Superhydrophobic Condensation Enhanced by Conical Hierarchical

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Superhydrophobic Condensation Enhanced by Conical Hierarchical Structures Steve Q. Cai, and Avijit Bhunia J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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The Journal of Physical Chemistry

Superhydrophobic Condensation Enhanced by Conical Hierarchical Structures Steve Q. Cai1 and Avijit Bhunia Teledyne Scientific, Thousand Oaks, CA 91360

Abstract: Droplets on micro/nano hierarchical structures exhibit extraordinary hydrophobic properties, such as large contact angles, low dynamic hysteresis and high mobility. Vapor condensation on such the surface may potentially achieve rapid condensate removal and surface cleaning, therefore, significantly enhancing the heat transfer coefficient. This article reports novel conical hierarchical structures (CHS) and their mechanisms for enhancing vapor/moisture condensation. Through a normal optical tomography, visualization images show, in spite of ultrahigh structural depth, condensate droplets are able to rapidly precipitate under capillary forces and maintain at the stable Cassie state in a dynamic condensation environment. Within CHS, the major condensation is advanced at where the incident moisture is cooled to its dew point. Compared with the traditional dropwise condensation, CHS reduces the mass transport resistance when moisture must diffuse through the entire non-condensable gas (NCG) layer. The stable Cassie state in dynamic condensation environment, as well as the CHS structural tolerance to NCG, enables high efficient vapor/moisture condensation in complicated industrial environment.

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the corresponding author, [email protected] 1

ACS Paragon Plus Environment


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NOMENCLATURE

function of moisture concentration/density gradient

h

height of roughness structure, m

hfg

vaporization latent heat, kJ/kg

m

weight, kg

heat transfer flux, W/cm2

r

ratio between actual and apparent surface

T

temperature, oC

x

structural depth, m

Greek symbols α

diffusivity, m2/s

θ

contact angle, degree

ρ

density, kg/m3

φ

area fraction in contact with liquid

Subscripts

a

atom

c

critical

dew

dew point

eff

effective

s

solid

0

initial

2


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INTRODUCTION Plants and animals living in highly humid environment, such as lotus and butterflies, evolved hierarchical roughness structures on their leaves or wings to accommodate moisture condensation and perform surface self-cleaning to remove attached condensate droplets1, 2. This unique superhydrophobic property can be introduced on a solid surface to enhance the condensation efficiency3. However, in a dynamic system (e.g. the condenser of a turbine generator), non-condensable gas (NCG) mixed/dissolved in the vapor stream aggregates on the condensation surface and retards mass transport across the diffuse boundary layer4-7, significantly reducing the effectiveness for applying this technologies. By increasing the character length of the micro/nano structures, one can utilize the roughness structure to conduct heat and to fully or partially bypass the NCG barrier layer, and therefore enhances the condensation efficiency. Moreover, the high surface roughness significantly increases the total surface energy for a droplet to wet the entire structure. It reduces the critical contact angle, θc, making condensate droplets more stable in Cassie state8, 9 for surface selfcleaning10, 11. cosθ c = (φs − 1) /( r − φs )

(1)

Here, φs is the fraction of the solid in contact with the liquid and r is the ratio between actual and apparent surface. Droplets in the Cassie state possess high mobility12-14. It can perform a fast condensate removal to expose the cold surface and enhance condensation efficiency15, 16. The traditional Cassie or the Wenzel model is based on macro scale droplets on roughness surfaces. For droplets initially starting in the deep micro/nano structures, they are much smaller and equivalent to the size of structural gaps. These droplets are only in contact with the less rough surfaces/sidewalls of the structural units. Their hydrophobic behaviors, mobility, and

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condensation locations in the 3D structure need to be identified. In addition, given a unique conical design of the micro/nanoscale structures, it is also very interesting to see whether and how the growing small condensate droplets can utilize capillary force to automatically elevate/precipitate from the high or ultra-high surface roughness, turning into the Cassie state. These interesting microscale condensation mechanisms, as well as its comprehensive impact on condensation heat transfer enhancement, motivate this research. In this article, novel conical micro-to-nano hierarchical surface structures with structural height 20 ∼ 30 times higher than those in nature (e.g. lotus leaf) are developed. Through a normal optical tomography, condensation within these structures is imaged to exhibit dynamic behaviors of microscale droplets and their effectiveness on mitigating the impact of NCG.

Experimental Samples Although given extended structural height, with lack of external perturbations, condensation in one-tier structures tends to be flooded and show in the Wenzel state17, which may significantly reduce mobility of large droplets and exposure of the cold surface to vapor/ moisture18-21. A heavy flooding may also push NCG out of the structure, veiling the roughness and disabling the structural conduction for condensation. To ensure condensate droplets stable in the Cassie state, additional two approaches are implemented. First, rather than using single-tier structures, we mimic biologically superhydrophobic surfaces of plants or animals to develop micro-to-nano hierarchical surface structures. Second, conical pillars/structural units are designed to reduce the fraction of the solid in contact with the liquid φs and further shift the hydrophobic state to the Cassie mode.

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As shown in Fig.1, CHS is composed of perpendicular, staggered silicon pillars 12µm in diameter and 150µm in height (20 ∼ 30 times higher than the micro structures on the lotus leaf) with pillar gaps of 6µm. The conical section starts from the top tip and extends to the middle of the pillars (∼ 50% depth), with an inclined angle of 3-5o. In additional to stabilizing the large ontop droplets in the Cassie state, within the structures, the conical pillars create a surface tension difference between the top and bottom sides of droplets. This capillary mechanism therefore amplifies external perturbations and direct condensate droplet motion upward to precipitate from the structures22, 23. Two different nano structures are created on the conical micro pillars. As shown in Fig.1a, silicon spikes with a diameter range from 200 to 500nm are etched on the tapered sidewalls of CHS 1. The silicon spikes in parallel with the pillar axis, which mimics nano structures appearing on the legs of the water striders24. In contrast, carbon nanotubes (CNTs) 100nm in diameter and 1.0∼ 2.0µm in height, are synthesized on CHS 2 (shown in Fig.1b). Limited by the sputtering seeding process, fewer and shorter CNTs are found on the pillar sidewall, as shown in Fig.1b (the conical pillar tip is removed for the cross-section view), particularly when approaching to the structural bottom. Compared with the flat tip of CHS 1, the uneven CNT growth creates sharper and furry tips of CHS 2. In order to minimize the surface energy, both CHSs are deposited by Dupont Teflon AF amorphous fluoropolymer with Young’s contact angle ∼ 120o. After the hydrophobic coating, the static apparent contact angle is measured over 175o on both the CHS structures with high droplet mobility and dynamic hysteresis less than 5o.

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FIG. 1 CHS samples made for superhydrophobic condensation studies: a) ∼ 6µm gaps between the staggered pillars in the CHS 1. Nano spikes are in parallel with the pillars (right), b) the conical pillar tips with CNTs in the CHS 2. Less and shorter CNTs are grown on the sidewall (right).

Experimental Setup A normal optical tomographic system25, composed of a Sensi-Cam and Nikon microscope, is developed to visualize microscale condensate droplets within CHS. As shown in Fig.2a, by adjusting illumination intensity and the microscope focal plane, the condensate droplets at different structural depths x (e.g. at the structural tips, 0.25 depth and the bottom) can be imaged. This approach can visualize condensate droplets with diameters less than a few micrometers and located between structural units. By counting the focused droplets, condensation spatial distribution can be quantified versus the structural depth. The CHS samples are fabricated in size of 2 cm × 2 cm, containing CHS 1 or 2 on the center area of 1.2 cm × 1.2 cm. The sample is horizontally set under the microscope and over a copper cold plate, as shown in Fig.2b. Thermal paste is applied to reduce thermal resistance between the sample and the cold plate. Chilled water at 3oC flows through the cold plate to condense

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moisture from air. A humidifier is employed to increase a relative humidity to 85% at the lab temperature of 21oC, setting dew point at 18oC.

Fig.2 Visualization of dropwise condensation on/in the roughness structures: a) a schematic of the normal optical tomography, b) visualization system, c) a CHS sample placed between the copper cold plate and the microscope objective. On-top large condensate droplets and reflective light from unfocused droplets are two major interferences for this observation. To balance relation between minimization of reflection light from unfocused droplets and sufficient illumination, a 40x ELWD HMC Plan Fluor objective with working distance 2.7 to 3.7mm is used (Fig.2c). At the steady state condition, visualization is selected on a “cleaned” surface area where a large on-top droplet rolls over and sweeps all other on-top droplets.

Results 7



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The flat tips of the micro pillars of CHS 1 increase the fraction of the solid in contact with the liquid φs. It reduces motion sensitivity from the external perturbations and extends the stagnating time of the on-top droplets. After experiencing multiple coalescences, droplets can grow to exceed 50µm in diameter, as shown in Fig.3a. As moisture diffuses into CHS 1, condensate droplets with diameter less than the pillar gaps (∼6.0µm) are imaged. Figure 3b shows small droplets at 0.25 h (25% structural height from its tip). Appearance of these small condensate droplets verifies the structure is non-flooded or air-entrapped, which substantiates the Cassis state of those on-top large droplets shown in Fig. 3a. By further moving the focal plane down to 0.32h and 0.42h (Fig.3c&d), condensate droplets in size of a few micrometers can also be found between the silicon pillars. At these structural levels, droplet coalescence or growth increases their sizes. Under constrain of the surrounding structures, these droplets deform/distort. To reduce surface energy, droplets tend to migrate to a place where it has larger spaces/gaps to maintain their spherical shape. The instability is eventually broken by external perturbations. Under the capillary force, droplet motion is directed to the upper structure until precipitating from the structure. No droplets larger than the structural gap are observed, which indicates that, within CHS 1, small condensate droplets exhibit very good mobility. Slight deformation/ increase of surface energy leads to migration of the droplet. With reduction of moisture humidity, quantity of condensate droplets starts to reduce from the depth of 0.42h. In the depth of 0.54 and 0.65h (Fig.3e&f), only a fewer droplets are imaged. Observation of CHS 1 is limited above 0.65h, due to the intensified reflection light from the upper levels.

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FIG. 3 Dropwise condensation on/in the CHS 1: a) Cassie-state droplets on top of the CHS, b) 0.25 h (CHS depth), c) 0.32 h, d) 0.42 h, e) 0.54 h, f) 0.65 h.

Due to the high emissivity of CNTs26, 27, high contrast images are obtained on CHS 2, as shown in Fig.4a-f. Reflection light from the upper condensate droplets is dramatically absorbed by CNTs. As a result, visualization can be extended to the bottom of the structure. Differing with CHS 1, the sharp structural tips and fine nanoscale roughness reduce the fraction of the solid in contact with the liquid, φs, enabling highly mobile droplets to frequently clean the top roughness surface. As a result, the precipitated droplets have less time stagnating

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over the structure, possessing smaller diameters hdew 1, temperature drop is dictated by the Fourier’s conduction law. = + ( − ℎ )/

(5)

Where, keff is the thermal conductivity of the CHS layer. Since the thermal conductivity of air is 2-3 orders of magnitude lower than silicon, heat conduction through air is negligible. keff approximately equals to the thermal conductivity of CHS material.

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FIG. 7 CHS reduces thermal resistances/temperature drops at different condensation heat fluxes.

Defined by Eq.(4) and (5), moisture temperature versus the structural depth can be illustrated in Fig. 7. For this case, fixed thermal boundary conditions are defined by ambient (point 0) and the substrate temperatures (point h). Moisture condenses when arriving at the first dew point, followed by a series of piecewise function curves with gradually reduced dew temperatures (Tdew 2,

Tdew 3, …) and humidity until reaching the bottom substrate. Condensation occurs in all the rest

structural depth after the first dew point appears. The co-existence of conduction (the straight lines after the curved diffusion lines) and diffusion heat transfer reduces the total heat transfer resistance for moisture/vapor penetrating the NCG layer. Given moisture and substrate temperatures, condensation heat flux becomes self-adjustable. As q1 and q2 curves (e.g. humidity of 85% and 75%) shown in Fig. 7, less saturated moisture with less vaporization latent heat condenses later/deeper in the structure. It balances the delta T fractions between the diffusion and conduction to meet a constant total. In comparison, on a flat surface where diffusion is the 15



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major mechanism, a much larger delta T is required for higher heat and mass transfer, referring to q1 and q2 curves with extended dash sections. A quantitative evaluation of CHS impact can be conducted when giving a constant condensation heat flux at 1W/cm2. For 85% and 75% humidity moisture, it requires 7.5 and 8.4oC for mass transport across a 150µm thick NCG layer on a flat condensation surface. Air conduction contributes about one tenth of the total heat transfer in the form of sensible heat, but zero on mass transfer of latent heat. In comparison, assuming average condensation depth is at 0.3 and 0.5h for humidity of 85% and 75% within CHS, moisture only requires 2.2oC and 4.2oC, respectively, for effective heat and mass transfer. CHS utilizes structural conduction to bypass diffusion heat transfer and to reduce the resistance of mass transport. Thus, the material thermal conductivity is also a key parameter. A critical thermal conductivity exists at each humidity level at which the delta T for both the conduction and diffusion heat transfers is equivalent (between point A and h). Only if the CHS thermal conductivity higher than the critical value, the enhanced condensation heat transfer coefficient will be achieved.

Conclusion In this article, we developed ultra-high conical hierarchical surface structures (CHSs) for boosting superhydrophobic dropwise condensation stable in the Cassie state. Experimental visualization through the normal optical tomographic system indicates that, 1. CHS with extended structural height, hierarchical structure, conical pillars, and low surface energy coating provided the capillary mechanism for directional droplet draining

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and achieved rapid precipitation. Large spherical droplets sitting over CHS can be stabilized at the Cassie state in dynamic condensation environments. 2. With achieving the steady precipitation of small condensate droplets, CHSs are able to reduce moisture diffusion depth and advanced condensation at the upper structures. As a result of this contribution, temperature difference for achieving effective condensation is reduce, which practically enables higher efficient heat transfer with the presence of NCG in the dynamic condensation systems. 3. Given a constant temperature difference between vapor/moisture and the substrate, both the vapor/moisture humidity and the material thermal conductivity of CHS affect the condensation heat flux.

Acknowledgement Authors appreciate Teledyne Scientific Company for sponsoring this research. Authors also appreciate Prof. Zhifen Ren and Dr. Hengzhi Wang for their assistance for development nanostructures on the test samples, and Dr. Chialun Tsai for silicon MEMS fabrication.

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Superhydrophobic Condensation Enhanced by Conical Hierarchical

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