English

• 1.   Introduction

  In the last decade, coffee ring effect is under research for their potential in a wide variety of applications, such as producing nanostructures [1, 2], diagnosing diseases [3], printed electronics [4] and other fields. Some research focuses on making use of coffee ring effect to modify ordered structure and sort colloid. Others research centers on how to solve the coffee ring problem to meet practical requirement. Since pioneering work from Deegan et al. reported the detailed investigation of solute transport during droplet drying [5], many methods have been developed to solve the coffee ring problem, such as tuning solvent composition [6], manipulating substrate temperature [7], controlling substrate wettability [8], modifying particle shape [9], using electrowetting [10] and utilizing polymer additives [11]. Despite these efforts, to our knowledge for the broad applications, there are few reports to date on adjustable coffee ring structure. Here, a simple, rapid and novel approach for controlling coffee ring structure is firstly reported via O₂ plasma treatment.

  Wettability is an important property of solid-gas interfaces [12, 13] and plays an important role on the evaporation behavior of the droplet placed on substrates. In general, for smooth hydrophilic and hydrophobic surface droplet evaporation follows the constantcontact-diameter mode and the constant-contact-angle mode, respectively. That is to say different wettability leads to different contact line (CL) behavior during droplet drying. Switching of surface wettability between superwetting and superantiwetting by external stimuli has been achieved by temperature, pH, optic, solvent, electric or mechanical force [14, 15]. Therefore, regulating coffee ring structure is performed on smart surface via responsive wettability. In this communication, coffee ring structure is controlled on polystyrene (PS) surface by O₂ plasma. Uniform and macroscale 33 mm² SiO₂ microspheres deposition without coffee ring structure and 2 mm² SiO₂ microspheres deposition with coffee ring structure are demonstrated. O₂ plasma treatment contributes to the wettability variation of PS surface from hydrophobic to hydrophilic. For hydrophilic PS surface the coffee ring structure is avoided relying on the motion of CL while SiO₂ microspheres are left. The motion of the CL is caused due to the viscosity and Marangoni effect by means of the addition of polymer additives. For hydrophobic PS surface coffee ring structure still exists even with polymer additives because SiO₂ microspheres transfer with the motion of the CL at the beginning stage of droplet evaporation and accumulate at the droplet edge at later stage with the pinning of the CL. This kind of non-contact real-time controllable coffee ring structure might inspire and facilitate the designs and applications in material deposition and biology fields. Moreover, the design principles disclosed in this work shall be a guidepost for the following researcher on manipulating coffee ring structure with smart liquid/solid interfaces with on-off switching wettability.

  2.   Experimental

  Materials: The average diameter of SiO₂ microspheres used in our work was 972 nm measured by SEM with a calibrated length. The silicon wafers (100) were cut into 20 mm × 20 mm pieces, were soaked in the mixture of 98% H₂SO₄/30% H₂O₂ (volumetric ratio 7:3) for 20 min under boiling (caution: strong oxide), and then were rinsed with Milli-Q water (18.2 MΩ cm^-1) and ethanol several times, at last were dried with N₂ stream. Polyethyleneoxide (PEO, Mw=4 × 10⁶) and polystyrene (PS, Mw=2.8 × 10⁵) were purchased from Sigma Aldrich. All the above chemical reagents in our work were used as received.

  Droplet deposition: Prior to each deposition, the latex suspension is sonicated for 30 min. In our experiments the volume of droplets is 2 μL controlled by a micropipette, with different polymer concentration. Furthermore, micrometer-sized SiO₂ spheres in them are all 972 nm in diameter and 0.5 wt% in concentration. The weight fraction of 0.5 wt% is selected by taking into account subsequently easy quantification of the SiO₂ microspheres density, and SiO₂ microspheres are monolayer deposition after coffee ring effect is destroyed. The experiment is carried out at the room temperature of 23~25 8C and humidity of 35%~45%.

  Characterization: SEM micrographs were examined using a JEOL JSM 6700F field-emission scanning electron microscope with an accelerating voltage of 5.0 kV. The samples were sputtered with a thin layer of Pt prior to imaging. Static emulsion droplet contact angles (CAs) were measured on a Krüss DSA100 (Krüss GmbH, Hamburg, Germany) drop shape analyzer at ambient temperature. The static CA was read by injecting 2 mL emulsion droplet. The average CA was obtained by measuring more than five different positions on the same sample. The optical microscope images were recorded using an Olympus fluorescence microscope (BX51). Viscosity data were recorded using a Julabo 536 10 Ubbelohde viscometer. The thickness of polystyrene film was determined using a Dektak 150 surface profiler (Veeco).

  3.   Results and discussion

  The experimental realization of the proposed tunable coffee ring structure has been accomplished. Fig. 1a shows uniform deposition of droplet containing SiO₂ microspheres (972 nm in diameter and 0.5 wt% in concentration) without the coffee ring structure on PS surface via O₂ plasma for 2 min, which is fabricated with adding polyethyleneoxide (PEO) additives (1.5 g/L in concentration, 2.5 in relative viscosity) according to our previous report [11]. The SiO₂ microspheres were prepared by the Stöber method as mentioned in the Refs. [16-18]. The weight fraction of 0.5 wt% is selected by takinginto account subsequently easy quantification of the SiO₂ microspheres density, and SiO₂ microspheres are monolayer deposition. The droplet is 2 μL in volume. PS film is prepared by spinning coating with the rate of 3000 rpm/min and its thickness is ca. 2 μm. The coffee ring effect is ameliorated and disappears (Fig.S1 in Supporting information and Fig. 1a) due to introducing PEO additives. Firstly, the PEO additives increase droplet viscosity, which provides a large resistance to capillary flow. In other words, small amounts of SiO₂ microspheres are deposited on droplet periphery, which is not advantageous to the pinning of the CL [19]. Secondly, Marangoni effect induced by PEO concentration inhomogeneity as solvent evaporating is the driving force to make the apparent CL recede [20, 21]. Thus, the coffee ring effect is prevented on hydrophilic PS surface.

  图 1

  

  图 1  (a, b) Images of the final distribution of SiO₂microspheres with PEO (1.5 g/L) additives on PS film with (a) and without (b) O₂ plasma. The scale bar is 1 mmin (a) and (b). (c, d) Photographs of a flat emulsion droplet with a CA of 10° (c) and a spherical emulsion droplet with a CA of 70° (d) with PEO additives after and before the PS films were exposed to O₂ plasma. (e) The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, on PS film with and without O₂ plasma.

  Figure 1.  (a, b) Images of the final distribution of SiO₂microspheres with PEO (1.5 g/L) additives on PS film with (a) and without (b) O₂ plasma. The scale bar is 1 mmin (a) and (b). (c, d) Photographs of a flat emulsion droplet with a CA of 10° (c) and a spherical emulsion droplet with a CA of 70° (d) with PEO additives after and before the PS films were exposed to O₂ plasma. (e) The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, on PS film with and without O₂ plasma.

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  In contrast with Fig. 1a, the inset of Fig. 1b exhibits the optical microscope image of SiO₂ microspheres deposition with PEO additiveson PSsurfacewithoutO₂plasmatreatment.However, the coffee ring effect does not vividly shown owing to the optical contrast. So, corresponding SEM image of SiO₂ microspheres deposition is obtained in Fig. 1b. Obviously, it shows coffee ring structure exists with most of SiO₂ microspheres distributed at droplet periphery. This will be verified in Fig. 1e.

  We believe that the coffee ring structure is adjusted with wettability change via O₂ plasma. The contact angle (CA) of droplet on PS surface after O₂ plasma treament is unstable. So, firstly, the PS surface is managed by O₂ plasma for 2 min. Then, in order to ensure the accuracy and comparability of experimental results, the experiments on PS surface are all performed after 30 min. To evaluate the wettability, Fig. 1c, d manifest the CAs of droplet with PEO additives on PS surface are 10° and 70° after and before O₂ plasma. And the CAs of droplet without PEO additives on PS surface are 10° and 78° after and before O₂ plasma shown in Fig.S2 in Supporting information. On PS surface the value of the CA for droplet with PEO additives is obviously lower than that for droplet without PEO additives. This result is in consistent with literature report and indicates introducing PEO additives will decrease droplet surface tension to result in low value of the CA [20, 21]. But, on hydrophilic PS surface PEO additives have no influence on the value of the CA due to small value of the CA.

  In order to assess the coffee ring structure controlling by O₂ plasma, the value of ρ_max/ρ_mid in Fig. 1a, b is explored, as shown in Fig. 1e. Specifically, SEM image analysis enables counting of the number of particles, N, in an area set by ca. 13600 μm², which is the area of SEM image with the magnification of 1000. The areal particle density ρ=N/A with A=13600 μm². ρ_max is the maximum value of ρ at the edge of droplet deposition and ρ_mid is the average value of ρ in the center of droplet deposition. Evidently, we can see that the value of ρ_max/ρ_mid in Fig. 1e is 13.1 and 1.2, respectively. And it confirms switching coffee ring structure has been successfully accomplished via O₂ plasma.

  The coffee ring effect is prevented on hydrophilic PS surface (Fig. 1a). As has been confirmed in Fig. 2, we calculate and plot ρ_max/ρ_mid. Clearly, Fig. 2 displays the values of ρ_max/ρ_mid evidently vary with droplet relative viscosity. The value of ρ_max/ρ_mid is 2.6 and 1.2 with corresponding droplet relative viscosity 1.47 and 2.5. This is in agreement with the optical microscope image in Fig. 1a and S1. The combination of the value of ρ_max/ρ_mid (1.2) and the optical microscope image in Fig. 1a indicates inhibiting coffee ring structure is realized via PEO polymer additives on hydrophilic PS surface resulted from O₂ plasma treatment.

  图 2

  

  图 2  The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, plotted for different relative viscosity.

  Figure 2.  The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, plotted for different relative viscosity.

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  In addition, so as to explore the coffee ring structure controlling via O₂ plasma in detail, the magnification optical microscope images of edge and centre of droplet deposition are determined in Fig. 3. From Fig. 3, we can see that SiO₂ microspheres distribute uniformly at droplet edge and centre in Fig. 3a. In comparison, SiO₂ microspheres aggregate at the CL and the number of SiO₂ microspheres at droplet edge in Fig. 3b is larger than that in Fig. 3a. Moreover, in present case the optical microscope of the droplet centre in Fig. 3a, b do not distinctly display different owing to optical contrast. Understanding of SiO₂ microspheres mechanism starts from analyzing the influence of wettability on the CL by measuring the optical microscope images and the motion of the CL during droplet evaporation, which will be discussed in following section.

  图 3

  

  图 3  (a, b) Optical microscope images of SiO₂ microspheres deposition on PS film with PEO additives with (a) and without (b) O₂ plasma. The scale bar is 1 mm in (a) and (b), and 250 mm in the others optical microscope images.

  Figure 3.  (a, b) Optical microscope images of SiO₂ microspheres deposition on PS film with PEO additives with (a) and without (b) O₂ plasma. The scale bar is 1 mm in (a) and (b), and 250 mm in the others optical microscope images.

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  To explore the possible mechanism for manipulating the coffee ring structure by means of O₂ plasma, typical optical micrograph sequence of droplet evaporation is characterized. And the optical microscope images are observed with a frame rate of one frame per minute. In this case, the time at which evaporation finishes, t_final, is readily identified as the time when the droplet radius shrinks to zero. Fig. 4a-d clearly reveals that for droplet with PEO additives on hydrophilic PS surface the motion of the CL begins at the time of t/t_final > 0.25, which is resulted from the small value of the CA and addition of PEO additives [11]. Whereas, on hydrophobic PS surface, Fig. 4e-h exhibit that the CL is basically pinning at early stage before t/t_final=0.25 and after t/t_final=0.75, moving at the time from t/t_final=0.25 to t/t_final=0.75. More importantly, SiO₂ microspheres do not anchor on PS surface during the recession of the CL. These behaviors have been shown in Fig. 4i. Concretely, on hydrophilic PS surface for droplet without PEO additives the motion of the CL starts until at the time of t/t_final=0.90 (red circle). That is to say the CL remains pinned until the final stage of evaporation, which is in consistent with literature report [6, 7]. However, the motion of the CL is at the time of t/t_final=0.25 for droplet with PEO additives (black circle), which has been investigated in our previous work. On hydrophobic PS surface for droplet without PEO additives (red square) the pinning of the CL is about at the time between t/t_final=0.60 and t/t_final=0.90 at the entire evaporation process (red square). By contrast, for droplet with PEO additives the pinning of the CL is approximately at the time between t/t_final=0.70 and t/t_final=0.90 (black square). From above results we can see that on hydrophobic PS surface the time for the motion of the CL for droplet with polymer additives is longer than that for droplet without polymer additives, which may be attributed to Marangoni effect [20, 21].

  图 4

  

  图 4  (a-h) Experimental optical microscope images at different times (t/t_final) with PEO (1.5 g/L in concentration) additives during the evaporation of a droplet of SiO₂ microspheres suspension; shown are data for SiO₂ microspheres on PS film with (a-d) and without (e-h) O₂ plasma. The scale bar is 1 mm. (i) The radius, R, of droplets of different suspensions is plotted versus time, t, for evaporating droplets. Suspensions of SiO₂ microspheres with PEO additives on PS film with (black circle) and without (black square) O₂ plasma. Suspensions of SiO₂ microspheres without PEO additives on PS film with (red circle) and without (red square) O₂ plasma. To facilitate comparisons, the time is normalized by the time evaporation ends (t_final), and R is normalized by the value of R at t=0 min.

  Figure 4.  (a-h) Experimental optical microscope images at different times (t/t_final) with PEO (1.5 g/L in concentration) additives during the evaporation of a droplet of SiO₂ microspheres suspension; shown are data for SiO₂ microspheres on PS film with (a-d) and without (e-h) O₂ plasma. The scale bar is 1 mm. (i) The radius, R, of droplets of different suspensions is plotted versus time, t, for evaporating droplets. Suspensions of SiO₂ microspheres with PEO additives on PS film with (black circle) and without (black square) O₂ plasma. Suspensions of SiO₂ microspheres without PEO additives on PS film with (red circle) and without (red square) O₂ plasma. To facilitate comparisons, the time is normalized by the time evaporation ends (t_final), and R is normalized by the value of R at t=0 min.

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  4.   Conclusion

  In summary, controllable macroscale coffee ring structure has been demonstrated on PS surface by employing O₂ plasma with the presence of PEO additives. O₂ plasma has allowed the wettability variation of PS surface from hydrophobic to hydrophilic. On hydrophilic PS surface the uniform SiO₂ microspheres deposition is achieved depending on PEO additives, which endows the increasing relative viscosity and Marangoni effect to make the CL recede with SiO₂ microspheres left during droplet evaporating. On the contrary, on hydrophobic PS surface, SiO₂ microspheres do not adhere to PS surface at early motion of the CL and aggregate at droplet edge to form coffee ring structure at following pinning of the CL. Therefore, we believe that this present system will open up new opportunities for manipulating coffee ring structure by combining smart surface.

  Acknowledgments

  This work is supported by the National Nature Science Foundation (Nos. 91123031, 20921003, 51403076, 21103112), and the National Basic Research Program of China (No. 2012CB933802).

  Appendix A. Supplementary data

  Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.07.028.

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  Figure 1  (a, b) Images of the final distribution of SiO₂microspheres with PEO (1.5 g/L) additives on PS film with (a) and without (b) O₂ plasma. The scale bar is 1 mmin (a) and (b). (c, d) Photographs of a flat emulsion droplet with a CA of 10° (c) and a spherical emulsion droplet with a CA of 70° (d) with PEO additives after and before the PS films were exposed to O₂ plasma. (e) The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, on PS film with and without O₂ plasma.

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  Figure 2  The maximum local density, ρ_max, normalized by the density in the droplet centre, ρ_mid, plotted for different relative viscosity.

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  Figure 3  (a, b) Optical microscope images of SiO₂ microspheres deposition on PS film with PEO additives with (a) and without (b) O₂ plasma. The scale bar is 1 mm in (a) and (b), and 250 mm in the others optical microscope images.

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  Figure 4  (a-h) Experimental optical microscope images at different times (t/t_final) with PEO (1.5 g/L in concentration) additives during the evaporation of a droplet of SiO₂ microspheres suspension; shown are data for SiO₂ microspheres on PS film with (a-d) and without (e-h) O₂ plasma. The scale bar is 1 mm. (i) The radius, R, of droplets of different suspensions is plotted versus time, t, for evaporating droplets. Suspensions of SiO₂ microspheres with PEO additives on PS film with (black circle) and without (black square) O₂ plasma. Suspensions of SiO₂ microspheres without PEO additives on PS film with (red circle) and without (red square) O₂ plasma. To facilitate comparisons, the time is normalized by the time evaporation ends (t_final), and R is normalized by the value of R at t=0 min.

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