Drop impact printing

Abstract

Hydrodynamic collapse of a central air-cavity during the recoil phase of droplet impact on a superhydrophobic sieve leads to satellite-free generation of a single droplet through the sieve. Two modes of cavity formation and droplet ejection have been observed and explained. The volume of the generated droplet scales with the pore size. Based on this phenomenon, we propose a drop-on-demand printing technique. Despite significant advancements in inkjet technology, enhancement in mass-loading and particle-size have been limited due to clogging of the printhead nozzle. By replacing the nozzle with a sieve, we demonstrate printing of nanoparticle suspension with 71% mass-loading. Comparatively large particles of 20 μm diameter are dispensed in droplets of ~80 μm diameter. Printing is performed for surface tension as low as 32 mNm^-1 and viscosity as high as 33 mPa∙s. In comparison to existing techniques, this way of printing is widely accessible as it is significantly simple and economical.

Fig. 1. Mechanism and explanation of drop-impact printing technique.

a Schematic illustration showing the drop-impact setup, a droplet (diameter D_o, velocity U_o) impacting on a superhydrophobic sieve (pore opening, L) to eject out a single smaller droplet (diameter D_p). The impacting drop gives rise to two modes of single-droplet ejection. b Impact cavity (IC) and (c) recoil cavity (RC). Scale bar: 200 µm. The time-lapsed images and schematic illustration for IC and RC modes show the mechanism of cavity formation and collapse using sieve #0.0045 with 65% glycerol water droplet and sieve #0.009 with pure water droplet, respectively. The drop-impact printing technique was explored in terms of the smallest ejected droplet that can be generated. d Shows a plot between water droplet diameter versus pore opening, and the insets show the corresponding patterned droplet (scale bar: 100 µm). Superhydrophobic sieves with different pore openings were used starting from sieve type #0.012 (pore opening L: 533.4 µm, wire diameter W: 304.8 µm) to #0.0020 (pore opening L: 76.2 µm, wire diameter W: 50.8 µm) and electroplated mesh (pore opening L: 25.2 µm, wire diameter W: 101.2 µm) marked in blue-dotted circle. e Scanning electron microscopy (SEM) images of sieves #0.009 and #0.0020 (scale bar: 100 µm, magnified image scale bar: 2 µm).

Fig. 2. Parametric studies showing the capabilities of drop-impact printing technique.

The extent of viscous liquid and low surface tension liquid printing was explored using glycerol water solution, polyethylene glycol (PEG) water solution, and ethanol-water solution. The ejected droplet diameter was plotted with (a) liquid viscosity and (b) liquid surface tension for a sieve with different pore openings. c The printable regime was observed in the plot between Ohnesorge and Reynolds number. The light-blue shaded part shows the printable region of drop-impact printing technique. The range gives us an idea of the extent of different liquids that can be used for printing. d The broad range of liquids is shown in terms of Z number with inset images showing the different liquid drops that can be printed. The drop-impact printing technique (shown with purple color bar) was compared to inkjet printing, electrohydrodynamic (EHD) printing and acoustophoretic printing represented with turquoise, blue, and yellow bars, respectively (scale bar: 100 µm). e The mechanism of different ejection modes was explained based on a timescale factor with varying Ohnesorge number. The critical Ohnesorge number that ensures transition from the inertial to viscous regime was 0.03, and the time- scale factor value that defines the transition from collapse-penetration mode (CPM) to impact-penetration mode (IPM) was found to be 0.04.

Fig. 3. Clogging-free printing with a large particle size and higher mass loading printing.

The clogging-free printing was demonstrated based on the ability to print a large particle size and high mass loading suspensions. a The larger particle size printing ability was shown in a linear L/D_p chart with the inset showing a different printed particle size for different L/D_p (scale bar: 100 µm) ratios. L/D_p can be as low as 3.81 for drop-impact printing, which is significantly smaller as compared to inkjet and electrohydrodynamic (EHD) printing. b The percent count to print a single and multiple beads in a drop is demonstrated. The probability of single- bead capturing in a single drop (80-µm diameter) was found to be 32%. The inset shows the number of beads in a single drop (scale bar: 100 µm). c Further, the printed droplet diameter with varying particle size is shown. The droplet diameter was independent of different particle-size suspensions. d The linear chart shows that as high as 71% mass loading suspension solution printing is possible using drop-impact printing as compared to inkjet and EHD printing. The inset shows the scanning electron microscopy (SEM) image of a printed droplet for different mass loading (scale bar: 100 µm). e The printed feature height is shown with varying mass loading (print substrate—glass). The inset shows the printed droplet with 71% mass loading having a base diameter of 990 µm (sieve used—#0.009). f Further, the printed droplet diameter was plotted with varying mass loading (inset image scale bar: 100 µm). The printed drop size was found to be independent with an increase in mass loading. Insets in both figures (e) and (f) show a higher mass loading printed drop.

Fig. 4. Drop-impact printing of biological solutions and biopolymers.

a Microscopic images of a single droplet patterned using cell (RBC)-laden PBS solution of different concentrations, scale bar: 50 µm. The cells are contained in an isolated droplet of volume 26 nL patterned using mesh type #0.009. b The number of cells per droplet for varying cell concentrations was examined for mesh types #0.009 and #0.0045. The single-cell printing was further demonstrated using drop-impact technique. c Single cells (MDA-MB-231) of average size ~17 µm were trapped in a 0.268-nL single drop. The drops were collected on an oil-coated glass slide. The concentration of cell solution was kept at 50 × 10⁴ cells per mm³ (scale bar: 50 µm). d Illustration showing printed DMEM droplet arrays using drop- impact printing technique. (1) Shows printed DMEM droplets on a hydrophobic Teflon surface. (2) Shows the arrays of MDA-MB-231 cells containing droplets after cell solution swipe, and a magnified image of a printed droplet containing cells (scale bar: 500 µm). Beside this, the technique’s ability was explored by using biopolymeric viscoelastic liquid (0.0125 g per mL polyacrylic acid mixed in water) for 3D printing applications. e The large patterned microposts of 875 µm diameter and 2 µm height were printed on APTES-coated glass slides and the corresponding scanning electron microscopy (SEM) image (scale bar: 400 µm).

Fig. 5. Printing of electrically conducting materials for large-area fabrication and flexible electronics applications.

Room-temperature printing of (a) silver ink (4% (v/v)) conductive line (scale bar: 200 µm) and the corresponding scanning electron microscopy (SEM) image (scale bar: 100 nm). b PEDOT:PSS-printed line (scale bar: 200 µm) and the corresponding SEM image showing the connectivity (scale bar: 20 µm). c IV characteristics of both silver and PEDOT:PSS-conducting lines. d (1) Silver ink and PEDOT:PSS were further used to form a junction to show the capability of the technique for electronic applications. (2) Optical microscopic and the SEM image showing the junction (scale bar: 250 µm). (3) In addition, IV characteristic was performed for the junction to check the connectivity. Further, as a demonstration (e) two silver-conducting lines are connected using drop- impact printing technique, and the voltage is applied at both ends to show the glowing LED. f Large-area droplet patterning (scale bar: 1 mm), g flexible printing, and h 3D pillars printed using sieves with different pore openings have been shown to demonstrate the wide applicability of this technique (scale bar: 100 µm).