Micro-nanostructured Surfaces Research Articles

Self-Driven Droplet Motions Below their Icing Points.

Liquid fluidity is a most key prerequisite for a broad range of technologies, from energy, fluid machineries, microfluidic devices, water, and oil transportation to bio-deliveries. While from thermodynamics, the liquid fluidity gradually diminishes as temperature decreases until completely solidified below icing points. Here, self-driven droplet motions are discovered and demonstrated occurring in icing environments and accelerating with both moving distances and droplet volumes. The self-driven motions, including self-depinning and continuous wriggling, require no surface pre-preparation or energy input but are triggered by the overpressure spontaneously established during icing and then continuously accelerated by capillary pulling of frosts. Such self-driven motions are generic to a broad class of liquid types, volumes, and numbers on various micro-nanostructured surfaces and can be facilely manipulated by introducing pressure gradients spontaneously or externally. The discovery and control of self-driven motions below icing points can greatly broaden liquid-related applications in icing environments.

Directional Drop Rebound on Adhesive-Gradient Micro-Nanostructured Surfaces Formed by a Femtosecond Laser.

The dynamic behavior of droplets hitting a solid surface has received extensive attention due to its broad application prospects. Additionally, controlling the rebound behavior of impacting droplets is an important research topic. Current methods for investigating this behavior focus on the construction of a differentiated wettability surface, which is characterized by contact angle measurements, or a differentiated topography surface, which is represented by geometric height. This information allows one to obtain the nonuniform kinetic energy distribution of rebounding droplets and to realize control of rebounding droplet behavior. In this paper, femtosecond laser processing is proposed for the fabrication of an anisotropic surface with differences in adhesion, which allows for the control of impacting droplet rebound behavior. The experimental results show that the micro-nanostructure of the surface affects its adhesion. By changing the micro-nanostructure of the solid surface, the difference in surface adhesion can be controlled, thereby realizing precise control of impacting droplet rebound behavior. This study demonstrates that the micro-nanostructured surface formed by a femtosecond laser can be used to control a droplet rebound direction and landing site, which is of great significance to the development of liquid transport, microfluidic devices, and other fields.

Modulation of MG-63 Osteogenic Response on Mechano-Bactericidal Micronanostructured Titanium Surfaces.

Despite recent advances in the development of orthopedic devices, implant-related failures that occur as a result of poor osseointegration and nosocomial infection are frequent. In this study, we developed a multiscale titanium (Ti) surface topography that promotes both osteogenic and mechano-bactericidal activity using a simple two-step fabrication approach. The response of MG-63 osteoblast-like cells and antibacterial activity toward Pseudomonas aeruginosa and Staphylococcus aureus bacteria was compared for two distinct micronanoarchitectures of differing surface roughness created by acid etching, using either hydrochloric acid (HCl) or sulfuric acid (H2SO4), followed by hydrothermal treatment, henceforth referred to as either MN-HCl or MN-H2SO4. The MN-HCl surfaces were characterized by an average surface microroughness (Sa) of 0.8 ± 0.1 μm covered by blade-like nanosheets of 10 ± 2.1 nm thickness, whereas the MN-H2SO4 surfaces exhibited a greater Sa value of 5.8 ± 0.6 μm, with a network of nanosheets of 20 ± 2.6 nm thickness. Both micronanostructured surfaces promoted enhanced MG-63 attachment and differentiation; however, cell proliferation was only significantly increased on MN-HCl surfaces. In addition, the MN-HCl surface exhibited increased levels of bactericidal activity, with only 0.6% of the P. aeruginosa cells and approximately 5% S. aureus cells remaining viable after 24 h when compared to control surfaces. Thus, we propose the modulation of surface roughness and architecture on the micro- and nanoscale to achieve efficient manipulation of osteogenic cell response combined with mechanical antibacterial activity. The outcomes of this study provide significant insight into the further development of advanced multifunctional orthopedic implant surfaces.

Comparative study of pool boiling heat transfer on different subtractive surfaces

• The subtractive micro-nano structured surfaces based on sandblasting were prepared. • The structured surface facilitates heat transfer depends on synergistic benefit of the influencing factors. • The characteristic parameters of the micro-nano surface have been quantitatively analyzed. • The visualization study has been carried out on the micro-nano structured surface. The preparation of stable surfaces to improve the heat transfer performance can make the equipment operate safely and bring significant economic benefits. Using sandblasting and ultrasonic wet etching techniques, copper surfaces were subtracted to prepare the microscale structures (MS) and nanoscale structures (NS) in this study. Then, these two special subtractive structures were composited together to form a hybrid micro-nano structured (HMS) surface. Nucleate pool boiling experiments were carried out on the prepared surfaces with different structures at atmospheric pressure. The effect of surface modification on key parameters such as critical heat flux (CHF) and heat transfer coefficient (HTC) were investigated. Through the quantitative analysis of roughness, surface area ratio and apparent contact angle, potential mechanisms affecting nucleate boiling were explored. Meanwhile, the surfaces were visualized and their enhancement mechanisms were analyzed from the perspective of bubble dynamics. The results show that compared with the smooth plain (SP) surface, the CHFs on NS surface, MS surface and HMS surface are improved by 22.8%, 48.2% and 33.8%, respectively. The maximum HTCs are improved by 42.9%, 83.7% and 77.6%, respectively. However, the boiling heat transfer performance of HMS surface is not improved compared with MS surface. According to the characterization results, this is the result of the combined effect of roughness, surface area ratio and wettability.

Boiling heat transfer on the micro-nano structured surface fabricated by mechanical sandblasting/alkali-assisted oxidation

In order to prepare stable and efficient surfaces to improve boiling heat transfer, the micro-nano structured surface (MNS) was prepared by mechanical sandblasting and alkali-assisted oxidation technique for saturated pool boiling experiment. For comparison purposes, experiments were also carried out on the plain copper surface (PCS), microstructured surface (MS) and nanostructured surface (NS). The results show that the heat transfer performance of different surfaces increases in the order PCS < NS < MS < MNS. Compared with the PCS, the critical heat flux (CHF) of MNS is increased by 55.6%, and the maximum heat transfer coefficient (HTC) is increased by 218%. In addition, the surface properties were characterized and the characterization results explained its excellent boiling performance. The formation of MNS enhances the surface roughness, effective surface area and wettability, thus promoting the supplement of liquid and delaying the occurrence of CHF. It can be seen from the visualization study results that the MNS has more nucleation sites, smaller bubble departure diameter and faster departure frequency. These promising results reveal the potential of micro-nano structured surfaces in various engineering applications.

How Different Are Fog Collection and Dew Water Harvesting on Surfaces with Different Wetting Behaviors?

As the clean water shortage becomes a serious problem for mankind, atmospheric water harvesting has emerged as a viable solution. Two main approaches to collect water from the atmosphere exist: the first is to capture it from fog, whereas the second is through condensation of vapor on surfaces with a temperature below the dew point. The water collection mechanism in these two modes is completely different. In this work, we develop a deeper understanding of the effect of surface wettability on gravity-assisted atmospheric water harvesting and a comparative study of the two collection modes (fog and dew). First, we present theoretical estimates for the maximum water mass available in each mode and introduce an efficiency factor η which enables the direct comparison among surfaces in different setups and modes. Then we fabricate a series of micronanostructured surfaces with different surface wetting properties from hydrophilic to superhydrophobic. Our results demonstrate that drop mobility, derived from the surface superhydrophobic properties and micronanotopography, is the most important factor affecting fog collection: superhydrophobic surfaces show 40-65% higher fog collection rates compared to flat hydrophilic surfaces, with the more mobile among superhydrophobic surfaces (hysteresis 2°, and air-liquid fraction fA-L > 0.9) showing higher water collection. On the other hand, dew harvesting efficiency depends on the combination of drop mobility and nucleation rate, with superhydrophobic surfaces exhibiting 40% higher water collection rate compared to the flat hydrophilic or hydrophobic surfaces due to their low hysteresis as well as high surface area available for nucleation.

Flow boiling heat transfer performance of copper-alumina micro-nanostructured surfaces developed by forced convection electrodeposition technique

The stability and adhesion of developed surfaces to the substrate is directly influenced the boiling performance with respect to the recurring trial runs. For the commercial application point of view, the stability of developed surfaces is an important concern. Therefore, a new multi-step forced convective electrocodeposition technique is proposed for further improvement in adhesion, nucleation sites, wettability, and flow boiling heat transfer. The uniform particles deposition on the surface is achieved by forced convection electrodeposition technique. The SEM images, porosity, wettability, coating thickness, EDS spectrum, and surface roughness are carried out in order to analyze the surface behaviors of the surfaces. The flow boiling tests are conducted on considered surfaces at different mass flow rates with DI water. The maximum variations of 5.3% in critical heat flux (CHF) and 1.3°C in surface temperature are monitored for micro-nanostructured (MS#2) surface between the first and fifth test run. The incipience of boiling (ONB) for MS#2 surface is occurred at surface temperatures of 100.3, 102.1, 103.4, and 106.5°C for the mass fluxes of 53, 113, 268, and 361 kg/m2s, respectively. The maximum augmentation in CHF (~176%) and heat transfer coefficient (~200%) are achieved on MS#2 surface at lower mass flux.Statement of novelty and Significance: The boiling heat transfer mechanism is an efficient heat transfer mode and able to achieve large amount of heat extraction with very small wall superheat temperature from the critical equipments though sensible heat and latent heat of vaporization. In boiling heat transfer process, being a vigorous process, the stability and adhesion of developed surfaces to the substrate is directly influenced the boiling performance with respect to the repetitive test runs. For the commercial application point of view, the stability of developed surfaces is an important concern. Therefore, In the present study, a new four-step (deposition-sintering-deposition sintering) forced convection electrodeposition technique is conceptualized and used in the present work for further improvement in adhesion, nucleation sites, wettability, and boiling heat transfer. The flow boiling tests are conducted on micro/nanostructured surfaces at different mass flow rates with DI water (working fluid). More improvements in CHF and HTC have been monitored with the developed micro- nanostructured Cu-Al2O3 (MS#2) surface.

One-step control of hierarchy and functionality of polymeric surfaces in a new plasma nanotechnology reactor

Hierarchical micro-nanostructured surfaces are key components of ‘smart’ multifunctional materials, used to control wetting, adhesion, tactile, friction, optical, antifogging, antibacterial, and many more surface properties. Hierarchical surfaces comprise random or ordered structures ranked by their length scale spanning the range from a few nanometers to a few micrometers, with the larger microstructures typically embedding smaller nanostructures. Despite the importance of hierarchical surfaces, there have been few studies on their precise and controlled fabrication or their quantitative characterization, and they usually involve multiple and complex fabrication steps. Here, we present a new plasma nanotechnology, which we term ‘nanoinhibit’, and a new plasma reactor for producing in one facile process-step-controlled hierarchy at will on polymeric surfaces. We couple the new plasma nanotechnology with detailed computational nanometrology based on the analysis of scanning electron microscopy images and targeted to specific functionality. We showcase the potential of ‘nanoinhibit’ for functional surface fabrication by controlling the wetting and optical functionality of the fabricated hierarchical surfaces and showing its dependence on surface morphology metrics. Finally, we observe that ‘nanoinhibit’ produces a new class of ‘strong hierarchical’ surfaces exhibiting spatially separated periodic and fractal-like components.

Bactericidal efficiency of micro- and nanostructured surfaces: a critical perspective.

Micro/nanostructured surfaces (MNSS) have shown the ability to inactivate bacterial cells by physical means. An enormous amount of research has been conducted in this area over the past decade. Here, we review the various surface factors that affect the bactericidal efficiency. For example, surface hydrophobicity of the substrate has been accepted to be influential on the bactericidal effect of the surface, but a review of the literature suggests that the influence of hydrophobicity differs with the bacterial species. Also, various bacterial viability quantification methods on MNSS are critically reviewed for their suitability for the purpose, and limitations of currently used protocols are discussed. Presently used static bacterial viability assays do not represent the conditions of which those surfaces could be applied. Such application conditions do have overlaying fluid flow, and bacterial behaviours are drastically different under flow conditions compared to under static conditions. Hence, it is proposed that the bactericidal effect should be assessed under relevant fluid flow conditions with factors such as shear stress and flowrate given due significance. This review will provide a range of opportunities for future research in design and engineering of micro/nanostructured surfaces with varying experimental conditions.

Superefficient and robust polymer coating for bionic manufacturing of superwetting surfaces with “rose petal effect” and “lotus leaf effect”

Mimicking the superwetting behaviors of nature creatures is advantageous for the development of multifunctional surfaces. Superhydrophobic surfaces are desired for applications such as self-cleaning, drag reduction, anti-staining, anti-corrosion, oil-water separation, etc. Herein, a superefficient approach is proposed to produce a robust ultrahigh molecular weight polyethylene (UHMWPE) coating on various substrates by dripping and drying UHMWPE solutions. The superhydrophobic surfaces can be tuned between Cassie impregnating state and Cassie state, which resembles the “rose petal effect” and “lotus leaf effect”, respectively, by simply adjusting the quenching temperature and the solvent system. UHMWPE/ortho-xylene biphasic solution resulted in a wrinkled morphology with a high-adhesion superhydrophobic surface, while the UHMWPE/ortho-xylene/cyclohexanone triple-phase solution led to a highly porous morphology with a low-adhesion superhydrophobic surface. The micro-nano structured surfaces displayed promising applicability in droplet transportation, selective oil absorption, anti-staining, and self-cleaning. They also possess superior stability and durability against various liquids and mechanical impact. This work provides a highly efficient approach to produce versatile bionic superhydrophobic coatings with tunable superwetting states.

Oxygen plasma micro-nanostructured PMMA plates and microfluidics for increased adhesion and proliferation of cancer versus normal cells: The role of surface roughness and disorder

Abstract Micro-nanostructured surfaces play a key role in the understanding of cell-surface interactions. The aim of this work was to study the effect of oxygen plasma micro-nanostructured commercial PMMA plates on the adhesion, morphology and proliferation of normal skin fibroblasts and A431 skin cancer cells. It was found that the adhesion and proliferation of normal skin fibroblasts was significantly inhibited, while their morphology and focal points formation were seriously impaired on plasma treated surfaces with peak-to-valley height ≥ 5 μm as compared to the untreated ones. On the other hand, the adhesion of cancer cells was 2.5–3 times higher in all rough surfaces as compared to the untreated ones, whereas their proliferation rate was considerably improved on the rough surfaces prepared through plasma treatment for 5 and 10 min. Besides, the morphology of cancer cells was not affected on all the micro-nanostructured surfaces tested. The preference of cancer cells for adhesion and proliferation on highly rough surfaces compared to normal cells was exploited for enrichment of cancer cells in co-cultures with normal ones. Thus, for cancer to normal cell seeding ratios from 1/50 to 1/200, after 3 days of culture, enrichment ratios of about 48.5 ± 1.6 were obtained for the 10-min treated micro-nanostructured surfaces as compared to enrichment ratios of 13.8 ± 0.9 obtained for the untreated surfaces. A detailed Fourier and multifractal analysis shows for the first time that this increased enrichment of cancer cells follows the increased surface disorder of the rough surfaces. Moreover, the enhanced adhesion of cancer cells with respect to normal ones onto plasma micro/nanostructured surfaces was investigated under flow conditions using microchannels with micro-nanostructured bottom. In this case, an enrichment ratio of cancer to normal cells of 1.5 was determined compared to 0.5 obtained for the untreated PMMA after 1-h culture with the surface. These results demonstrate the great potential of plasma micro-nanostructured PMMA plates for enrichment of cancer cells from mixtures with normal ones.

Anodized anisotropic titanium surfaces for enhanced guidance of gingival fibroblasts.

Ensuring the formation of a robust trans-mucosal soft-tissue seal at the dental abutment surface is crucial towards protecting the underlying dental implant associated tissues from the external microbial-rich oral environment. The ability to mechanically enhance fibroblast functions at the dental abutment-mucosa interface, without the use of bioactive agents, holds great promise towards reducing the ingress of oral pathogens into the dental implant microenvironment. We hereby propose fabrication of unique anisotropic titania nanopores (TNPs) on the surface of titanium (via electrochemical anodization, EA) towards enhancing the soft-tissue integration and wound healing abilities of the conventional abutments. Using optimized EA, mechanically robust TNPs of varied diameters were fabricated on Ti surfaces with preserved underlying substrate micro-features: dual micro-nanostructured surfaces. Next, we evaluated the mechanical stability of such structures and demonstrated the ease of fabrication on commercial abutment geometries. The functions of primary human gingival fibroblasts (GFs) cultured on these surfaces in vitro were evaluated from 1h to 7days, and were compared between TNPs and clinically relevant titanium controls: as-received irregular rough Ti (Rough Ti) and mechanically prepared micro-rough Ti (Micro Ti). Improved cell viability was observed on TNPs as compared to controls. Additionally, cellular spreading morphology indicated cell alignment along the direction of the nanopores with strong anchoring evident by enhanced filopodia and stress fibers. RT-PCR showed improved wound healing, cell migration/adhesion and angiogenesis related mRNA, especially for TNPs with large diameters. This study provides a proof-of-concept towards using anodization for improving soft-tissue sealing around dental abutment surfaces, with implications towards reducing implant failure/peri-implantitis and achieving long-term success, especially in compromised patient conditions.

Hierarchical Micro-Nanostructured Surfaces for Isotropic/Anisotropic Liquid Transport.

The control of liquid transport using hierarchical micro-nanostructured surfaces is of significant interest for a broad range of applications, including thermal management, digital lab-on-chip, self-cleaning surfaces, antifogging, and water harvesting, among others. Although a variety of fabrication techniques can be utilized to produce micro/nanostructured patterns for controlling liquid transport, each sample usually needs to be patterned and developed separately, making the micro/nanofabrication process tedious and expensive. Here, based on scalable template stripping and chemical oxidation techniques, we demonstrate hierarchical micro-nanostructured surfaces for isotropic or anisotropic liquid transport. Furthermore, the overall structure is thin and flexible, making it ideal for applications where geometry and weight are constrained, such as aerospace, flexible, and wearable devices.

A comparative study of the icephobic and self-cleaning properties of Teflon materials having different surface morphologies

Materials having fluorocarbon bonds are among the best candidates for the fabrication of superhydrophobic surfaces. Here, two facile, non-expensive, and industrialized approaches were described to produce superhydrophobic Teflon materials having ultra-water repellency, icephobic, and self-cleaning properties. Direct replication and plasma-treatment approaches produced Teflon sheets having very different surface patterns, i.e. microstructures and micro-nanostructures. Neither approach altered the chemical composition of the original Teflon surfaces. Rice leaf–like microstructures were produced on the replicated surface, whereas lotus leaf–like hierarchical micro-nanostructures characterized the plasma-treated surface. Water droplets rolled off the micro-nanostructured surfaces ˜10% faster than off the microstructured surfaces. The micro-nanostructured surface also produced more rebounds for a water droplet during the impact test. Although both surfaces possessed similar self-cleaning properties, the micro-nanostructured surface reduced ice adhesion to a greater degree than the microstructured surface. The more effective ice repellency of the micro-nanostructured surface was due to its surface morphology that reduced the interlocking of ice inside the surface asperities. However, the microstructured surface delayed considerably the onset of freezing of a water droplet due to the larger micro-air pockets trapped within its surface asperities.

Condensation on solid surfaces with amphiphilic micro-nanostructures by lattice Boltzmann simulation

The chemical properties and topography of solid surfaces exert a large effect on droplet condensation efficiency. Some scholars have devised a kind of amphiphilic microstructure with hydrophobic top and hydrophilic bottom, and their experiment results show that surfaces with this kind of amphiphilic microstructure can effectively reduce the size of condensed droplets and enhance condensation. In this paper, the Lattice Boltzmann method (LBM) is utilized to investigate this experiment numerically, and a corresponding conclusion is obtained. Further, nanostructures are added to this amphiphilic microstructure, and two kinds of amphiphilic micro-nanostructured surfaces were designed. The results by numerical studies demonstrate that the wettability of nanostructures on the top of amphiphilic micro-nanostructures are critical to condensation efficiency. Amphiphilic micro-nanostructured surfaces with hydrophilic nanostructures (amphiphilic micro-nanostructure I) possess better condensation efficiency than amphiphilic micro-structured surfaces and amphiphilic micro-nanostructured surfaces with hydrophobic nanostructures (amphiphilic micro-nanostructure II), which can significantly enhance droplet condensation. The findings of this paper provide guidance for the optimal design of surface structures to promote condensation.

Preparation of micro-nanostructure on titanium implants and its bioactivity

Surface modification of medical implants was considered as an effective method to improve the cellular behaviors and the integration of tissue onto materials. The micro-nanostructured surface on the titanium alloy was prepared by laser treatment and multiple acid etching. The surface morphologies of different titanium alloy substrates were characterized by scanning electron microscopy (SEM). The effects of micro-nanostructured surfaces on the cellular responses were investigated in vitro by observing hydroxyapatite formation, cell morphology and cell adhesion. The results indicate that the micro-sized structure promoted the adhesion and proliferation of cultured osteoblasts. Furthermore, the micro-nanostructured surface was more conducive to cell adhension stretching compared with the micro-structured surface. All results suggest that the micro-nanostructured surface improved the biocompatibility and integration of tissue onto titanium alloy implants.

Artificial petal surface based on hierarchical micro- and nanostructures

In this paper, we present an artificial rose petal composed of hierarchical micro- and nanostructures on a polymethyl methacrylate surface. The petal effect implies that the surface has a high adhesion force in spite of being in a super-hydrophobic state, while the lotus effect implies that the surface has a low adhesion force when it is in a super-hydrophobic state. We have fabricated four different types of surfaces, namely, smooth, nanostructured, microstructured, and hierarchically micro-nanostructured surfaces. Microstructures and nanostructures have a quadrangular pyramid shape (one-side length: 15 ± 2 μm, height: 10.6 ± 1 μm) and a circular bump shape (diameter: 130 ± 10 nm, height: 100 ± 10 nm), respectively. The four types of surfaces are also chemically treated with trichlorosilane in order to reduce the surface energy. The contact angles of the smooth, nanostructured, microstructured, and hierarchically micro-nanostructured surfaces are measured to be 104° ± 2°, 112° ± 2°, 138° ± 4°, and 159° ± 2° after the chemical treatment. In the case of the super-hydrophobic micro-nanostructured surfaces, water droplets remain attached to the surface, even when the surface is turned upside down.

Drag reduction in ultrahydrophobic channels with micro-nano structured surfaces

A series of experiments have been performed to demonstrate the significant drag reduction of the laminar flow in the ultrahydrophobic channels with dual-scale micro-nano structured surfaces. However, in previous experiments, the ultrahydrophobic surfaces were fabricated with micro-structures or nano-structures and the channels were on the microscale. For the drag reduction in macro-scale channels few reports are available. Here a new method was developed to fabricate ultrahydrophobic surfaces with micro-nano hierarchical structures made from carbon nanotubes. The drag reductions up to 36.3% were observed in the macro-channels with ultrahydrophobic surfaces. The micro-PIV was used to measure the flow velocity in channels. Compared with the traditional no-slip theory at walls, a significant slip velocity was observed on the ultrahydrophobic surfaces.