Xiaowei Pei

Grafting poly(ionic liquid) brushes for anti-bacterial and anti-biofouling applications

Polymer brushes provide an ideal platform for studying biofouling and screening anti-biofouling materials. In the present work, high-density poly(ionic liquid) brushes based on imidazolium salts were successfully grafted to surfaces via surface-initiated ring-opening metathesis polymerizations from catecholic initiator and their anti-microbial activity was evaluated. Very uniform poly(ionic liquid) coatings with thickness up to 80 nm were obtained on TiO2. Various characterization techniques including infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis and transmission electron microscopy were used to characterize poly(ionic liquid) brushes modified TiO2 nanomaterials. Subsequently, the anti-bacterial and anti-biofouling properties of poly(ionic liquid) brushes were evaluated. It was found that poly(ionic liquid) brushes can obviously resist adhesion of Chlorella spores and the counter-anions have a key impact on the anti-microbial property. The hexafluorophosphate anion poly(ionic liquid) coated TiO2 nanomaterials have excellent anti-bacterial properties compared to pristine TiO2 nanoparticles against both E. coli and S. aureus.

Bioinspired Self-Healing Organic Materials: Chemical Mechanisms and Fabrications

Design and preparation of organic materials having the ability to automatically restore their mechanical and physical properties are of great importance because of the extensive application ranging from aerospace components to microcircuitry, where the accessibility is highly limited and the reparability of materials is lower. The self-healing behavior is actually a dynamic property of material, resembling what is possessed by nature living systems. Therefore, fabrication of most self-healing materials is actually inspired by nature. This tutorial review focuses on the basic chemical mechanisms that have been successfully adopted in designing self-healing organic materials. It specially covers recent development in the design of materials with durable, easy repairable or self-healing superhydrophobic surfaces and coatings.

How Solid–Liquid Adhesive Property Regulates Liquid Slippage on Solid Surfaces?

The influence of solid-liquid adhesive property on liquid slippage at solid surfaces has been investigated using experiment approach on well-defined model surfaces as well as theoretical analysis. Based on a classical molecular-kinetic description for molecular and hydrodynamic slip, we propose a simple theoretical model that directly relates the liquid slip length to the liquid adhesive force on solid surfaces, which yields an exponential decay function. Well-defined smooth surfaces with varied surface wettability/adhesion are fabricated by forming self-assembled monolayers on gold with different mole ratios of hydrophobic and hydrophilic thiols. The adhesive force of a water droplet and the molecular slippage on these surfaces are probed by surface force apparatus and quartz crystal microbalance measurements, respectively. The experiment results are well consistent with our theoretical prediction. Our finding benefits the understanding of the underlying mechanism of liquid slippage on solid surfaces at molecular level and the rational design of microfluidics with an aim to be frictionless or highly controllable.

Bio‐Inspired Renewable Surface‐Initiated Polymerization from Permanently Embedded Initiators

Herein, we describe a simple and robust approach to repeatedly modify surfaces with polymer brushes through surface-initiated atomic transfer radical polymerization (SI-ATRP), based on an initiator-embedded polystyrene sheet that does not rely on specific surface chemistries for initiator immobilization. The surface-grafted polymer brushes can be wiped away to expose fresh underlying initiator that re-initiates polymerization. This strategy provides a facile route for modification of molded or embossed surfaces, with possible applications in the preparation of fluidic devices and polymer-embedded circuits.

Accelerating the healing of superhydrophobicity through photothermogenesis

We describe a novel approach to efficiently achieve self-healing superhydrophobicity on textiles by spontaneous deposition of polydopamine (PDA) encapsulated octadecylamine (ODA) and Fe3O4 nanoparticles. The healing of superhydrophobicity was accelerated by the photothermogenesis effect of Fe3O4 nanoparticles under near-infrared (NIR) light within 40 seconds (s). The coated fabric is also robust and durable against washing and mechanical abrasion without apparently changing the superhydrophobicity. In addition, NIR light can be used to realize controllable wettability gradients and patterns on this kind of textile.

Switching friction with thermal- responsive gels.

The thermosensitive graphene oxide (GO)/poly(N-isopropyl acrylamide) (pNIPAM) composite hydrogels are prepared, and their tribological properties in response to external stimuli are evaluated. The frictional coefficient of the hydrogels is closely related to the gel composition and ambient temperature. When the gel is in swelling state below the low critical solution temperature (LCST), it shows ultra-low friction and exhibits high friction at a shrunk state above the LCST. The huge difference of frictional coefficient under two states can be reversibly switched many times by altering the temperature. The incorporation of a nonthermal sensitive monomer into pNIPAM could change the LCST and thus the transformation point of frictional coefficient can be altered. These reversible and tunable frictional hydrogels have potential application in the design of intelligent control equipment.

Switching Fluid Slippage on pH-Responsive Superhydrophobic Surfaces

Two stimuli-responsive polymer brushes, poly(dimethylaminoethyl methacrylate) and poly(methacrylic acid), were grafted from initiator-modified anodized alumina substrates to prepare two pH-responsive surfaces. By regulating the swelling states of the two polymers, water droplets can roll off or adhere onto the textured surface because of different adhesion forces. These forces also strongly affect boundary slippage. To determine the different slippage effects of fluid on our pH-responsive surfaces, a series of rheological experiments are carried out on two kinds of surfaces. A large slip length is obtained and reversibly regulated by changing the fluid pH. These responsive superhydrophobic surfaces with considerable slip length and pH-responsive properties have extensive potential applications in intelligent micro- and nanofluidic devices or biodevices, which can solve fluid flow problems.

Bio-Inspired Design and Fabrication of Micro/Nano-Brush Dual Structural Surfaces for Switchable Oil Adhesion and Antifouling

The underwater superoleophobic surfaces play a significant role in anti-oil contamination, marine antifouling, etc. Inspired by the Geckos feet and its self-cleaning property, a hierarchical structure composed of poly (acrylic acid) gel micro-brushes is designed by the liquid-infused method. This surface exhibits underwater superoleophobicity with very low oil adhesion. It is then modified with stimuli-responsive polymer nano-brushes via surface-initiated atom transfer radical polymerization from the embedded initiator. The micro/nano-brush dual structural surfaces can switch the underwater oil adhesion between low and high while keeping the superoleophobicity. The antifouling properties against algae attachment under different mediums are also investigated to show a strong link between oleophobicity and antibiofouling property. The model surface will be very useful in directing the design of marine self-cleaning coatings to both living and non-living species.

Slip flow of diverse liquids on robust superomniphobic surfaces

Water slips exist over superhydrophobic solid surfaces, but the slip flow of diverse liquids on a single surface has not been deliberately studied to date. Here, we report the slip flow behavior of a variety of liquids with different surface tensions and viscosities on a robust omniphobic surface. This surface displayed a dramatic slippage effect and thus a high drag reduction efficiency of approximately 10-20% for all liquids, depending on both liquid viscosity and surface energy. The observed liquid slip was attributed to the surface dual micro/nanostructure and the low-surface-energy coating.

Contribution of Surface Chemistry to the Shear Thickening of Silica Nanoparticle Suspensions

Shear thickening is a general process crucial for many processed products ranging from food and personal care to pharmaceuticals. Theoretical calculations and mathematical simulations of hydrodynamic interactions and granular-like contacts have proved that contact forces between suspended particles dominate the rheological characteristic of colloidal suspensions. However, relevant experimental studies are very rare. This study was conducted to reveal the influence of nanoparticle (NP) interactions on the rheological behavior of shear-thickening fluids (STFs) by changing the colloidal surface chemistries. Silica NPs with various surface chemical compositions are fabricated and used to prepare dense suspensions. Rheological experiments are conducted to determine the influence of NP interactions on corresponding dense suspension systems. The results suggest that the surface chemistries of silica NPs determine the rheological behavior of dense suspensions, including shear-thickening behavior, onset stress, critical volume fraction, and jamming volume fraction. This study provides useful reference for designing effective STFs and regulating their characteristics.