Article pubs.acs.org/Langmuir
Anomalous Thermally Induced Pinning of a Liquid Drop on a Solid Substrate Srinivas Mettu,† Mandakini Kanungo,† and Kock-Yee Law* Xerox Corp., Xerox Research Center Webster, 800 Phillips Rd, 147-59B, Webster, New York 14580, United States S Supporting Information *
ABSTRACT: The effect of substrate temperature on the wetting and spreading behavior of a UV ink monomer has been studied as a surrogate for the ink on four different substrates: DTC (digital top coat)-coated BOPP (biaxial oriented polypropylene), Flexo-coated BOPP, DTC-coated SGE (semigloss elite) paper, and Flexo-coated SGE paper. Results show that the dynamic contact angles of the monomer decrease exponentially over time after contacting the surface, and the rate of spreading is consistently higher at 95 °C than at 22 °C. This observation indicates that spreading is controlled by the viscosity of the monomer as it decreases with temperature. An anomalous temperature effect is observed for the static contact angle on the DTC-coated BOPP substrate. The static contact angle at 95 °C is significantly larger than that at 22 °C (52° versus 30°). This is counterintuitive, as the surface tension of the monomer is shown to decease with increasing temperature. Microscopy (SEM and AFM) studies suggest that there is little interaction between the DTC coating solution and the BOPP substrate during the fast-drying coating process. This results in a smooth coated surface and, more importantly, voids between the BOPP nanofibers underneath the DTC coating. As the DTC−BOPP substrate is heated to 95 °C, fiber expansions occur. Microscopy results show that nanosized protrusions are formed on the DTC surface. We attribute it to fiber expansions in the vertical direction. Fiber expansions in the lateral direction causes little surface morphology change as the expanded materials only fill the voids laterally between the nanofiber network. We suggest that the protrusions on the surface create strong resistance to the wetting process and pin the monomer drop into a metastable wetting state. This interpretation is supported by the sliding angle and sessile drop height experiments.
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size in the fabrication of a thin-film-transistor (TFT) array.6 A jetted ink drop is also known to form the so-called strained coffee ring due to unoptimized spreading and drying, which is detrimental to the quality of the printed image.8 The use of phase-change UV-curable ink in inkjet printing is a promising approach to achieve high-speed, high-quality commercial printing.9,10 Similar to the work reported by Arias and co-workers on inkjet printing of TFT array and Kim et al. on fountain pen writing, controlling wetting and spreading of the ink on the print substrate are obviously important in the image quality of the final output. In this work, we study the wetting and spreading behavior of a UV-curable ink and its monomer (the main component in the ink formulation) by time-dependent contact angle measurements. The temperature of the liquid droplets was set at 95 °C, the optimal jetting temperature for the ink for rheological reasons. Four commercially available label stocks, namely, Flexo-coated BOPP (biaxial oriented polypropylene), DTC (digital top coat)-coated BOPP, Flexo-coated SGE (semigloss elite) paper,
INTRODUCTION Fundamental understanding of wetting and spreading of liquid on a solid surface is not only an important topic in surface science but it also has tremendous value in many applications, such as painting, coating, printing, lubrication, and liquid transportation in microfluidic devices. For instances, Whitesides and co-workers1−4 reported the creation of millimeter-sized hydrophilic channels in paper by patterning it with a hydrophobic polymer (PDMS) solution. The patterned paper comprises a hydrophilic region and a hydrophobic region, and the latter defines and controls the wetting and spreading of blood or other bodily fluid samples in bioassay and other microfluidic diagnostic devices. Even for simple ink writing with a pen, Kim and co-workers5 found that the line width of the ink image on paper depends on the speed of the pen as well as the physicochemical properties of both ink and paper. Controlling ink wetting and spreading is crucial in avoiding blog and ink spreading. As for printing technology, inkjet printing has become a manufacturing technology of choice for large-area electronic devices, such as organic transistors and backplanes in display and solar cell.6,7 Arias et al. showed that a balance between pinning and overspreading of printed liquid ink on the solid surface is crucial in defining the position, resolution, and © 2013 American Chemical Society
Received: March 15, 2013 Revised: June 15, 2013 Published: July 30, 2013 10665
dx.doi.org/10.1021/la400991y | Langmuir 2013, 29, 10665−10673
Langmuir
Article
constant of 30 N/m was used. The cantilever was oscillated in its resonance frequency (290 kHz). The amplitude of the oscillation decreased when the tip was brought close to the surface, and the tip measured either attractive or repulsive forces. The surface topography was determined by the height of the images. In the phase imaging mode, the phase lag of the cantilever oscillation, relative to the signal sent to the piezo driver of the cantilever, was monitored and recorded. The phase lag was very sensitive to variations in material properties at very high resolution, providing better contrasts relative to the topographic images. In this work, in situ assessment of the morphologies of uncoated BOPP, DTC-coated BOPP, and Flexocoated BOPP was carried out both at room temperature (22 °C) and at 95 °C. Each sample was placed inside a small portable heater. The morphology of the substrate was measured at 22 °C and the sample was then heated to 95 °C, where the high-temperature morphology was recorded. To demonstrate that the morphological change is reversible, the sample was cooled to room temperature after measurement at 95 °C. The morphology at 22 °C was measured again and found to be identical to that of the beginning of the experiment. The surface properties were also found to be unchanged.
and DTC-coated SGE paper, were studied. Two substrate temperatures, room temperature (22 °C) and 95 °C, were investigated. Initial experiments indicate that the wetting and spreading behaviors of the UV ink are dictated by the monomer. Since the UV ink is a gel at room temperature, we use the monomer as a surrogate to investigate the wetting and spreading behavior under various potential printing conditions. Results show that the rates of wetting and spreading increase as the temperature increases on all four substrates. The increase in rates is attributable to the decreases in both surface tension and viscosity of the monomer (Figure S1 in Supporting Information).11 An anomalous temperature effect is observed for the static contact angle on the DTC-coated BOPP substrate. A static contact angle of 52° is observed at 95 °C substrate temperature and is significantly larger than that at 22 °C (30°). This observation is counterintuitive and can be attributed to neither the surface coating chemistry nor the substrate itself. Subsequent systematic SEM and atomic force microscopy (AFM) study reveals that the anomalous temperature effect is due to the formation of nanosized protrusions on the DTC− BOPP surface as the substrate is undergoing thermal expansion. The large contact angle observed at 95 °C is discussed in terms of the friction produced by these nanoprotrusions, which stop the advance of the contact line and pin the monomer droplet into a metastable state during wetting and spreading.
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RESULTS AND DISCUSSION Comparison of Wetting and Spreading Behavior between the Ink and Its Monomer on Various Substrates. The objective of this study is to identify the major component in the ink formulation that controls the wetting and spreading behavior of the ink drop on various substrates. This is done by comparing the time-dependent contact angles of the ink and monomer sessile droplets on the four substrates: DTC-coated BOPP, Flexo-coated BOPP, DTCcoated SGE paper, and Flexo-coated SGE paper. Figure 1a,b plots the contact angles of the ink and the monomer droplets over time on these substrates. Both the liquid and the substrate temperature are kept at 95 °C, the optimal temperature for jetting the ink. The results show that the contact angles of both the ink and the monomer decrease rapidly upon contacting the Flexo-coated BOPP, the DTC-coated SGE paper, and the
EXPERIMENTAL SECTION
Materials. The experimental ink used in this work comprises an UV-curable acrylate monomer, neopentyl glycol diacrylate (SR-9003 from Sartomer Inc.), a cyan pigment, and a proprietary additive and was obtained from an internal source. The preparative procedure for the ink has been described elsewhere.9,10 Four different label printing stocks, namely, DTC-coated BOPP, Flexo-coated BOPP, DTC-coated SGE paper, and Flexo-coated SGE paper, were studied, and they were from Label World, Rochester, NY. All the materials were used as received. Measurements. Contact angle measurements were conducted on an OCA20 goniometer (Dataphysics, FDS) which consists of a computer-based image-processing system and a heating system. All the measurements were carried out with ∼5 μL droplets inside a heating chamber where the temperature inside the chamber was carefully controlled to be within 1 °C. The temperatures of the ink and monomer were set at 95 °C, which was the jetting temperature used in inkjet printing. The substrate temperature varied from 22 to 95 °C. The substrate was mounted onto a microscope glass slide to ensure no wrinkling during heating and cooling. The distance between the tip of the microsyringe and the substrate was set at 4 mm unless specified. The wetting and spreading behavior of the sessile drops of the ink and the monomer on various substrates were studied by time-dependent contact angle measurements. The contact angles were recorded in side view at 25 frame/s using a CCD camera attaching to the goniometer and were automatically measured as a function of time at every 40 ms using the SCA20 software available in the goniometer. Each data point was an average of more than five independent measurements to ensure reproducibility between runs. The measurement error for the static contact angle was estimated to be better than ±2°. Sliding angle measurement was carried out by tilting the base unit at a rate of 1°/s with a ∼5 μL droplet using tilting base unit TBU90E. All reported values were an average of five to eight measurements using fresh area for each measurement, and the estimated error is better than ±3°. The tilted angle is defined as the angle where the liquid droplet starts to move or slide. The morphology of various substrates was studied by scanning electron microscopy (Hitachi S-4800 FESEM) and atomic force microscopy (Veeco Dimension 3100). Tapping mode AFM for both surface topography and phase imaging was used. In the tapping mode, a stiff silicon cantilever 130 μm long and 29 μm wide with a spring
Figure 1. Plots of the time-dependent contact angles for (a) ink and (b) monomer on various substrates (temp = 95 °C for both liquid and substrate). 10666
dx.doi.org/10.1021/la400991y | Langmuir 2013, 29, 10665−10673
Langmuir
Article
Figure 2. Plots of the time-dependent contact angles for the monomer at substrate temperatures of 22 and 95 °C on (a) DTC-coated BOPP, (b) Flexo-coated BOPP, (c) DTC-coated SGE, and (d) Flexo-coated SGE.
Flexo-coated SGE paper.12 The sessile drops then reach their respective static states in 5−20 s. Usually when a drop is placed on a solid surface, it spreads spontaneously to its static state as the kinetic energy in the sessile drop is totally dissipated. In the time-dependent contact angle measurement, it usually starts with a dynamic contact angle close to 180° and then decreases rapidly to its static contact angle. Wetting and spreading of a liquid drop on a solid surface is a complex process. In this section, we focus our attention to compare the wetting and spreading behavior between the ink and the monomer first. Both molecular-kinetic theory and molecular dynamics simulations13−19 suggest that the contact angle during wetting would decreases exponentially as t−3/10 or t−3/7 over time (t) depending on the modes of energy dissipation. The exponential decreases in contact angles over time observed on Flexo-coated BOPP, DTC-coated SGE paper, and Flexo-coated SGE paper (Figure 1) indicate that (a) the wetting and spreading behavior of the ink and monomer drops are comparable to other liquids reported in the literature13−15,19 and, more importantly, (b) the monomer is dictating the wetting and spreading behavior of the ink in the ink formation because of their similar spreading curves. Different results were obtained for both ink and monomer drops on the DTC-coated BOPP surface. Their droplets are found to reach their static contact angles instantly. The most unexpected observation here is the size of the static contact angles on the DTC-coated BOPP substrate. They are found to be larger than those on the other three substrates, and this point will be discussed later. As for the wetting and spreading behavior, the ink and the monomer are very similar. The overall results lead us to conclude that the monomer in the ink formulation dominates the wetting and spreading behavior of the ink. Since the UV ink studied in this work is a gel at temperatures
