A Study of Drop-Microstructured Surface Interactions during Dropwise Condensation with Quartz Crystal Microbalance

Enhanced wettability, known as superhydrophobicity or superhydrophilicity has drawn extensive attention in the past for wide range potential applications such as superhydrophobic surfaces for self-cleaning, anti-icing, dropwise condensation, and drag reduction. This research focuses on the investigation of the frequency responses of quartz crystal microbalance (QCM) devices coated with micropillars to the different wetting states of drops. A theoretical model was developed to correlate the resonant frequency shifts of QCMs with the penetrated (Wenzel state) and suspended (Cassie state) states based on the Euler-Bernoulli beam theory. In the experimental validation of the theory, Poly(methyl methacrylate) (PMMA) micropillars were fabricated on the QCMs using nanoimprint lithography (NIL) method and the different wetting states were generated by plasma treatment and chemical coating. The frequency shifts of the QCM device were measured by a network analyzer. A good agreement between experimental measurements and theoretical predictions was obtained. It was found that the micropillars operating in the penetrated state results in one order of magnitude higher frequency shift of QCM than the micropillars in suspended state. There exists a highly nonlinear vibrating behavior of micropillars with different heights in both penetrated and suspended states. The QCM based technology is a valuable tool for studying the wettability of different superhydrophobic or superhydrophilic surfaces.

This work reports a novel Quartz Crystal Microbalance (QCM) based method to analyze the droplet-micropillar surface interaction quantitatively during dropwise condensation. A combined nanoimprinting lithography and chemical surface treatment approach was utilized to directly fabricate the micropillar based superhydrophobic surface on the QCM substrate. The normalized frequency shift of the QCM device and the microscopic observation of the corresponding nucleation, drop growth, and drop coalescence processes clearly demonstrate the different characteristics of these condensation states. In addition, a synchrosqueezed wavelet spectrum based multi-resolution technique was utilized to analyze the resonant signal from the QCM sensor in both time and frequency domains simultaneously. An integrated discrete system modeling along with a hybrid signal and image processing approach was adopted to identify the response of the micropillars under different stages of dropwise condensation (DWC). The outcome of this signal processing research leads to a fundamental understanding of DWC spanning multiple time and length scales. The proposed study will also contribute to an in-depth understanding of different hydrophobic surfaces and DWC through this advanced signal processing and surface treatment. The developed QCM system provides a valuable tool for the dynamic characterization of different condensation processes.

Water droplets on rugged hydrophobic surfaces typically exhibit one of the following two states: (i) the Wenzel state [Wenzel RN (1936) Ind Eng Chem 28:988-994] in which water droplets are in full contact with the rugged surface (referred as the wetted contact) or (ii) the Cassie state [Cassie, ABD, Baxter S (1944) Trans Faraday Soc 40:546-551] in which water droplets are in contact with peaks of the rugged surface as well as the "air pockets" trapped between surface grooves (the composite contact). Here, we show large-scale molecular dynamics simulation of transition between Wenzel state and Cassie state of water droplets on a periodic nanopillared hydrophobic surface. Physical conditions that can strongly affect the transition include the height of nanopillars, the spacing between pillars, the intrinsic contact angle, and the impinging velocity of water nanodroplet ("raining" simulation). There exists a critical pillar height beyond which water droplets on the pillared surface can be either in the Wenzel state or in the Cassie state, depending on their initial location. The free-energy barrier separating the Wenzel and Cassie state was computed on the basis of a statistical-mechanics method and kinetic raining simulation. The barrier ranges from a few tenths of k(B)T(0) (where k(B) is the Boltzmann constant, and T(0) is the ambient temperature) for a rugged surface at the critical pillar height to approximately 8 k(B)T(0) for the surface with pillar height greater than the length scale of water droplets. For a highly rugged surface, the barrier from the Wenzel-to-Cassie state is much higher than from Cassie-to-Wenzel state. Hence, once a droplet is trapped deeply inside the grooves, it would be much harder to relocate on top of high pillars.

Dropwise condensation on single-micro-scale roughness hydrophobic surfaces

Improved humid air condensation heat transfer through promoting condensate drainage on vertically stripe patterned bi-philic surfaces

Study on the wetting transition of a liquid droplet sitting on a square-array cosine wave-like patterned surface

Liquids on a solid surface patterned with microstructures can exhibit the Cassie-Baxter (Cassie) state and the wetted Wenzel state. The transitions between the two states and the effects of surface topography, surface chemistry as well as the geometry of the microstructures on the transitions have been extensively studied in earlier work. However, most of these work focused on the study of the free energy landscape and the energy barriers. In the current work, we consider the transitions in the presence of a shear flow. We compute the minimum action path between the Wenzel and Cassie states using the minimum action method [W. E, W. Ren, and E. Vanden-Eijnden, Commun. Pure Appl. Math. 57, 637 (2004)]. Numerical results are obtained for transitions on a surface patterned with straight pillars. It is found that the shear flow facilitates the transition from the Wenzel state to the Cassie state, while it inhibits the transition backwards. The Wenzel state becomes unstable when the shear rate reaches a certain critical value. Two different scenarios for the Wenzel-Cassie transition are observed. At low shear rate, the transition happens via nucleation of the vapor phase at the bottom of the groove followed by its growth. At high shear rate, in contrary, the nucleation of the vapor phase occurs at the top corner of a pillar. The vapor phase grows in the direction of the flow, and the system goes through an intermediate metastable state before reaching the Cassie state.

Explosive boiling of argon nanofilms in the Wenzel or Cassie state on high-temperature nanopillar-arrayed surfaces

A liquid droplet on a micropatterned substrate equalizes into either the Cassie-Baxter (also called Cassie for short) or the Wenzel state. This paper investigates the wetting phenomena on ideal micropatterned surfaces consisting of straight micropillars at different pillar dimensions and spacings (the word "ideal" refers to being chemically homogeneous and free of submicron-scale roughness all over the micropatterned surface). Two modeling approaches are used: (1) a thermodynamic approach analyzing the Gibbs energy of the droplet-solid-gas system and (2) a computational fluid dynamics (CFD) approach studying the three-dimensional dynamic wetting process to validate the results of the first approach. The thermodynamic approach incorporates three creative submodels proposed in this paper: (i) a sagging model explaining the pillar edge effect, (ii) a touchdown model transitioning the droplet's partial penetrating condition toward its full penetrating condition, i.e., the Wenzel state, and (iii) a liquid-volume model dynamically computing the liquid volume between the pillar valleys while in the partial penetrating condition or in the Wenzel state. The results of the thermodynamic approach reveal (1) a small energy barrier between the Cassie and Wenzel states, (2) no partial penetration and sagging of the liquid in the Cassie state on the ideal straight micropillared surface, and (3) that the apparent contact angle in the most stable Wenzel state can be 5° or more lower than the prediction of the Wenzel equation when the pillar height is equal or greater than 75 μm. To the best of our knowledge, this paper presents the theoretical explanation of this Wenzel deviation on micropatterned surfaces for the first time in the literature. Utilizing the state-of-the-art continuum model developed by the authors in previous studies, the CFD approach investigates the same wetting conditions and confirms the same findings.

Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry, with applications ranging from condensation heat transfer to water harvesting to the prevention of icing and frosting. The mobility of a liquid droplet on a rough solid surface has long been associated with its wetting state. When liquid drops are sitting on the top of the solid textures and air is trapped underneath, they are in the Cassie state. When the drops impregnate the solid textures, they are in the Wenzel state. While the Cassie state has long been associated with high droplet mobility and the Wenzel state with droplet pinning, our work challenges this existing convention by showing that both Cassie and Wenzel state droplets can be highly mobile on nanotexture-enabled slippery rough surfaces. Our surfaces were developed by engineering hierachical nano- and microscale textures and infusing liquid lubricant into the nanotextures alone to create a highly slippery rough surface. We have shown that droplet mobility can be maintained even after the Cassie-to-Wenzel transition. Moreover, the discovery of the slippery Wenzel state allows us to assess the fundamental limits of the classical and recent Wenzel models at the highest experimental precision to date, which could not be achieved by any other conventional rough surface. Our results show that the classical Wenzel eq (1936) cannot predict the wetting behaviors of highly wetting liquids in the Wenzel state.

We perform large-scale molecular dynamics simulations to measure the contact-angle hysteresis for a nanodroplet of water placed on a nanopillared surface. The water droplet can be in either the Cassie state (droplet being on top of the nanopillared surface) or the Wenzel state (droplet being in contact with the bottom of nanopillar grooves). To measure the contact-angle hysteresis in a quantitative fashion, the molecular dynamics simulation is designed such that the number of water molecules in the droplets can be systematically varied, but the number of base nanopillars that are in direct contact with the droplets is fixed. We find that the contact-angle hysteresis for the droplet in the Cassie state is weaker than that in the Wenzel state. This conclusion is consistent with the experimental observation. We also test a different definition of the contact-angle hysteresis, which can be extended to estimate hysteresis between the Cassie and Wenzel state. The idea is motivated from the appearance of the hysteresis loop typically seen in computer simulation of the first-order phase transition, which stems from the metastability of a system in different thermodynamic states. Since the initial shape of the droplet can be controlled arbitrarily in the computer simulation, the number of base nanopillars that are in contact with the droplet can be controlled as well. We show that the measured contact-angle hysteresis according to the second definition is indeed very sensitive to the initial shape of the droplet. Nevertheless, the contact-angle hystereses measured based on the conventional and new definition seem converging in the large droplet limit.

Three-dimensional lattice Boltzmann simulations of microdroplets including contact angle hysteresis on topologically structured surfaces

A unique sensing device, which couples microscale pillars with quartz crystal microbalance (QCM) substrate to form a resonant system, is developed to achieve several orders of magnitude enhancement in sensitivity compared to conventional QCM sensors. In this research, Polymethyl Methacrylate (PMMA) micropillars are fabricated on a QCM substrate using nanoimprinting lithography. The effects of pillar geometry and physical properties, tuned by molecular weight (MW) of PMMA, on the resonant characteristics of QCM-micropillars device are systematically investigated. It is found that the resonant frequency shift increases with increasing MW. The coupled QCM-micropillars device displays nonlinear frequency response, which is opposite to the linear response of conventional QCM devices. In addition, a positive resonant frequency shift is captured near the resonant point of the coupled QCM-micropillars system. Humidity detection experiments show that compared to current nanoscale feature based QCM sensors, QCM-micropillars devices offer higher sensitivity and moderate response time. This research points to a novel way of improving sensitivity of acoustic wave sensors without the need for fabricating surface nanostructures.

Wetting on textured solids has gained much attention in the past decade due to increasing interest in artificial superhydrophobic surfaces. (Bahadur & Garimella, 2007; Boreyko & Chen, 2009; Forsberg, Nikolajeff, & Karlsson, 2011; Heikenfeld & Dhindsa, 2008) On textured surfaces, the wetting liquid can be in either the Cassie–Baxter state, which the liquid does not fill the surface texture; or the Wenzel state, which the liquid completely wets the surface and fills the recesses. For a hydrophobic micro-scale rough surface, the Cassie state is usually a more favorable state since it requires less energy. However, due to contact angle hysteresis, the Wenzel state can also be meta-stable. By controlling the roughness of the texture and initial droplet position, both Cassie and Wenzel states can be stable simultaneously. (Koishi, Yasuoka, Fujikawa, Ebisuzaki, & Xiao, 2009) However, with the proper energy input, the droplets can be induced to transition between states. While multiple methods have been developed to switch from Cassie to Wenzel states (Bormashenko, Pogreb, Whyman, & Erlich, 2007; Krupenkin et al., 2007; Kumari & Garimella, 2011; Ran, Ding, Liu, Deng, & Hou, 2008), it is much more difficult to switch from the Wenzel state to the Cassie state. Wenzel-Cassie transitions have been achieved by changing the surface structure to destabilize the Wenzel state (Krupenkin et al., 2007)(Ran et al., 2008) or by changing the ambient fluid. (Dhindsa et al., 2006)

Polyacrylonitrile (PAN) nanofiber deposited on quartz crystal microbalance (QCM) substrate as solvent vapor sensing has been successfully developed. The absorption and swelling behavior has been assumed to be responsible for sensing mechanism in vapor sensing. In this study, we aim to investigate the correlation between the swelling degree (polymer-solvent affinity) and the sensor response. The PAN nanofiber has been successfully deposited on QCM substrate with relatively homogenous nanofiber diameter about (260 ± 20) nm. The tests vapor solvent was included dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene glycol (EG), toluene, ethanol, and distilled water. The results indicated that the sensor response for various vapor solvent clearly influences by its polymer-solvent affinity. The highest sensor response was achieved with DMF vapor due to its highest affinity with PAN polymer. The swelling behavior of polymer can be a potential candidate for developing vapor sensors with a polymer as an active layer.