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Investigation of micro-sized droplet impinging on sharp wettability contrast surface

C Y Lim and Y C Lam*

School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.

*Email: myclam@ntu.edu.sg

Abstract

Experimental investigation of a micro-sized droplet jetted on a surface with sharp wettability contrast was conducted. The dynamics of micro-sized droplet impingement on a sharp wettability contrast surface, which is critical in inkjet printing technology, has not been investigated in the literature. Hydrophilic lines with line width ranging from 27µm to 53µm and contact angle ranging from 17° to 77° were patterned on a hydrophobic surface with contact angle of 107°. Water droplets with 81µm diameter were impinged at various offset distances from the center of the hydrophilic line. The evolution of the droplet upon impingement can be divided into three distinct phases, namely the kinematic phase, the translating phase where the droplet moves towards the center of the hydrophilic line, and the conforming phase where the droplet spreads along the line. The key parameters affecting the conformability of the droplet to the hydrophilic line pattern are the ratio of the line width to the initial droplet diameter and the contact angle of the hydrophilic line. The droplet will only conform completely to the hydrophilic pattern if the line width is not overly small relative to the droplet and the contact angle of the hydrophilic line is sufficiently low. The impact offset distance does not affect the final shape and final location of the droplet, as long as part of the droplet touches the hydrophilic line upon impingement. This process has a significant impact on inkjet printing technology as high accuracy of inkjet droplet deposition and shape control can be achieved through wettability patterning.

1. Introduction

Droplet impingement and spreading on a solid substrate are key processes in inkjet printing technology. Materials including metals [1], ceramics [2] and polymers [3], in the form of particles suspension or dissolved in appropriate solvents, can be deposited at the desired locations on a substrate through inkjet printing. Various applications of inkjet technology have been demonstrated such as fabrication of organic light emitting diodes (OLED) [4], field-effect transistors (FET) [5], polymer solar cells [6] and micro electro-mechanical system (MEMS) devices [7]. The minimum size of the printed features is generally restricted by the size of the inkjet droplet, which typically ranges from 10µm to 100µm in diameter [8]. An obvious way to print finer features would be to reduce the nozzle size to generate smaller droplets. However, this is not practical because the nozzle will be prone to clogging, especially when a suspension of particles is involved.

The wettability of the substrate surface affects the final printed features significantly. Surface modification through wettability patterning on the substrate has been exploited to guide and restrict the spreading of droplet to achieve small printed features [9-10]. The droplet is repelled from the hydrophobic region and spreads on the hydrophilic region. The early experimental study on the dynamics of this effect was conducted with a liquid ridge straddling across two regions with different static contact angle [11]. The liquid ridge migrates towards the hydrophilic area because the two edges of the ridge are not in equilibrium and are subjected to uncompensated Young forces. The morphologies of liquid droplet on hydrophilic stripes on a hydrophobic surface were studied by Gau et al [12] through water condensation on the hydrophilic stripes to form cylindrical ridges.

Theoretical analyses [13-14] and numerical simulations [15-19] had been performed to study the interaction of droplet with the hydrophilic-hydrophobic wettability pattern. For a micro-sized droplet impinging on a wettability pattern, the impact and spreading processes occur in a very short time scale due to the small displacement involved as opposed to a millimeter-sized droplet. The experimental observation of this process is difficult because it demands high speed image capturing with good resolution of the micro-sized feature. The final outcome of the printed feature was typically studied [20] instead of the dynamics of the impact and spreading behavior.

In recent years, high speed imaging and/or flash photography has been employed to observe droplet impact and spreading dynamics. Through flash photography. Rioboo et al [21] identified the different phases for droplet spreading on a dry homogenous solid surface upon impingement. Eddi et al [22] captured the initial spreading dynamics when a millimeter-sized water-glycerine droplet came in contact with a glass surface, with a capturing rate as high as 250,000 frame per second (fps). Briones et al [23] studied the impact and spreading of a micro-sized water droplet on an aluminum thin film at a maximum frame rate of 120,000 fps. Brown et al [24] investigated the effect of surface roughness on the impact and spreading of a picoliter water droplet, with a frame rate of 90,000 fps. Vadillo et al [25] captured the impact of millimeter-sized droplet on a surface with various equilibrium contact angles at a frame rate of 10,000 fps. Based on short duration flash photography, Jung and Hutchings [26] captured the ink-jet printed droplet on non-porous surfaces at an equivalent frame rate as high as 10 million fps. These studies mainly focused on surfaces with uniform wettability. Hitherto, the movement of impinging droplets on a surface with a sharp wettability contrast has not been investigated with high speed imaging. This study focuses on the capturing of micro-sized droplet impacting on a sharp wettability contrast surface and the associated spreading dynamics.

Some studies focused on the droplet movement on a surface with a continuous wettability gradient [27-29] instead of a sharp wettability contrast. On the surface with a continuous wettability gradient, the droplet attains a steady velocity when the driving force due to the wettability gradient is balanced by the viscous drag on the wetted surface [28]. The migration velocity is proportional to the gradient of interfacial tension difference and inversely proportional to the fluid viscosity. Other investigations demonstrated the spreading behavior of a millimeter-sized droplet spanning across multiple chemically heterogeneous stripes [30-31]. The droplet exhibits anisotropic spreading and is elongated in the direction of the stripes. It should be highlighted here that the droplet behavior on a wettability pattern with a sharp contact angle contrast is distinctively different from a continuous wettability gradient. A wettability gradient moves the droplet in a certain direction or to induce droplet elongation. In contrast, a sharp wettability contrast dictates the droplet final position and restricts its spreading. Hitherto, the dynamics of accurate positioning of a jetted droplet on a specific location and shaping the droplet into a desired micro-sized feature have not been investigated.

This investigation reveals the dynamics of a micro-sized droplet impinging on a line pattern with sharp wettability boundaries through high speed imaging. The understanding of this process can potentially enhance the accuracy of material deposition through inkjet printing and improve the resolution of printed features.

2. Experimental methods

2.1. Surface patterning

The substrate employed in the experiments is polyethylene terephthalate (PET) sheet, which is a common material for the roll-to-roll manufacturing process [32]. The water contact angle of pristine PET sheet was measured to be 75°. To increase the contact angle of the surface, the PET sheet was first dip-coated with 3M Novec 1700 electronic grade coating, which is a clear solution with fluorochemical crylic polymer carried in a hydrofluoroether solvent. The concentration of the solution was diluted to 0.1 weight % from the original concentration of 2% to reduce the coating thickness and time for selective removal of coating. The solvent was evaporated in an oven at 120oC for 10 seconds. The average contact angle for the hydrophobic coated PET sheet was 107°. The thickness of the hydrophobic coating on PET sheet has been measured by an atomic force microscope (AFM) in our previous paper to be in the range of 45nm to 126nm [33].

To create hydrophilic patterns on the coated PET sheet, a nickel mask of 50µm thick with long slits of various widths (25µm, 35µm and 50µm) was placed on top of the PET sheet. Magnet was placed below the PET sheet to hold the nickel mask firmly against the PET surface. The sandwich was then placed into an ultra-violet (UV) ozone exposure unit (Senlight PL 16) which generates 10.2mW/cm2 at 185nm wavelength and 32.7mW/cm2 at 254nm wavelength for 25 minutes. The treatment selectively removed the hydrophobic coating according to the slits on the nickel mask and modified the exposed areas on the PET surface into hydrophilic regions. The length of the slit is 5mm. The droplet size (81µm) is much smaller than the length of the slit even after the droplet has fully spread (to approximately 0.5mm) on the hydrophilic line. Therefore, the length of the slit will not affect the droplet spreading behavior. The average contact angle of the UV ozone modified PET surface was measured to be 17°. A sharp and high wettability contrast of 90° between the line pattern and its surrounding surface was thus achieved.

To study the effect of wettability contrast on droplet spreading behavior, similar patterns with other contact angles were created through plasma treatment system (March PX-500). Oxygen plasma treatment would also remove the hydrophobic coating and render the surface hydrophilic. At a power of 100W, a pressure of 70mTorr and 45 seconds exposure time, the hydrophobic coating was removed, with little modification to the PET surface. The water contact angle was measured to be 77° which is similar to the pristine PET surface. Increasing the exposure time to 2 minutes lowered the contact angle to 40°.

2.2. Experimental setup

Water droplet was jetted on the surface with an inkjet dispenser head (MD-K-130 Microdrop Technologies) with a 70µm nozzle diameter. A driving voltage pulse of 105V with 30µs pulse width generated a droplet diameter D of 81µm. The average impact velocity was measured by capturing the side view images of the droplet at 100,000 fps with a high speed camera (Photron Fast cam SA5 monochrome). The movement of the droplet before impact was tracked and analyzed with MATLAB based on the captured images and the velocity of the droplet can be calculated. The average impact velocity U was measured to be 1.2 ms-1. Weber number for the impingement process, defined as ρU2D/σ (where ρ is the density of the water droplet and σ is the surface tension of water), is 1.6. Reynolds number for the impact process, which is defined as ρUD/µ (where µ is the viscosity of the droplet) is 109.

The inkjet dispenser was mounted on an XY precision translation stage (Unice 06DTS-1M) to control the deposition of droplet accurately. Droplet was impinged at a lateral offset from the center of the hydrophilic lines to investigate the droplet movement induced by the wettability contrast (see figure 1). The distance from the nozzle to the substrate was set to 5 mm to minimize the disturbance of ambient air flow during the droplet flight.

The bottom view of the droplet impingement and spreading process was captured through the high speed camera attached to an inverted microscope (Nikon Ti-S Eclipse). Images of 320 x 192 pixels were captured at 100,000 fps with an exposure time of 2µs. With a 10X objective lens, the spatial resolution of the image is 2µm per pixel. Due to the short exposure time, a high intensity light source was required to capture the images at high speed. The white light illumination was supplied by ultra high pressure mercury lamp (Nikon Intensilight 130W). A UV cut-off filter was attached to the filter cube of the microscope to remove the UV component from the light source.

[pic]

Fig. 1 Experimental setup to capture bottom view of micro-sized droplet impinging on PET surface with hydrophilic line

3. Results and discussions

Figure 2 shows the bottom view or the footprint of the droplet impingement and spreading at different offset distances from the center of a hydrophilic line with an average line width of 53µm and a contact angle of 17o (see supplementary video). For the first 50µs after impact, the droplet impinged at a negligible offset (4µm) from the line covered a large area of hydrophilic surface while the droplet impinged farther from the line (70µm) was on the hydrophobic surface. The droplet impinged at 29µm from the line covered both hydrophobic and hydrophilic surfaces with an almost equal proportion. Despite the differences in the proporition of hydrophobic to hydrophilic surfaces, these droplets maintained an approximately circular footprint, regardless of contact angle of the wetted surface.

According to Rioboo et al [21], droplet spreading after impingement on a dry homogeneous surface can be characterized by four phases: kinematic phase, spreading phase, relaxation phase and wetting/equilibrium phase. The kinematic phase is the first phase after impact, and occur during t* ~ 0.1, where t* = tU/D. During the kinetic phase, droplet is not affected by the surface wettability and the dynamic behavior is purely described by impact velocity and droplet initial diameter [21]. For the micro-sized droplet in this investigation, the kinematic phase occurs at t < 6.8µs. The frame rate of our high speed camera is insufficient to capture the complete dynamic process during the kinematic phase because at 100,000 fps it only captures an image every 10µs. However, the characteristic of the kinematic phase is clearly shown in the first row of images in figure 2, where the droplet maintains a circular footprint regardless of the surface wettability contrast.

The bright white ring around the edge of the droplet was formed by the reflection of light from the interior of the droplet. The total amount of light reflected to the camera and hence the brightness of the ring is related to the shape of the droplet and its contact angle at the time frame. It is likely that the droplet shape and contact angle at t = 0.05ms are the most optimal for light reflection. The initial droplet spreading (kinematic phase) after impact does not depend on the surface condition. Therefore, all the droplets seem to demonstrate maximum brightness at t=0.05ms.

After the kinematic phase, surface wettability starts to influence the droplet spreading behavior on a homogenous surface for t* > 1 [21]. For a droplet impinging on a surface with sharp wettability contrast, subsequent evolution of the droplet can be divided into two distinct phases, namely the translating phase and the conforming phase.

[pic]

Figure 2 Bottom view image sequence of droplets (diameter D = 81µm) impinging at various offset distances from middle of hydrophilic line (width W = 53µm). Contact angles of hydrophilic line and surrounding surface are 17o and 107o respectively. Dimensionless offset distance F* is the offset distance divided by droplet initial diameter D.

3.1. Translating phase

Droplets which were not impinged exactly at the center of the hydrophilic lines moved towards the center of the line. The droplet migration was driven by the difference in surface energy between the hydrophobic and hydrophilic regions. The mechanics of droplet movement under wettability gradient was investigated in detailed in the literature [28] and briefly discussed here. The solid-gas interfacial energy γSG and solid-liquid interfacial energy γSL for hydrophobic and hydrophilic surfaces are denoted with subscript A and B respectively in figure 3. For a droplet on a homogeneous surface, the equilibrium contact angle is given by Young’s equation:

[pic] (1)

Since cos θB is larger than cos θA, (γSGB – γSLB) is then larger than (γSGA – γSLA). The difference in surface energy at the two edges of the droplet imposes a net force on the droplet and induces migration towards the hydrophilic region.

[pic]

Figure 3 Droplet spanning cross two surfaces with sharp contact angle contrast.

Figure 4 shows the displacement of the droplet center from the middle of the hydrophilic line at various initial offset distances. The dimensionless droplet center displacement d* and initial offset distance F* are defined as the corresponding quantities divided by the initial droplet diameter D (81µm). Each data point represents an average of 2 to 3 experimental runs, with approximately similar initial offset distance (see supplementary document for experimental details and data before non-dimensionalized). The displacement error is assigned to be the larger value between the pixel resolution (2µm) and the standard deviation from repeated experimental runs. The results demonstrate high repeatability with an average error of less than 12%.

All droplets did not demonstrate any translation process when t* < 0.3 because the droplet dynamic behavior is not affected by the surface wettability during the kinematic phase. The droplet impinged at 4µm offset (F* = 0.05) shows negligible translation throughout the whole process because its contact surface was almost symmetrical about the droplet center line (see figure 2). Both the upper and lower edges of the droplet were on the hydrophobic surface and thus, the difference in interfacial tension as depicted in figure 3 was absent.

When the impingement offset was increased to 29µm (F* = 0.36) and 47µm (F* = 0.58), the droplet showed significant translation process towards the center of the hydrophilic line after t* > 0.5. Subsequently, the droplet began to spread sideway along the hydrophilic surface and resulted in an elongated droplet shape. The translation process for a droplet impinged at 70µm offset (F* = 0.86) was relatively slow due to the small contact area with the hydrophilic surface. The droplet also appeared to be elongated in the direction of migration (see figure 2) due to minimal spreading in the left and right directions on the hydrophilic surface initially.

[pic]

Figure 4 Displacement of droplet center relative to center of hydrophilic line, d at various initial offset distances. Inset shows definition of d. d and initial offset distance are non-dimensionalized with droplet initial diameter, D (81µm). Dimensionless time t*= tU/D with impact velocity U = 1.2ms-1. Hydrophilic line width W = 53µm. Contact angles of hydrophilic line and surrounding surface are 17o and 107o respectively. Error bars are not shown for clarity.

Residual offset of 5µm to 7µm persisted for all the droplets except for the droplet with an initial offset of 4µm. The lower edge of the wettability contrast impeded the droplet contact line movement. Contact line movement from a surface with a lower contact angle to another surface with a higher contact angle causes additional local energy dissipation at the contact line [17]. The movement of the droplet lower edge was hindered by the wettability contrast and remained pinned at the lower edge of the hydrophilic region (see figure 5). This residual offset will eventually diminish during the subsequent conforming phase.

During the initial stage of droplet impact (kinematic phase), the contact line can actually overcome the wettability contrast due to the high kinetic energy. For the droplet impinged at F* = 0.05, the contact line has overcome the contact angle contrast and the droplet edges moved out of the hydrophilic-hydrophobic barrier right after the impact (see t = 0 of figure 2). This behavior has been demonstrated in our previous numerical investigation [17] of droplet impinging on a circular hydrophilic pattern surrounded by hydrophobic surface. Based on the simulation, during the initial stage of droplet impact, the contact line of the impinging droplet (with We ranging from 0.19 to 12) still moves out of the circular pattern with a similar diameter as the droplet, even when the contact angle contrast is as high as 70°. This process was not captured in our experiment because it required a much higher capturing rate and a stronger light source. As the kinetic energy is being dissipated throughout the spreading process, the contact line no longer has sufficient energy to overcome the wettability contrast at the later stages of spreading. Therefore, the droplet will be constrained within the hydrophilic regions and will conform to the pattern.

3.2. Conforming phase

After migration towards the hydrophilic line, the droplet was elongated along the line and conformed to the edges of the hydrophilic pattern. Typically, after the translating phase, part of the droplet would still lie outside of the hydrophilic pattern, especially when the line width is smaller than the droplet base diameter. For example, the translation phase for a droplet impinged at F* = 0.36 and 0.86 ended when the lower contact line of the droplet was pinned at the edge of the wettability contrast at t* = 1.6 and 20 respectively (see figure 5). However at that time, the upper contact line of the droplet was located outside of the hydrophilic stripe. As the droplet spread along the direction of the hydrophilic line, liquid from the center was directed away to the left and right of the line. As a result of mass continuity, the upper and lower edges of the droplet were pulled into the hydrophilic region and conformed to the hydrophilic pattern.

Although evaporation process could be considered as one of the reason the droplet was being transported into the hydrophilic region due to the shrinkage of total droplet volume, it was not significant in the studied time frame (0.5 seconds after impact). This could be easily verified by observing the impinged droplet on a homogenous surface with contact angle of 17° or 107°. Droplet base shrinkage due to evaporation has not been observed in both cases within 0.3 seconds after impact. Briones et al[23] demonstrated that during the early stages of micro-sized droplet evaporation, only the apex or droplet height reduces while the droplet diameter is constant. Base diameter shrinkage due to evaporation only happens after 0.5 seconds from impact.

A summary of the different phases after droplet impingement and their characteristics are illustrated in figure 6. The kinematic phase starts from the time of impact to t* = 0.1 ~ 0.3, during which the droplet shows a circular footprint with the absence of translation. The translating phase ensues until the time when one of the droplet contact line is pinned at the wettability contrast edge at t* = 1.6 ~ 20, depending on the offset distance and contact angle difference. The subsequent conforming phase is approximately two to three orders slower than the translating phase and it was completed at t* = 4000 when the contact lines of the droplet coincide with the wettability pattern (see figure 5).

The conforming phase has been demonstrated in figure 5 to be independent of the initial offset distance. The final displacement of the edges of droplets impinging at various offsets both coincided with the edges of the hydrophilic line. The residual offset from the translating phase was also eliminated. The droplet impinged at F*= 0.36 and 0.86 initial offsets appeared to be no different from the droplet impinged at F* = 0.05 after t* = 4000. This behavior is important for inkjet printing technology. The final printed feature is independent on the initial impact location as long as part of the droplet touches the hydrophilic line. Despite an offset or inaccuracy of the initial impact location, the droplet will migrate towards the line and conform to the required line width. This effect enhances the robustness of the inkjet printing process, resulting in more accurate final droplet deposition and good control over the printed line width.

[pic]

Figure 5 (a) Definition of upper and lower contact lines with respect to center of hydrophilic line. (b) Dimensionless displacement of upper and lower edges of droplets impinging at various initial offset distances. Both the displacement and offset are non-dimensionalized with droplet initial diameter, D (81µm). Dimensionless time t*= tU/D with impact velocity U = 1.2ms-1. Hydrophilic line width W = 53µm (indicated by grey dashed lines). Contact angles of hydrophilic line and surrounding surface are 17o and 107o respectively. Error bars are not shown for clarity.

[pic]

Figure 6 Displacement of droplet center, upper and lower contact lines to illustrate different spreading processes upon impingement on surface with wettability pattern. Horizontal dashed grey line represents the position of the wettability contrast.

3.3. Effect of line width

The effect of hydrophilic line width on the droplet spreading behavior was investigated. Figure 7 shows the bottom view image sequence of droplets impinging on hydrophilic lines with various line widths (see supplementary video). The dimensionless line width W* is defined as W/D. Hence, the line widths W of 27µm, 37µm and 53µm correspond to W* of 0.33, 0.46 and 0.65 respectively. The contact angles for the lines and the surrounding surface are 17° and 107° respectively. All droplets were impinged on the center of the hydrophilic lines with negligible offset (smaller than 5µm) for ease of comparison.

[pic]

Figure 7 Bottom view image sequence of droplets (diameter D = 81µm) impinging on hydrophilic lines with different widths. Contact angles of hydrophilic line and surrounding surface are 17o and 107o respectively. Dimensionless line width W* is the line width divided by droplet initial diameter D.

The droplet conformed to the hydrophilic line with W* = 0.65 as both the upper and lower edges coincided with the wettability boundary at approximately t* = 4000 after impact. However as the line width was reduced, the droplets were not confined completely within the hydrophilic region even after 0.5 second (t*=7400). Although the droplet impinged on the line with W* = 0.46 showed significant spreading and elongation along the line, the liquid transport on the thinner hydrophilic region was insufficient to fully divert the fluid away from the central blob. This is clearly shown in figure 8 where the upper and lower edges of the droplet for W* = 0.46 were not reduced significantly over time as compared to W* = 0.65. The droplet impinged on the line with W* = 0.33 was only elongated slightly and most of the liquid stayed on the hydrophobic surface. The small hydrophilic area did not provide sufficient capillary force to distort the shape of the droplet and divert the fluid from the central blob.

Figure 9 shows the translation and conforming process of the droplet impinged at the same offset distance (F* = 0.54) from the hydrophilic line with different widths. Good conformability was achieved for W* = 0.65. However, the smaller line width (with W* of 0.33 and 0.46) induced slower and weaker migration towards the hydrophilic line due to the limited contact area between the droplet and the hydrophilic zone. For W* 1, both droplets started moving towards the hydrophilic lines. The velocity of the upper (receding) and lower (advancing) contact line were obtained by determining the derivative of the displacement curves after fitting them with smoothing splines. The contact line velocities at t* 5, the receding contact line velocities increased steadily, and at t*=10, surpassed the advancing contact line velocities. Both advancing and receding velocities gradually reduced with time until the contact lines were pinned. The advancing and receding contact line velocities for 17o line were higher than the corresponding contact line velocities for the 40o line throughout the whole process.

Theoretical analysis of droplet migration velocity on a wettability gradient has been conducted by Subramanium et al [28]. The droplet migration velocity on a surface with continuous wettability gradient was known to increase with increasing contact angle difference between the opposite edges of the droplet [28, 36]. However, this theoretical analysis is only valid if the droplet maintains the shape of a spherical cap throughout the translation phase. The non-uniformity of contact line velocities caused the distortion of the droplet shape and, thus significant deviation from the spherical cap. Hence, the theoretical prediction of the droplet translation velocities cannot be performed based on the existing simplified model. Numerical simulations would be conducted in our future studies to model the droplet translation velocities and its morphological changes during the translating phase.

[pic]

Figure 11 (a) Displacements of upper and lower contact lines and (b) contact line velocities of droplets impinged on hydrophilic lines with contact angle of 17o and 40o. Contact angle of surrounding hydrophobic surface = 107o. Initial impact offset F* = 0.84. Hydrophilic line width W* = 0.65 (indicated by grey dashed lines). All contact line displacements, impact offset and line width are non-dimensionalized with droplet initial diameter D (81µm)

The lower (advancing) contact line of the droplet in both cases was pinned at the lower edge of the hydrophilic-hydrophobic barrier at t*=20 (see figure 11). However, the droplet sideway spreading on the 40o line was insufficient to transport the remaining liquid into the hydrophilic lines, resulting in an asymmetrical blob (see figure 12b). On the contrary, the droplet on the surface with 17o contact angle spread continuously along the line and eventually dragged all liquid into the hydrophilic pattern (see figure 12a).

[pic]

Figure 12 Bottom view of droplet impinged on hydrophilic line with contact angle of (a) 17o and (b) 40o after 0.3 seconds. Initial impact offset is 68µm. Contact angle of surrounding hydrophobic surface = 107o. Hydrophilic line width W = 53µm. Droplet initial diameter D = 81µm. Black dashed lines serve as visual guide for the wettability contrast edges.

Brinkmann and Liposky [13] analytically derived the criteria for the transition between the two regimes of droplet spreading on a hydrophilic strip over a surrounding hydrophobic surface. The droplet only stays within the stripe when the contact angle of the stripe is less than ~38o [13]. This criterion is applicable when the contact angle of the hydrophobic surface surrounding the strip is larger than 90o. When the contact angle of the hydrophilic stripe is larger than 38o, a droplet with diameter larger than the width of the stripe would stay as a central blob with limited spreading along the line. Our experimental results agree with the theoretical prediction of the transition between the two types of droplet morphologies. At low contact angle (17o), the droplet conform to the hydrophilic line while at contact angle higher than 38o (40o and 77o), a central blob is formed (see figure 12). A high contact angle contrast between the hydrophilic pattern and its surrounding is thus a critical parameter for ensuring good conformability of a droplet to the desired pattern.

4. Conclusion

Images of a micro-sized water droplet impinging on a hydrophilic line with a surrounding hydrophobic surface have been captured with a high speed camera. The droplets were impinged at various offset distances from the center of the hydrophilic line. The movement and spreading of the droplet upon impingement can be divided into three phases, namely kinematic phase (t*~ 0.1-0.3), translating phase (t* ~ 1-20) and conforming phase (t* ~ 4000). During the initial kinematic phase, the droplet retains a circular footprint and unaffected by the wettability of the surface. During the translating phase, the droplet moves towards the centre of the hydrophilic line if part of the droplet touches it. However, the center of the droplet is not aligned exactly to the center of the hydrophilic line as one of the droplet contact line is pinned on the edge with sharp contact angle contrast. In the subsequent conforming phase, the droplet spreads along the hydrophilic line and conforms to the edges of the hydrophilic line. The conforming phase is typically much slower than the translating phase.

Effects of impact offset distance, line width and contact angle contrast on the droplet movement and final droplet shape were investigated. The contact angle of the hydrophobic surface was kept constant at 107o throughout the study. Droplets impinged on the hydrophilic line with W*= 0.65 and contact angle of 17o at various offset distances moved towards line center. The final shapes of these droplets conformed to the line width regardless of the initial offset distance after 0.3s. However, when W* was reduced to 0.33 or 0.46, the droplet failed to conform completely to the line width. A residual blob of liquid remained at the initial impact location due to the lack of spreading on the hydrophilic line with a smaller width. When the contact angle of the hydrophilic line was increased to 40o and 77o, the droplet did not conform completely to the line width, even though W* was 0.65. The translating phase was also slower when the contact angle of the hydrophilic line was increased.

Acknowledgement

The authors gratefully acknowledge the Agency for Science, Technology and Research (A*STAR) Singapore for its financial support (grant no. 102 170 0140).

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