Controlling the evaporation lifetimes of sessile droplets on superhydrophobic paper by simple stretching

Abstract

Superhydrophobic paper was prepared by a soot template method. After being simply stretched at a strain of ∼7.2% under humid conditions, its high water repellency was maintained, but its wetting adhesion remarkably increased. The evaporation mode of a 5 μL droplet also transitioned from the constant contact angle (CCA) to the constant contact line (CCL) mode, which shortened the evaporation lifetime from ∼3560 s to ∼2380 s. This work shows significant advances for improving the analytical efficiency of lab-on-paper applications.

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Paper is a common and essential material in our lives, which can be written or printed on, cut, folded, coated, embossed, colored, packed, and so on. Owing to its low-cost, biodegradability, wide availability, and the ability to modify and functionalize its cellulose fibers, paper has been recognized as an appealing substrate for application in more technical and specialized purposes. In recent years, researchers have demonstrated a wide range of paper-based sensors and devices.¹ They have shown increased interest in devices for biochemical analysis or less expensive health and point-of-care (POC) diagnosis, lab-on-paper (LOP), and two-dimensional (2D) systems to transport, mix, and store liquid samples, where the paper can support capillary flow enabling sample transfer onto desired reaction spots in a controlled manner without any external pumps. The wettability, as characterized by contact angle and contact angle hysteresis, therefore plays an important role in these applications.^2–7 Attempts have been made to obtain paper-based devices by changing the wettability of the paper, i.e. by using hydrophobic–hydrophilic contrast.^8–10 However, the use of paper has proven to be challenging due to its typical hydrophilic nature. Many physical or chemical methods have been proposed to fabricate hydrophobic papers, for example, photolithography, plasma treatment, inkjet printing and wax printing approaches.^11–14 Among these methods, transforming a sheet of paper into a superhydrophobic substrate has shown significant advantages for overcoming/delaying liquid penetration into the substrate, especially for paper applications such as LOP devices.⁶ It may increase the wettability contrast between wetting and repellent regions, prevent water and moisture absorption, and avoid pathogenic contamination.² Therefore, investigating the wetting behaviour of superhydrophobic paper is meaningful for practical applications.¹⁵

Evaporating droplets on superhydrophobic substrates have attracted scientists’ attention recently. Researchers have devoted a lot of time and energy to investigating the factors that affect the evaporation mode, pattern, lifetime and rate.^16–24 Achievements have been applied to control the evaporation of drops in order to serve our daily lives, including adjusting the humidity, temperature, vapor pressure, droplet size, wettability of the solid surface, etc.^19,24 Due to the globe-like profile of sessile droplet, which reduces the surface area, the evaporation lifetime on superhydrophobic surfaces is substantially prolonged.¹⁶ This is beneficial to LOP applications where very small amounts of materials must be analyzed over short durations.¹⁷ However, the efforts of industrial engineers are always directed towards greater efficiency and further studies have demonstrated that the evaporation lifetimes or rates of droplets can be tuned via the structure of patterns on superhydrophobic surfaces.^25–28 Therefore, it seems there is an urgent need to be able control the evaporation lifetimes or rates of sessile droplets on superhydrophobic surfaces by varying surface microstructures. In this communication, superhydrophobic paper was prepared by a candle soot template method. We then stretched the superhydrophobic paper in a humid environment to adjust its surface microstructures. The wettability of the superhydrophobic paper was subsequently changed by this stretching process, which induced a different droplet evaporation mode. This method may open a new pathway to control the evaporation lifetimes of sessile droplets depending on their application.

The typical experimental process is shown in Fig. 1. A piece of common printing paper was skilfully held above the flame of a paraffin candle and a soot layer was deposited on one side of the paper. The soot coated paper was then placed in a desiccator together with two open glass vessels containing tetraethoxysilane (TES) and aqueous ammonia solution, respectively. After chemical vapor deposition (CVD) of TES, silica was formed by hydrolysis and then condensation of TES was catalyzed by ammonia. To transform this into a superhydrophobic coating, CVD of semi-fluorinated silane onto the paper was performed.^29,30 Fig. 2a shows an optical image of a droplet on the coated paper. Some globe-like droplets develop on the paper surface, implying it is highly water repellent. As shown in the inset of Fig. 2a, scanning electron microscope (SEM) images show that dense soot particles with silica (SPs) are deposited on the surface. The magnified SEM image shows these SPs can wrap the paper fibers well. These SPs are highly aggregated and deposited with a thickness of 10–20 μm. X-ray photo-electron spectroscopy (XPS) measurements were performed to confirm that the SPs were coated with perfluorooctyltriethoxysilane. The elements F (binding energy, BE = 688.1 eV) and Si (BE = 103.1 eV) can be seen on the paper surface. In the total elemental content, F and Si reach 39.5 wt% and 19.5 wt%, respectively. The ATR-FTIR spectrum of the coated paper was also measured in order to fully evaluate its chemical composition. However, we could not observe any extra absorptions (for example, absorption from the C–F group) from the paper due to its thickness (see Fig. S1†). So a stable coating of fluorosilane has already been produced on the surfaces of the SPs. After that, the wettability of the coated paper was examined by contact angle measurements, in which a 5 μL water droplet was used. The paper possesses highly hydrophobic properties with a contact angle of 160.2 ± 1.5°. In addition, the droplet could easily roll off the surface with a rolling-off angle of 5.5 ± 0.5°, which suggests low adhesion for a droplet. Interestingly, the coated paper was reflective after being immersed into water for several seconds. Moreover, it remained completely dry after being taken out, which illustrates the existence of an air cushion around the paper, i.e. a Cassie state. However, the uncoated side of the paper was partly wet.

Fig. 1 The typical procedure for the preparation of the superhydrophobic paper and the variation of its microstructures by the stretching process.

Fig. 2 (a) Optical image of droplets on the coated paper and SEM images (inset) of the paper at different magnifications. (b) The XPS spectrum of the coated paper. (c) Optical image of the coated paper when it was immersed to water. Note that some reflective light can be seen. (d) A high contact angle and low rolling-off angle was seen for a 5 μL droplet on the coated paper.

Inspired by the fact that the wettability of soft materials can be adjusted by a stretching process,³¹ we tried to change the surface morphology of the superhydrophobic paper by stretching it at different strains using a tensile testing machine. However, the maximum strain at break of the paper was only about 2.6% in air, which is much less than that of other soft materials for example, elastomers or fabric (which can extend itself several times or ten times). Such a tiny strain cannot cause any obvious surface deformation to affect the microstructures (see Fig. S2†). However, the maximum strain at break of the paper can be changed when it is placed in a humid environment. We placed the uncoated side of the paper over a vessel containing water which was heated at 50 °C (see Fig. 1). The paper remained in place for 5 min and was wetted a little. The maximum strain at break (∼8.3%) of the paper under these humid conditions substantially increased. Surprisingly, a globe-like droplet was still observed on the paper until it broke (see Fig. 3a). In this case, the fractured paper cannot be used, so we decided to stretch the paper at a moderate strain of ∼7.2%. The microstructures of the stretched paper were checked by SEM. Note that a lot of cellulosic paper fibers were exposed on its surface, as shown in Fig. 3b. Due to the penetration of water vapor, the bonding among these cellulosic fibers may be damaged. After being stretched, the fibers can slide around and this causes the SPs layer to be thinner. Some smooth areas without any SPs aggregation can be seen on the paper fibers, which is very different from the initial (un-stretched) paper shown in Fig. 2b. The contact angle of droplets on the paper was tested. The angle decreased to 154.2 ± 1.1° in comparison with the initial value (see Fig. 3c). The rolling-off angle on the paper was interestingly beyond 90° as shown in Fig. 3d. Even when we turned the paper over, a 5 μL droplet did not fall off. This means that a droplet on the stretched paper is in a Wenzel state. Therefore, such mechanical stretching induces variations in the wettability of the coated paper.

Fig. 3 (a) An optical image of a droplet on the stretched paper, even after it was extended to break. (b) SEM images of the paper after it was stretched at a strain of ∼7.2% under humid conditions. (c) The large contact angle of a droplet on the stretched paper. (d) The measured contact angles and rolling-off angles of the paper before and after it was stretched.

Droplet evaporation is a diffusion-controlled “quasi-stationary” process, i.e. it involves the transfer of mass (or volume) and heat. Typically, the evaporation modes of superhydrophobic substrates are divided by contact angle and contact line into a constant contact angle (CCA), constant contact line (CCL) or mixed mode.^32–37 Furthermore, rough microstructures produce more complex evaporation processes due to their effect on the surface wettability.³³ The volume of a sessile droplet (V) on a solid surface can be expressed by:

where θ is the contact angle and r_b is the contact radius. It can be deduced from eqn (1) that processes that vary the contact angle, contact radius or height of the droplet (caused by evaporation modes), really determine the evaporating volume and even the droplet’s disappearance (i.e. its evaporation lifetime). The evaporation of ultra pure water droplets with volumes of 5 μL on the coated paper was monitored in a closed chamber attached to a contact angle measurement instrument. Evaporation of the droplets was conducted at a temperature of 28 ± 0.1 °C, which was controlled by the heating holder of the chamber. Nitrogen (N₂) gas was used to pass through water to form a N₂/water mixing gas in a closed vessel in order to maintain a constant relative humidity of 50 ± 2% inside the chamber. Contact diameters, contact angles, and volumes were calculated using analysis software (SCA20, version 4.1.10). All data were obtained by repeating each experiment five times. The variations of the contact angle and contact diameter of a pure water droplet with time on the initial paper are given in the left-hand side of Fig. 4a. Error bars are displayed for the data. Successive optical images of droplet evaporation on the paper surface are displayed in the insets. During the evaporation lifetime of ∼3560 s, the contact diameter of the droplet reduced with increasing evaporation time. The CCA mode is maintained for an evaporation period of ∼3200 s. After that time, the evaporation mode of the droplet transistioned into a mixed mode where the contact diameter and contact angle simultaneously decrease. This situation may be caused by the transition of the wetting state from the Cassie to the Wenzel state.^18,32 The volume of the droplet in the mixed mode is very small, which implies that the superhydrophobicity of the coated paper is very stable.³² In comparison, after the paper was stretched at a stain of ∼7.2%, the contact diameter of a droplet on the paper was almost unchanged (see the right-hand side of Fig. 4a). However, the contact angle decreases with the increase in evaporation time. Optical images show that the contact line is always pinned during almost the whole evaporation time, i.e., it maintains the CCL mode. At a lifetime of ∼2300 s, we nearly see the shift of the contact line. Fig. 4b illustrates the transition mechanism of the evaporation mode. Initially, the cellulose fibers are random stacked layer by layer on the paper surface, which are the primary microstructures. SPs are thickly deposited onto these fibers to construct secondary microstructures on the paper, moreover, the SPs develop fractal-like composite interfaces when a droplet is dropped upon them (see Fig. S3†). The droplet is suspended on the SPs aggregates, not touching the paper fiber. Therefore, there is a small interface area in contact with the droplet, representing a Cassie state. During evaporation, the contact line of the droplet can easily recede along the surface composed of SPs aggregates. Therefore, it retains the CCA mode. However, after the paper is stretched, the SPs aggregation is partly damaged and some flat areas of the paper fiber are exposed. These areas are hydrophobic and not able to suspend the droplet. Once these areas are in contact with the bottom of the droplet, they can be strongly adhesive, which enhances the interaction of the droplet/paper interfaces. During evaporation, the contact line is partly or completely limited by these areas and cannot recede to maintain the constant contact angle and thus leads to the development of the CCL mode. So the ability of the paper to move the contact line becomes the key to controlling the evaporation mode. In order to display the effects of stretching on the motion of the contact line, we hung a droplet just in contact with the paper. Once in contact, the droplet was drawn upwards to leave the paper. The movement of contact line can therefore be observed (see Fig. S4†). The contact line easily moved on the un-stretched paper, but the droplet was drawn to break after it contacted the stretched paper. The contact line barely moved during the process. Looking at another aspect, the adhesion force between droplets and the coated paper was also measured using a high sensitivity micro-electrodynamic balance system.^38,39 The maximum adhesion force (about 129.2 μN) of the stretched paper is significantly more than that (approximately zero) of the initial paper (see Fig. S5†). The high level of adhesion greatly affects the evaporation of droplets on the superhydrophobic paper after it is stretched.

Fig. 4 (a) Plots of contact angle and contact diameter against evaporation time before (left) and after (right) the superhydrophobic paper was stretched. Red circles represent the contact angles and black squares indicate the contact diameters. The insets show optical images of a droplet on the paper during evaporation. (b) Illustration of the mechanism for the transition of the droplet evaporation mode. (c) Plots of the droplet volume versus evaporation time for the initial and stretched paper.

Fig. 4c shows the volume variation of a droplet over time on the superhydrophobic paper before and after it has been stretched. The evaporation rate (−dV/dt) exhibits similar behaviour for the initial and stretched paper before an evaporation time of ∼1200 s. The evaporation rate of the droplet on the stretched paper is faster than that on the initial paper once its contact angle decreases to the critical angle of ∼128°. In contrast with a hydrophilic solid surface, a sessile droplet placed on a hydrophobic surface will evaporate slower owing to the existence of the surface wall preventing the formation of vapor, which can diffuse in a downward direction. Retardation of evaporation is increased because a greater area of the droplets globe-like shape is exposed preventing downward evaporation, especially for superhydrophobic surfaces with the contact angles of above 150°. When the droplet is evaporated in the CCA mode (initial paper), the retardation changes proportionally because it always maintains the same scaled-down shape during the period of the evaporation (i.e. it has a constant contact angle). However, when the droplet is evaporated in the CCL mode (i.e. when deposited on the stretched paper), its evaporation has remarkably reduced retardation (giving rise to a faster rate) due to the decrease of the contact angle with the increase in evaporation time. More areas of the droplet are shrunk increasing downward evaporation. Obviously, when the contact angle of an evaporated droplet decreases to 90°, retardation completely disappears causing a remarkable increase in the evaporation rate. In fact, such differences in retardation happen between the initial and stretched paper when the contact angle reaches a moderate hydrophobicity (in our case, it is ∼128°). As such the droplet evaporation lifetime on the stretched paper is largely shortened. When a sessile droplet placed on a solid surface evaporates in the CCA mode, its evaporation lifetime (t) can be given as,¹⁸

where c_s is the concentration of vapor at the droplet sphere surface, c_∞ is the concentration at an infinite distance from the droplet, D is the diffusion coefficient, ρ_L is the density of the drop of liquid, V₀ is the initial volume of the droplet and f(θ) represents the retardation of evaporation in the downward direction due to the presence of a horizontal surface given by:^16,19,40,41

f(θ) = (4.4785 × 10⁻⁵) + 0.31665θ + (5.8 × 10⁻²)θ² − (4.439 × 10⁻²)θ³ + (5.165 × 10⁻³)θ⁴.

Once eqn (1) is introduced into eqn (2), we can obtain the evaporation lifetime:

On the other hand, when a droplet evaporates in the CCL mode, its lifetime can be expressed by,¹⁷

where ΔA_LV is the surface area variation of the sessile droplet (A_LV = 2πr_b²/(1 + cos θ)). When f(θ) is assumed to be constant, the evaporation lifetime, t can be calculated using:

For a similar contact angle, contact diameter and volume before evaporation, a comparison can be made between eqn (3) and (5) that the evaporation lifetime of a sessile droplet in the CCL mode is shorter than in the CCA mode due to the relationship of (1 − cos θ)²(2 + cos θ)/sin² θ (≈34.8) > 2/(1 + cos θ) − 1 (≈32.8). The lifetime of the droplet is calculated to be 3337 s (on the initial paper) and 2472 s (on the stretched paper) according to eqn (3) and (5), which is in agreement with observations. However, the effects of contact angle (not on evaporation) are not neglected in the calculation of evaporation lifetime. For example, if the initial contact angle is 160.2° on the un-stretched paper, the calculated evaporation lifetime is 1.6 times more than that with an angle of 154°. Therefore, deviations in the computation of evaporation lifetimes are mainly a result of measurements of the contact angle. Moreover, the function of f(θ) greatly decreases with a reduction in contact angle, thus decreasing the evaporation lifetime more in the CCL mode, as predicted in eqn (5) (see Fig. S6†). Nevertheless, it should be stressed that droplet lifetimes on the stretched paper can be greatly shortened during evaporation. This is significant for providing better efficiency to process a relatively short-time analysis. In one possible case, a simple stretching by hand operation for the paper can be conducted to serve LOP applications. Nevertheless, it should be stressed that tuning the evaporation mode of a droplet on superhydrophobic paper is an effective method to control its lifetime. In our case, the evaporation mode is determined by the adhesion of droplet to the paper surface, which is directly affected by its microstructures. Therefore, evaporation lifetimes are easily adjusted by stretching the superhydrophobic paper, which is believed to be important for applications.

Furthermore, only extreme samples are shown in this communication where the strain of the paper reaches ∼7.2%, which is very close to being broken. An attempt has been made to display powerful control of the evaporation lifetime of a droplet on our superhydrophobic paper. In fact, more complex evaporation phenomena were observed if we adjusted the strain between 0 and ∼7.2%. For example, two stages were seen when the paper was stretched at a strain of ∼5.3%: the droplet initially retained the CCA mode, then at a certain evaporation time, it transitioned to the CCL mode. Thus, this strain controlled method can be performed to realize complex evaporation processes (different lifetimes or rates, etc.) to satisfy the diverse requirements of LOP applications.

Conclusions

In summary, we prepared superhydrophobic paper by a soot template method. Highly hydrophobic properties and low adhesion were realized on this prepared paper. During stretching under humid conditions, the superhydrophobic paper became stickier, but it still possessed a large contact angle. Evaporation of a droplet on the coated paper maintained a CCA mode. In contrast, the evaporation mode transferred to the CCL mode with a shorter evaporation lifetime after the paper was stretched at a strain of ∼7.2%. The shift in the contact line is the key to determining the evaporation mode, and is associated with the adhesion of the droplet. Stretching induces strong adhesion of the droplet on the paper, which causes the transition of the evaporation mode and shortens its lifetime. This offers insight into realizing the control of droplet evaporation lifetimes for LOP applications by a simple mechanical action.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25745j

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