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Recent progress in the preparation, properties and applications of superhydrophobic nano-based coatings and surfaces: A review

are the liquid density and surface tension).This law is also observed to hold on partially wettable surfaces, provided that liquidsof low viscosity (such as water) are used. The law is interpreted as resulting fromthe eﬀective acceleration experienced by the drop during its impact. Viscous dropsare also analysed, allowing us to propose a criterion for predicting if the spreading islimited by capillarity, or by viscosity.

Maximal deformation of an impacting drop

Surface wettability plays an important role in regulating biomolecule adhesion behaviors. The biomolecule adhesion behaviors of superwettable surfaces have become an important topic as an important part of the interactions between materials and organisms. In addition to general research on the moderate wettability of surfaces, the studies of biomolecule adhesion behaviors extend to extreme wettability ranges such as superhydrophobic, superhydrophilic and slippery surfaces and attract both fundamental and practical interest. In this review, we summarize the recent studies on biomolecule adhesion behaviors on superwettable surfaces, especially superhydrophobic, superhydrophilic and slippery surfaces. The first part will focus on the influence of extreme wettability on cell adhesion behaviors. The second part will concentrate on the adhesion behaviors of biomacromolecules on superwettable surfaces including proteins and nucleic acids. Finally, the influences of wettability on small molecule adhesion behaviors on material surfaces have also been investigated. The mechanism of superwettable surfaces and their influences on biomolecule adhesion behaviors have been studied and highlighted.

Bioinspired surfaces with wettability: biomolecule adhesion behaviors

We fabricated biomimetic hierarchical superhydrophobic surfaces using the Tobacco mosaic virus and investigated the role of each length scale during droplet impact by decomposing the micro and nanoscale components. We found that 10 μl water droplets rebounded at impact velocities greater than 4.3 m/s on the hierarchical surfaces, outperforming the nanostructured surfaces, which underwent an observable wetting transition at an impact velocity of 2.7 m/s. This finding demonstrates that each length scale plays a distinct, but complementary, role in maximizing water repellency during droplet impact and, thus, provides insight into the evolutionary development of highly water-repellant hierarchical plant leaves.

Biotemplated hierarchical surfaces and the role of dual length scales on the repellency of impacting droplets

In this work, we report a large-area fabrication of a flexible superhydrophobic bactericidal surface decorated with copper hydroxide nanowires. This involves a simple two-step method which involves growth followed by transfer of the nanowires onto the polydimethylsiloxane (PDMS) surface by mechanical peeling. Additional roughness in PDMS is obtained through incomplete wetting of the nanoscale gaps which leads to dual-scale roughness and superhydrophobicity with a contact angle of 169° and hysteresis of less than 2°. The simplicity of the process makes it low-cost and easily scalable. The process allows fabrication of nonplanar 3D surfaces. The surface shows blood repellence and antibacterial activity against Escherichia coli with more than 5 log reductions in bacterial colony. The surface also shows hemocompatible behavior, making it suitable for healthcare applications. The fabricated surface is found to be extremely robust against stretching, twisting, sandpaper abrasion, solid weight impact, and tape p...

Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial Surface with Dual-Scale Roughness

Hydrodynamic collapse of a central air-cavity during the recoil phase of droplet impact on a superhydrophobic sieve leads to satellite-free generation of a single droplet through the sieve. Two modes of cavity formation and droplet ejection have been observed and explained. The volume of the generated droplet scales with the pore size. Based on this phenomenon, we propose a drop-on-demand printing technique. Despite significant advancements in inkjet technology, enhancement in mass-loading and particle-size have been limited due to clogging of the printhead nozzle. By replacing the nozzle with a sieve, we demonstrate printing of nanoparticle suspension with 71% mass-loading. Comparatively large particles of 20 μm diameter are dispensed in droplets of ~80 μm diameter. Printing is performed for surface tension as low as 32 mNm−1 and viscosity as high as 33 mPa∙s. In comparison to existing techniques, this way of printing is widely accessible as it is significantly simple and economical. Printing small droplets for a wide range of applications remains a challenge. Here, the authors propose a simple drop-on-demand printing technique which replaces the use of a nozzle with a sieve, enabling printing of nanoparticle suspension with 71% mass-loading, performed for surface tension range of 72–32 mNm-1 and viscosity up to 33 mPas.

Drop impact printing.

When a droplet impacts a superhydrophobic sieve, a part of the droplet penetrates through it when the dynamic pressure (ρU2) of the impinging droplet exceeds the breakthrough pressure (γΓ/A). At higher impact velocities, the ejected-jet breaks and separates from the main droplet. The remaining part of the droplet bounces off the surface showing different modes (normal bouncing as a vertically elongated drop or pancake bouncing). In this work, we have studied the effect of different geometrical parameters of superhydrophobic copper meshes on different modes of droplet rebound. We observe three different effects in our study. Firstly, we observe pancake like bouncing, which is attributed to the capillary energy of the rebounding interface formed after the breaking of the ejected-jet. Secondly, we observe leakage of the droplet volume and kinetic energy due to the breaking of the ejected-jet, which leads to reduction in the contact times. Finally, we observe that for flexible meshes, the transition to pancake type bouncing is induced at lower Weber numbers. Flexibility also leads to a reduction in the volume loss from the ejected-jet. This study will be helpful in the design of superhydrophobic meshes for use under impact scenarios.

Effect of geometrical parameters on rebound of impacting droplets on leaky superhydrophobic meshes

Hydrodynamic collapse of a central air-cavity during the recoil phase of droplet impact on a superhydrophobic sieve leads to satellite-free generation of a single droplet through the sieve. Two modes of cavity formation and droplet ejection was observed and explained. The volume of the generated droplet scales with the pore size. Based on this phenomenon, we propose a new drop-on-demand printing technique. Despite significant advancements in inkjet technology, enhancement in mass-loading and particle-size have been limited due to clogging of the printhead nozzle. By replacing the nozzle with a sieve, we demonstrate printing of nanoparticle suspension with 71% mass-loading. Comparatively large particles of 20 micrometer diameter were dispensed in droplets of 80 micrometer diameter. Printing was performed for surface tension as low as 32 mNm-1 and viscosity as high as 33 mPa-s. In comparison to existing techniques, this new way of printing is widely accessible as it is significantly simple and economical.

Drop Impact Printing

The global emergence of antimicrobial resistance poses a serious risk to patients by increasing the cost of healthcare with prolonged stay in hospitals, serious clinical complications, and even death. The ever-increasing challenges in discovering antibacterial agents with novel mechanisms of action necessitates the development of smart antibacterial surfaces that have the potential to minimize colonization of common hospital surfaces with bacterial pathogens. In this work, we report the antibacterial properties of flexible poly(dimethylsiloxane) (PDMS) polymer decorated with copper hydroxide nanowires (PDMS_Cu) against a panel of drug-resistant bacterial pathogens isolated from patients with bloodstream infection. The fabricated PDMS_Cu surface showed superior antimicrobial activity against both Gram negative (Escherichia coli and Klebsiella pneumoniae) and Gram positive (Staphylococcus aureus) bacterial strains as compared to flat PDMS and glass coverslip, which were used as controls. RAW macrophage and ...

Arvind Kumar

Papers

Fabrication of Low-Cost Flexible Superhydrophobic Antibacterial Surface with Dual-Scale Roughness

Drop impact printing.

Effect of geometrical parameters on rebound of impacting droplets on leaky superhydrophobic meshes

Drop Impact Printing

A Nanowire-Based Flexible Antibacterial Surface Reduces the Viability of Drug-Resistant Nosocomial Pathogens