Morphological Transformation of Surface Femtodroplets upon

Jan 12, 2017 - ... Department of Science and Engineering, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box...
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Morphological Transformation of Surface Femtodroplets upon Dissolution Shuhua Peng,† Bat-El Pinchasik,‡ Hao Hao,¶ Helmuth Möhwald,§ and Xuehua Zhang*,†,∥ †

Soft Matter & Interfaces Group, School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia Department of Physics at Interfaces, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany ¶ Electrical and Computer Engineering, School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia § Emeritus Group of Interfaces, Max-Planck Institute of Colloids and Interfaces, Golm/Potsdam D14476, Germany ∥ Physics of Fluids Group, Department of Science and Engineering, Mesa+ Institute, and J. M. Burgers Centre for Fluid Dynamics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ‡

S Supporting Information *

ABSTRACT: Constructing controllable liquid patterns with high resolution and accuracy is of great importance in droplet depositions for a range of applications. Simple surface chemical micropatterns have been popularly used to regulate the shape of liquid droplets and the final structure of deposited materials. In this work, we study the morphological evolution of a dissolving femtoliter droplet pinned on multiple microdomains. On the basis of minimization of interfacial energy, the numerical simulations predict various symmetric droplet profiles in equilibrium at different liquid volumes. However, our experimental results show both symmetric and asymmetric shapes of droplets due to contact line pinning and symmetry breaking during droplet dissolution. Upon slow volume reduction, the deposited microdroplet arrays on one single type of simple surface prepatterns spontaneously morphed into a series of complex regular 3D shapes. The findings in this work offer insights into design and prepararion of the rich and complex morphology of liquid patterns via simple surface premicropatterns.

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beautifully predicted the liquid morphology on either chemically patterned or physically microstructured substrates.18,19,21 However, the prediction remains to be confirmed for the morphological evolution of droplets with extremely small volumes (order of femtoliters). Understanding the interplay of pinning, liquid volume, and dimension of micropatterns will provide insights for design and control of the morphology of femtoliter droplets. In this work, we investigate both theoretically and experimentally the morphological evolution of femtoliter droplets in response to slow droplet volume reduction. These femtoliter droplets were formed by solvent exchange and were pinned by multiple microdomains on an immersed substrate.22−24 Gradual volume reduction of the droplets was controlled by diffusive droplet dissolution. Our experimental results show that the morphology of the droplet depends mostly on three control parameters: the spatial arrangement, the size of the microdomains, and the droplet volume. Remarkably, both a symmetric droplet shape in agreement with simulations and an asymmetric droplet shape are possible for a given droplet volume. Insight from this work is helpful for

inning on the rim of a droplet is common in daily phenomena, such as stain formation from spilled coffee or hanging rain drops on window glass. The pinning effect is critical for droplet growth from vapor in phase transition processes.1,2 Droplet pinning is also of great importance to control microdroplet shapes for many droplet-based applications, for instance, concentrating analytes inside of an evaporating drop in detection of a single biomolecule3−5 or templating 3D surface microstructures in fabrication of functional materials.6−9 Chemical micropatterns on the surface have been often exploited to manipulate the contact line of the droplets and the shapes of deposited materials.10−12 A small droplet can be possibly pinned by molecular heterogeneities or nanodefects on the surface.13−15 Simply classified into strong or weak pinning sites by de Gennes,16,17 the pinning strength of surface microdomains varies with the wettability of the droplet liquid therein. The droplet liquid can form an extremely low contact angle on smooth chemical microdomains but an extremely high contact angle on the surrounding area as the system is immersed in an immiscibe liquid phase. Under such wetting conditions, the microdomains at the droplet boundary act as strong pinning sites. Great effort has been devoted to predict the thermodynamic equilibrium shapes of droplets based on minimization of the interfacial energy.18−20 Interfacial energy minimization has © XXXX American Chemical Society

Received: December 6, 2016 Accepted: January 12, 2017 Published: January 12, 2017 584

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outside of the microdomains. A contact angle of 142° is used for the background surface, and 17° is for the circular microdomains. For all patterns, the center-to-center distance between two adjacent domains is 3r, with r as the domain radius. Figure 2 shows the simulated morphological evolution of a droplet on patterned surfaces consisting of two, three, four, five,

harnessing the pinning and the symmetry breaking to produce various polygonal shapes and asymmetric liquid structures. As demonstrated, we show that a plethora of liquid morphologies are used as precursors for the creation of a library of sophisticated 3D surface microstructures on the surface simply patterned with circular microdomains. Design of Surface Micropatterns and Numerical Simulations. The five arrangements of surface micropatterns in this work are listed in Figure 1C. They include a line of two domains, a

Figure 2. Numerically calculated evolution of droplet morphology driven by interfacial energy minimization on the patterns consisting of (A) two, (B) three, (C) four, (D) five, and (E) six circular microdomains. The contact angle is 17° inside of the domain and 142° outside. The initial droplet configuration is shown on the left side. The volume of the droplet decreases from left to right until the original droplet splits into subdroplets on the individual domains, as seen on the right side. The normalized volumes for evolving droplets from left to right are (A) 1, 0.50, 0.38 and 0.35; (B) 1, 0.46, 0.34, and 0.15; (C) 1, 0.31, 0.18, and 0.14; (D) 1, 0.31, 0.19 and 0.13; (E) 1, 0.30, 0.18 and 0.15.

Figure 1. Experiment setup and surface pattern design. (A) Sketch of the experimental setup for the dissolution process. (B) Sketch of four smooth circular domains arranged in a square pattern on the surface and a droplet on this four-domain pattern. The diameter of the domain is 2r, and the edge-to-edge distance between neighboring circular domains is d. (C) List of arrangements of microdomains in the patterns used in our experiments.

and six circular domains. In each case, the circular domian radius is equal to the interdomain spacing. In all pattern configurations, the droplet volume decreases from left to right. On the leftmost is the initial configuration of a droplet situated on the patterned surface. For each state of the droplet evolution, the corresponding volume is normalized to the initial volume of the droplet. At a certain volume, the initial droplet splits into subdroplets on individual domains. The critical volume for droplet splitting is defined as the volume, for which a further decrease of 1% results in splitting of the original droplet to multiple subdroplets. We will later compare the droplet volume and morphology in simulations with experimental data. Our experimental results will show that the symmetric droplet morphology is predicted in the above numerical simulations. Reversible and Symmetric Droplets on Large Microdomains. The time course snapshots in Figure 3 show the reversible symmetric droplets during growth and dissolution on large circular microdomains. The diameter of each domain (2r) is 10 μm with a constant interdomain spacing of 2.5 μm, as represented by domain numbers (N) of 3 and 4. As the volume

triangle of three domains, a square of four domains, a pentagon of five, and a hexagon of six domains surrounded by the hydrophilic flat area. The domain diameters (2r) of the hydrophobic circular micropattens are 3, 5, and 10 μm, respectively, with a constant edge-to-edge domain spacing of 2.5 μm for all patterns. The advancing and receding contact angles of the polymerizable liquid droplets in the water environment are 20 and 13° on the hydrophobic domains and 144 and 140° on the hydrophilic area. First we calculated numerically the liquid droplet shape on patterned surfaces for a given volume using the free available finite element software Surface Evolver.21,25 It should be noted that only the part of the droplet shapes on the symmetric patterns was calculated. The total surface energy of the spreading droplet can be expressed as Etotal = γLSinALSin + γLSoutALSout + γLAALA, with γLSin and γLSout as the interfacial energy between the solid surface and the droplet inside and outside of the microdomains, respectively. In our calculation, we use line integrals to describe the contact angles inside and 585

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Figure 3. Droplet growth and dissolution on micropatterned surface. (A,C) Droplet growth on circular square (green) and triangle (red) microdomain surfaces, respectively, by solvent exchange (2r = 10 μm); (B,D) corresponding droplet dissolution process on the same patterns of 4 and 3 domains. Scale bar: 15 μm. The droplet volume decreases from top to bottom.

Figure 4. Time course snapshots of dissolving droplets on the patterns consisting of 3−6 circular domains (2r = 5 μm). Scale bar: 15 μm. The symmetry breaks in the snapshots are in color frames. Blue, red, green, and yellow frames represent the first to fourth instances of liquid rupture in the colored frames. Initially, the droplets were large enough to cover all N circular domains of the surface patterns (N = 2, 3, 4, 5, 6). t1, t2, and t3 correspond to the drop dissolution dynamics in Figure 5A,B.

increases (Figure 3A,C), the droplets first grow independently on each domain in a constant contact radius mode. For a given growth time, the droplet size is the same on each domain. As the droplet size becomes large enough to contact each other, the individual droplets merge into a large polygonal droplet of square or triangle shape, covering all of the domains in the pattern. The triangle or square droplet becomes more and more spherical upon further growth. A reversible morphological transformation was observed in the droplet dissolution on the patterns with a domain size of 10 μm. An array of large spherical cap droplets was uniformly distributed on the patterned surface after the formation. Those spherical cap droplets, which cover all of the microdomains in the pattern, were subjected to a gentle continuous flow of pure water for dissolution. A typical droplet dissolution process on a pattern of four circular microdomains is shown in Figure 3B. One observes three consecutive main phases in the droplet

dissolution. In phase one, starting from a spherical cap droplet covering all four domains in Figure 3B, the droplet initially shrank in a shape of the spherical cap. As the boundary was pinned by the domains, the droplet evolved into a square shape after dissolution. In phase two, with the droplet volume reduction, the corners of the droplet developed, giving rise to four outstretched arms. The contact line of the droplet on the boundary of the domain remained pinned while the section of the boundary on the hydrophilic surface area receded, resulting in narrowing of the bridge between the three domains and the droplet center. At the beginning of phase three, the four bridges connecting the domains and the droplet center ruptured simultaneously. The liquid was distributed into four separate small droplets on the domains. The relaxation time from the rupture until the droplet assumed that a spherical cap shape on the individual domain was too short to be captured in our experiments. The spherical cap droplet further dissolved on 586

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dissolution is drastically different from the expected asymmetric pattern. In particular, the large droplet from the splitting has nonspherical cap shape. Symmetry breaking observed in our experiments is attributed to the fact that the droplets seek the local lowest energy state at their volume reduction. A marginal local heterogeneity or asymmetry conditions on the substrate or of the surrounding environment may have led to the bifurcation of the droplet to a small and large one instead of N small droplets with an equal size. Similar symmetric breaking of dissloving droplets was also observed for the pattern surfaces with 2r = 3 μm (Figure S5). In analogy, the symmetry breaking occurs in cooling or volume reduction of droplets in emulsion.28,29 Although similar surface heterogeneity may still exist for the circular patterns with 2r = 10 μm, the increased ratio between the hydrophobic domain area and hydrophilic area may overtake the effect from random heterogeneities due to a stronger pinning force from the larger domain size. Hence, the droplet on the larger domain can be stretched more before it splits equally into subdroplets. Time Evolution of Symmetric Droplets. The top-view videos were processed (see an example in Supporting Figure S7), and the base area of the droplets as a function of time is plotted in Figure 5A,B. For a given pattern, different droplets dissolve in a similar way in terms of jumps in the dissolution curve, in contrast to the individuality in the dissolution of surface nanodroplets on an unpatterned substrate with random heterogeneities.30 The results suggest that the well-defined pinning sites on the surface lead to the controlled dissolution of nanodroplets on the patterns. The three-phase dissolution on the two-domain pattern is evident in the dissolution curve; before the droplet boundary is pinned by the domain boundary (base area >125 μm2), the spherical droplet dissolves in a constant contact angle mode from t0 to t1. As the base area is reduced below 125 μm2 when the droplet boundary reaches the two domains, the dissolution rate of the droplet base area decreases, resulting from the pinning effect of the domains from t1 to t2. More than one transition can occur in the dissolution curve of pinned droplets. The general feature is that the dissolution rate deceases with time for droplets on all patterns. We further compare the droplet morphology at a given volume in simulations and in experiments. The volume of the evolving droplet was extracted from AFM images of polymerized droplets, as shown in Figures 5C and D and Figure S8. All volumes are normalized to the initial volume of the droplet. There is good agreement between the measurements and the simulation results in terms of the primary morphological features of the droplets before symmetry breaking, suggesting that the droplet morphology was close to equilibrium and can be well predicted based on minimization of the interfacial energy. The plots in Figure 5E,F show two profiles of the droplet on the six-domain pattern with different heights and volumes, demonstrating excellent agreement between AFM measurements and Surface Evolver simulations in these locations. The volumes extracted from AFM images are slightly larger than the numerically calculated volumes, possibly due to the convolution in the images; the droplet boundary outside of the circular domain has a contact angle larger than 90°, as shown in the SEM images in Figure S9 and predicted by numerical calculations; therefore, the AFM tip was unable to accurately follow the droplet surface in these areas. 3D Microstructures Templated by Pinned Droplets. The above quantitative agreement between experiments and simulations for symmetrical droplets can guide the control of liquid

each domain and eventually vanished. The three phases in the droplet dissolution also occurred on the three-domain pattern, and consequently, an individual small droplet ended up on each of the three domains. The rupture of the dissolving droplet was the same in the pattern of N = 5, 6 (Figure S6). In the growth and dissolution of droplets as above, the change of the droplet volume is slow due to the diffusiondominated process.26 Therefore, the droplet morphology represents a series of states close to equilibrium states for infinitely small volume change. The main features of the droplet morphology for a certain volume of droplet liquid are in good agreement with the simulation results based on the minimization of interfacial energy in Figure 2. Irreversible Droplet Morphology on Small Microdomains. When the diameter of each domain (2r) was reduced to 5 μm with a constant interdomain spacing (d) of 2.5 μm, similar droplet growth behavior was observed. However, dramatically different dissolution behaviors were observed on these patterns. As represented by the four-domain pattern in Figure 4 and Figures S1 and S2, the features in phase one and two are similar to those of symmetric dissolution on the patterns of large domains. However, the arms connecting the droplet center and the domains on the corners did not extend as much, while the bridges were not narrowed down near the center. Instead of the simultaneous rupture of all of the bridges, the droplet split into a small spherical cap droplet on one domain and a large anchor-shaped droplet with three corners stretched by the other three domains. The boundary quickly zipped off from the second domain. Now, two small spherical cap droplets and a large sausage-shaped droplet formed. The second detachment was always from the domain adjacent to the first released domain. There was also a probability for the star-shaped droplet to detach from two domains in one go and form a sausageshaped droplet in the film rupture. In further dissolution, the sausage-shaped droplet turned into a dog bone shape. Eventually, the dog bone shape split into two same-sized droplets on the last two domains. Symmetry breaking was observed on the patterns with more than two domains shown in Figure 4 and Figures S3 and S4. For the patterns with more than three domains, the symmetry breaking was observed N − 2 times. That is, the symmetry broke twice for four domains, three times for five domains, and four times for six domains. On the pattern with five or six domains, the droplets can also zip off from two domains continuously. Interestingly, from the size of four individual spherical cap droplets, we could determine which domain was released first. The top-view images of the droplets show that the droplet size is the smallest on the first released domain (Figure S2). As the droplet ruptures, the large and small droplets should have the same mean curvature and Laplace pressure. The larger isolated droplet suggests a lower Laplace pressure at the second rupture. After symmetry breaking, the hysteresis on the boundary of the large drop is evident from the sausage shape of the large droplet on two remaining microdomains. The shape became more symmetric with further volume reduction, approaching the symmetric morphologies of the droplet in equilibrium before the next breakup. According to interfacial energy minimization, as the volume of the droplet, V, decreases, the droplet attains a certain critical volume, at which the wetting layer undergoes a transition to an asymmetric pattern with one small spherical cap droplet on a circular pattern and a larger spherical cap droplet covering N − 1 domains.19,27 The above symmetry breaking in the droplet 587

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corresponding derived morphologies were also successfully prepared by templating of polymerized droplets at different dissolution stages, demonstrating that this is a general approach to build complex 3D microstructures. We show the morphological transformation of femtoliter droplets on chemical patterns consisting of extremely strong pinning microdomains. There is excellent agreement between experiments and simulations in the morphology of symmetric droplets. However, for a given domain spacing, symmetry breaking was observed for the droplets on small domains in response to droplet volume reduction at a critical value, resulting in a small spherical cap droplet on an individual pattern and a large droplet covering the remaining domains on the pattern. The symmetry breaking occurs N − 2 times, with N being the total number of the domains. By harnessing pinning and symmetry breaking, various polygonal shapes and asymmetric liquid shapes can be produced on the surface patterned with simple circular microdomains. Droplets of different shapes can be used as the precursor for a range of microstructures, opening an entirely new route for engineering surface microstructures. As handling liquid with extremely small volume is highly relevant to various practical applications in additive micromanufacturing, single molecular diagnosis, or liquid−liquid microextractions, the findings in this work provide many prospects of making use of femtoliter droplets as microtemplates, microreactors, microlenses, or sensors.

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EXPERIMENTAL SECTION Chemicals and Substrates. Octadecyltrichlorosilane (OTS, > 90%) and 1,6-hexanediol diacrylate (HDODA, > 80%) were purchased from Sigma-Aldrich. Photoresist AZ1512 HS and developer AZ400K were from MicroChemicals GmbH. All chemicals were used without further purification. A standard photolithography and chemical vapor deposition process was employed to prepare circular patterns of OTS on silicon wafer substrates. Formation and Dissolution of Droplets. In the solvent exchange process, a good solvent of oil was displaced by a poor solvent in the presence of a substrate on which droplets of oil form, as sketched in Figure 1A. Details about the solvent exchange process for micro- and nanodroplet formation were described in our previous work.23 The patterned substrates were exposed to a good solvent of ethanol/water (50%/50%) containing oil (HDODA), which was then displaced by a poor solvent of water. The flow rate of solvent exchange was controlled by a syringe pump. A very slow flow rate of 5 μL/min was applied for droplet formation. After droplets with appropriate sizes were formed on the prepatterned surface in the fluid cell, pure water with a flow rate of 1000 μL/min was injected into the fluid cell to dissolve the formed droplets. The dissolution process was monitored by optical microscopy (Huvitz HRM-300, Scitech Pty. Ltd., Australia). The evolving droplet morphology at different dissolution stages was fixed by polymerization and characterized by atomic force microscopy (AFM, Asylum Research, Santa Barbara, CA). Micropillars were fabricated by an anisotropic dry etching process (PlasmaPro Estrelas 100, Oxford Instruments) with the polymerized droplets as templates. The morphology of the resulting micropillars was investigated by scanning electron microscopy (SEM, FEI Nova, NanoSEM, Oxford X-MaxN 20 EDXS).

Figure 5. Dynamics of droplet dissolution. (A,B) Droplet base area as a function of dissolution time. The number of microdomains is indicated by the insets, and the blue, black, and red lines are the data from three repeating droplets. The images of the droplets at times of t1, t2, and t3 on each pattern are labeled in Figure 4. (C,D) Evolution of the droplet volume and morphology on four and six circular hydrophobic domains with 2r = 5 μm from AFM imaging (black line) and numerical calculation using Surface Evolver (red line). The error bars are due to uncertainty from the polymerization time. (Inset) Corresponding top view for each droplet volume. (E,F) Comparison of the cross-sectional profiles of the droplets from experiments and simulations.

structures. The morphology of evolving polymerizable droplets can be captured by photopolymerization, serving as versatile templates for microstructure fabrication. The advantage here is that there is no need to make a mask for every microstructure with a specific dimension, as required in standard photolithography. A single type of surface micropattern is sufficient to produce a variety of liquid shapes, determined by the pinning and symmetry breaking during the droplet shrinkage. For instance, ellipsoid, rod, and dumbbell microstructures are produced by using the same two-domain patterns, as shown in Figure S8. The stretched arms of the starfish microstructures based on the four-domain patterns can be regulated by the droplet dissolution duration. A library of polymeric microstructures with symmetric and asymmetric morphologies is prepared in Figure S8, demonstrating the flexibility and versatility of the droplet dissolution method to prepare complex 3D microstructures. The polymeric microstructures prepared as above are used as templates for building more complicated surface structures. As shown in Figure 6, a range of 3D morphologies are constructed based on hexagons. 3D micropillars with star (Figure 6B), square (Figure 6C), and triangle shapes (Figure 6D) and their 588

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Figure 6. Top-view SEM images of complex micropillars by templating of polymerized dissolving droplets on circular patterns with different circle diameters: (A−D) 2r = 10 μm; (A-VI) is the side-view of (A-V), and the top droplets are colored green; (E) 2r = 5 μm; (F) 2r = 3 μm. Scale bars: (A−D), 10 μm; (E,F), 30 μm.

Imaging Analysis. Video analysis was used to track the change of the base area of droplets with dissolution time. We selected video frames with a fixed interval (15−30 s) based on the rate in droplet dissolution. The frame images were converted to grayscale. We manually outlined the target droplets in an image of a multiple droplet array, shown in Figure S7. The captured videos show low contrast and random noise. A median filter was used to remove the noise before calculation of the threshold.31 Although the videos were recorded under the same conditions, each frame image had slight differences in terms of the contrast and the presence of small floating objects in the background. To eliminate the interference on threshold calculation for consistency in processing of each frame, we applied an averaged threshold calculated from the first frame and the subsequent frame on the images to extract a mask. Morphological dilation and erosion were used to smooth the mask and fill small holes in the image. The size change of the mask reflects the change of the droplet base area. We tracked these changes and calculated the area value based on image pixels. After a droplet split into several small parts, only the biggest one was tracked further. Then we converted the area into 1 μm2 based on experimental specification (1 μm = 9.4 pixels). Figure S7 in the Supporting Information shows an example of the base area change of a masked droplet.

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Video of droplet dissolution on a hexagon circular pattern (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuehua Zhang: 0000-0001-6093-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.H.Z. acknowledges support from the Australian Research Council (FT120100473, DP140100805) and S.P. from an ARENA Research Fellowship. B.-E.P. would like to thank Dr. Ciro Semprebon from Northumbria University for fruitful discussions and aid with the numerical calculations. We thank Dr. Shuying Wu for SEM measurements and Dr. Lei Bao for design and prepration of micropatterned substrates. We also acknowledge the use of facilities and the associated technical support at the RMIT MicroNano Research Facility (MNRF) and the Microscopy and Microanalysis Facility (RMMF).

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ASSOCIATED CONTENT

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02861. SEM, optical, and confocal microscopy images of polymerized droplets and an example of the droplet base area process (PDF) Video of droplet dissolution on a square circular pattern (AVI) Video of droplet dissolution on a pentagon circular pattern (AVI) 589

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