Transparency sheets, which are normally associated with use on overhead projectors, offer lowered costs and high amenability for optical diagnostics in microplate instrumentation. An alternative microplate design in which circles are scribed on the surface of the transparency to create the boundaries to hold the drop in place is investigated here. The 3D profile of the scribed regions obtained optically showed strong likelihood of affecting three-phase contact line interactions. During dispensation, the contact angle (≈95°) was larger than the drop advancing state (≈80°) due to a period of nonadhesion, where the contact angle later reduced to the drop advancing state followed by increase in the liquid area coverage on the substrate. It was established that 50 μL was needed to fill the well fully, and the maximum volume retainable before breaching was 190 μL. While the tilt angle needed for displacement reduced significantly from 50 to 95 μL, this was markedly better than nonscribed surfaces, where tilt angles always had to be kept to within 30°. It was found that there was greater ability to fill the well with smaller volumes with dispensation at the center. This was attributed to the growing contact line not meeting the scribed edge in parallel if liquid was dispensed closer to it, wherein pinning reduction in some directions permitted liquid travel along the scribed edge to undergo contact angle hysteresis. Fluorescence measurements conducted showed no performance compromise when using scribed transparency microplates over standard microplates.

Stretching liquid bridges with bubbles: the effect of air bubbles on liquid transfer

Liquid bridges containing bubbles are relevant to industrial printing and are also a topic of fundamental scientific interest. We use flow visualization to study the stretching of liquid bridges, both with and without bubbles, at low capillary numbers. We find that whereas the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force air bubbles selectively toward the least hydrophilic plate confining the liquid bridge.

Spectral tuning of conjugated polymer colloid light-emitting diodes

In recent years, the importance of the polymer light-emitting diode (PLED) has grown immensely, proving very desirable in numerous applications because of very high efficiencies, low power consumption, and ease of fabrication. Typically, these devices have been constructed in a layered, thin-film fashion consisting of either electron- and hole-transport materials doped with a luminescent dye (Hebner, T. R.; Sturm, J. C. Appl. Phys. Lett. 1998, 73, 1775. Jiang, X.; Register, R. A.; Killeen, K. A.; Thompson, M. E.; Pschenitzka, F.; Hebner, T. R.; Sturm, J. C. J. Appl. Phys. 2002, 91, 6717. Yeh, K. M.; Chen, Y. Org. Electron. 2008, 9, 45-50. Oh, G. C.; Yun, J. J.; Park, S. M.; Son, S. H.; Han, E. M.; Gu, H. B.; Jin, S. H.; Yoon, Y. S. Mol. Cryst. Liq. Cryst. 2003, 405, 43-51. Lee, J. I.; Chu, H. Y.; Kim, S. H.; Do, L. M.; Zyung, T.; Hwang, D. H. Opt. Mater. 2003, 21, 205-210. Hwang, D.-H.; Park, M.-J.; Lee, C. Synth. Met. 2005, 152, 205-208) or a conjugated polymer that can be engineered to tune the emission of the PLED to particular wavelengths. Stable PLED aqueous colloidal dispersions were prepared containing poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], (MEH-PPV), poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), and a binary poly(9,9-di-n-octylfluorenyl-2,7-diyl)/poly(2,5-dioctyl-1,4-phenylenevinylene) (PFO/POPPV) particle dispersion. Red-, green-, and blue-light-emitting colloidal dispersions could then be combined to achieve color-tailored emissions spanning the visible spectrum.

Ultrasmall liquid droplets on solid surfaces: production, imaging, and relevance for current wetting research

The investigation of micro- and nanoscale droplets on solid surfaces offers a wide range of research opportunities both at a fundamental and an applied level. On the fundamental side, advances in the techniques for production and imaging of such ultrasmall droplets will allow wetting theories to be tested down to the nanometer scale, where they predict the significant influence of phenomena such as the contact line tension or evaporation, which can be neglected in the case of macroscopic droplets. On the applied side, these advances will pave the way for characterizing a diverse set of industrially important materials such as textile or biomedical micro- and nanofibers, powdered solids, and topographically or chemically nanopatterned surfaces, as well as micro-and nanoscale devices, with relevance in diverse industries from biomedical to petroleum engineering. Here, the basic principles of wetting at the micro- and nanoscales are presented, and the essential characteristics of the main experimental techniques available for producing and imaging these droplets are described. In addition, the main fundamental and applied results are reviewed. The most problematic aspects of studying such ultrasmall droplets, and the developments that are in progress that are thought to circumvent them in the coming years, are highlighted.