Huiling Li

Super-hydrophobic film retards frost formation

Hydrophobic and super-hydrophobic isotactic polypropylene (i-PP) films were prepared and the process of frost formation was investigated. It was found that frost formation is greatly retarded on the super-hydrophobic i-PP surface. During the frost formation, the wettability on the super-hydrophobic film oscillates, while the wettability on the hydrophobic film increases steadily. This special oscillation is due to the micro- and nanometre structures, which leads to the delay of the solidification of liquid water at the three-phase line (TPL) region. This result gives a new explanation for the retardation of frost formation on the super-hydrophobic film surface and will be of great significance for the effective design of anti-frost materials.

Super-hydrophobic surfaces to condensed micro-droplets at temperatures below the freezing point retard ice/frost formation

Retarding and preventing ice/frost formation has an increasing importance because of the significant energy and safety concerns nowadays. In this paper, super-hydrophobic surfaces with ZnO nanorod arrays were fabricated. These surfaces were super-hydrophobic not only to sessile macro-droplets at room temperature but also to condensed micro-droplets at temperatures below the freezing point. The effects of these ZnO surfaces towards ice/frost formation were investigated. The results show that the time of condensed droplets maintaining the liquid state (t) increases with the decrease of the growth time (tZnO) of ZnO nanorods which determines the surface wettability, clearly indicating the retardation of ice/frost formation. An explanation is proposed based on classic nucleation theory and the heat transfer between condensed droplets and super-hydrophobic surfaces. These results make clear that superhydrophobicity to condensed micro-droplets at temperatures below the freezing point is desirable for effectively retarding ice/frost formation. In addition, they are significant for understanding the effect of superhydrophobicity at surface temperatures lower than the equilibrium freezing point on retarding and preventing ice/frost formation and will be beneficial for the design of effective anti-ice/frost materials.

Investigating the Effects of Solid Surfaces on Ice Nucleation

Understanding the role played by solid surfaces in ice nucleation is a significant step toward designing anti-icing surfaces. However, the uncontrollable impurities in water and surface heterogeneities remain a great challenge for elucidating the effects of surfaces on ice nucleation. Via a designed process of evaporation, condensation, and subsequent ice formation in a closed cell, we investigate the ice nucleation of ensembles of condensed water microdroplets on flat, solid surfaces with completely different wettabilities. The water microdroplets formed on flat, solid surfaces by an evaporation and condensation process exclude the uncontrollable impurities in water, and the effects of surface heterogeneities can be minimized through studying the freezing of ensembles of separate and independent water microdroplets. It is found that the normalized surface ice nucleation rate on a hydrophilic surface is about 1 order of magnitude lower than that on a hydrophobic surface. This is ascribed to the difference in the viscosity of interfacial water and the surface roughness.

Hierarchical Porous Surface for Efficiently Controlling Microdroplets' Self‐Removal

Removing condensed water from a cold surface can improve the surface heat-exchange coeffi cient by at least one order of magnitude, compared with the case of condensed water staying on the surface, [ 1–3 ] which is very important for all cooling systems due to increasing energy concerns nowadays. Because of their excellent water-repellent properties, superhydrophobic surfaces with low adhesion to water are promising candidates for effi cient removal of condensed water microdroplets. [ 4–7 ] It has been reported that coalescing microdroplets can self-remove from superhydrophobic surfaces when powered by the released surface energy, [ 8 , 9 ] which has aroused interest. [ 10–13 ] Though the self-removal of condensed microdroplets is energy saving, its effi ciency depends on the growth rate and the coalescing frequency of condensed droplets. Generally, faster growth and more frequent coalescence will lead to higher self-removal effi ciency of condensed droplets. However, though hydrophilic surfaces are favorable for the quick nucleation and growth of condensed water, they are unfavorable for the self-removal of condensed microdroplets. [ 1 , 14 , 15 ] While the high nucleation energy barrier on hydrophobic or superhydrophobic surfaces slows down the growth of condensed droplets, [ 16 ] the uncontrollable distance between condensed microdroplets decreases the coalescing frequency, resulting in a low self-removal effi ciency. Thus, how to control the condensation and coalescence processes and accelerate the self-removal of condensed microdroplets remains a great challenge for developments of new antifogging, anti-icing materials and heat exchangers. Herein, inspired by the peculiar hydrophilic/hydrophobic structures on a beetle’s elytra, [ 14 , 17 , 18 ] a micro-/nanoporous superhydrophobic surface modifi ed with hydrophilic polymer was designed for effi ciently controlling microdroplet self-removal. The hierarchical micro-/nanoporous structure was fabricated on aluminum by integrating microcontact printing [ 19 , 20 ] and chemical bath deposition. [ 11 , 21 ] After modifying the bottom of

Hierarchically structured porous aluminum surfaces for high-efficient removal of condensed water

Hierarchically structured porous aluminum surfaces for the high efficient removal of condensed water microdroplets are prepared via simply immersing aluminum sheets in hot water followed by modification with a low surface energy chemical. A correlation between the work of adhesion with the self-removal of condensed water microdroplets is established.

Superhydrophobic surface at low surface temperature

Superhydrophobic surfaces have aroused great attention for promising applications, e.g., anti-ice/frost. However, most surfaces which are superhydrophobic at room temperature lose their superhydrophobicity at low surface temperatures. Here, surfaces with different area fractions of the solid surface in contact with the liquid (f1) were designed. It is found that surfaces with f1 equal to or smaller than 0.068 maintain the superhydrophobicity when the surface temperature approaches the dew-point. These results are crucial to understand the correlation between the surface morphology and the superhydrophobicity around the dew-point, and design effective surfaces with desired wettability.

Concentrated-Mass Cantilever Enhances Multiple Harmonics In Tapping-Mode Atomic Force Microscopy

The natural frequencies of a cantilever probe can be tuned with an attached concentrated mass to coincide with the higher harmonics generated in a tapping-mode atomic force microscopy by the nonlinear tip-sample interaction force. We provide a comprehensive map to guide the choice of the mass and the position of the attached particle in order to significantly enhance the higher harmonic signals containing information on the material properties. The first three eigenmodes can be simultaneously excited with only one carefully positioned particle of specific mass to enhance multiple harmonics. Accessing the interaction force qualitatively based on the high-sensitive harmonic signals combines the real-time material characterization with the imaging capability

Investigating the Adhesion of Water Droplets at Low Temperatures

Adhesion of droplets to solid surfaces at low temperatures is crucial for antifogging and antifreezing, etc. So far, most reports on adhesion measurements have been carried out in air-liquid-solid systems, but it remains difficult to precisely investigate the adhesion at low temperatures due to the uncontrollable condensation. On the basis of the liquid-liquid-solid system, a new method to measure the adhesion of water droplets at low temperatures was developed and employed. Moreover, the reported method could be viable in other liquid-liquid-solid systems with wider temperature window; thus, it will find applications in broad fields such as crude oil recovery, ore-dressing, and transfer printing.