Experiments and analysis on self-motion behaviors of liquid droplets on gradient surfaces

Zhu, Xun; Wang, H.; Liu, Qiang; Ding, Yue; Gu, Yuheng

doi:10.1016/j.expthermflusci.2009.02.009

Cited by 55 publications

(36 citation statements)

References 22 publications

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“…The reduced velocity in the case of the tilted surface is due to the influence of gravity which opposes the droplet movement. Comparable velocities have been reported on other surfaces with chemical gradients, but the reported materials lack the ease of tunability demonstrated here. With tunable surfaces based on photosensitive materials light‐controlled wettability changes and liquid transport can be achieved, but the reported velocities are much lower than observed here .…”

supporting

confidence: 50%

Inscribing Wettability Gradients Onto Superhydrophobic Carbon Nanotube Surfaces

Babu

Varanakkottu

Eifert

et al. 2014

Adv Materials Inter

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Superhydrophobic surfaces with ultra-low contact angle hysteresis fi nd a wide range of applications in the fi elds of selfcleaning, anti-icing, anti-fogging, or microfl uidics. [ 1,2 ] Here, the feasibility of a novel type of superhydrophobic surface for guided droplet transport, which is a key issue for microfl uidic systems, is demonstrated. Among such applications, open-surface digital microfl uidics (OSDM) offers specifi c advantages in terms of remote controllability and programmability of droplet transport. [ 3,4 ] One of the major concerns while manipulating aqueous droplets at solid surfaces is the large contact angle hysteresis which arises due to chemical or physical heterogeneities of the surface. [ 5 ] Moreover, it is usually desirable to locally tune the surface properties in a well-controlled and reversible manner. Although several examples have been reported demonstrating the fabrication of chemically or physically modifi ed superhydrophobic surfaces, [ 6,7 ] tunability and fast control over the surface properties remains a challenge. Among the different materials used for the fabrication of superhydrophobic surfaces, carbon nanotubes (CNTs) are interesting because of their chemically well-defi ned nature and response in wettability to different stimuli (using plasma [ 8 ] or UV light). [ 9,10 ] Vertically aligned CNT structures gain additional signifi cance since transport properties of corresponding surfaces are strongly anisotropic, [ 11 ] which facilitates localized changes in temperature or electric fi eld. Several studies have been made in the past to tune the hydrophobic nature of aligned carbon nanotubes to produce superhydrophobic surfaces. In general, two methods have been developed to increase the hydrophobicity of CNT surfaces a) coating with a low surface energy material [ 12,13 ] and b) micropatterning of the CNT surface structure [ 14 ] so as to increase their surface roughness. Desirable characteristics of CNTs like, for example, thermal conductivity are compromised on coating with a foreign material. CNTs thus act as mere template structure for providing the desired nanoscale roughness. Micropatterning, on the other hand, is able to induce controlled surface roughness but relies on complex and expensive masking/lithography techniques involving multiple process steps. Superhydrophobic materials, including CNTs, exhibit two types of wetting behavior at high contact angles: a) very low contact angle hysteresis (like on lotus leaves) where the water droplet easily rolls off [ 10,15,16 ] or b) very high hysteresis (like on rose petals) where the water droplets pins to the surface. [ 17,18 ] While most of the surface modifi cation processes result in either one of these states, it is highly desirable to achieve a reversible switching between these two states on the same material. A reversible and localized switching between states with different contact angles and different levels of contact angle hysteresis has an enormous potential, for instance, for microfl uidic applications. By cre...

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supporting

confidence: 50%

Inscribing Wettability Gradients Onto Superhydrophobic Carbon Nanotube Surfaces

Babu

Varanakkottu

Eifert

et al. 2014

Adv Materials Inter

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“…4.5. Once the velocity peak has passed, the droplet motion is controlled by the balance of three forces, resulting in overall decline of measured velocities to a few mm/s, in agreement with other publications [76,77,81].…”

Section: Capillary Spreading Over Striped Patterns: Second and Third supporting

confidence: 90%

“…The surface tension increases in a specific direction, changing from hydrophobic (γ sv1 ) to hydrophilic (γ sv2 ). Owing to the different surface energy, the difference in dynamic contact angles (CAs) will induce movement in the direction of increasing γ sv [76][77][78][79]. Experimental motion studies reveal the velocities to depend linearly on the radius of the wetted area; moreover the droplet will move to the hydrophilic region only when the radius is above a certain critical value [80,81].…”

Section: Introductionmentioning

confidence: 99%

Directional wetting on patterned surfaces

Bliznyuk

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“…38 In the current design, however, this made an apparent difference between the advancing contact angle and the receding contact angle of the droplet, Fig. 10 The relative position of the speaker, device, and droplet and the direction of displacement: ͑a͒ side view; ͑b͒ top view.…”

Section: Droplet Movement On the Arrowed Surfacementioning

confidence: 91%

Fabrication and testing of surface ratchets primed with hydrophobic parylene and hexamethyldisilazane for transporting droplets

Chung

Hess

Yeh

et al. 2010

J. Micro/Nanolith. MEMS MOEMS

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We demonstrate a way to transport droplets by an arrowed micropillar array surface with hydrophobic parylene. The lightly hydrophilic parylene surface could be changed to hydrophobic one by treating with fluorine-based plasma ͑CF 4 or SF 6 ͒. The droplet on this hydrophobic parylene surface with arrowed ratchets could be transported by a speaker, and the average measured velocity was 29 mm/ s. Moreover, the authors compared driving performance of the parylene surface ratchets with the ones modified by the hexamethyldisilazane ͑HMDS͒ vapor. Both the results of using hydrophobic parylene and HMDS were better than the previous work.

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Experiments and analysis on self-motion behaviors of liquid droplets on gradient surfaces

Cited by 55 publications

References 22 publications

Inscribing Wettability Gradients Onto Superhydrophobic Carbon Nanotube Surfaces

Inscribing Wettability Gradients Onto Superhydrophobic Carbon Nanotube Surfaces

Directional wetting on patterned surfaces

Fabrication and testing of surface ratchets primed with hydrophobic parylene and hexamethyldisilazane for transporting droplets

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