Duyang Zang

Viscoelastic properties of silica nanoparticle monolayers at the air-water interface

We have investigated the rheological behaviour of silica nanoparticle layers at the air-water interface. Both compressed and deposited layers have been studied in Langmuir troughs and with a bicone rheometer. The compressed layers are more homogeneous and rigid, and the elastic response to continuous, step and oscillatory compression are similar, provided the compression is fast enough and relaxation is prevented. The deposited layers are less rigid and more viscoelastic. Their shear moduli deduced from the oscillatory uniaxial compression are much smaller than those deduced from pure shear deformation suggesting that the effective shear rate is smaller than expected in the compression measurements.

Impact dynamics of droplets with silica nanoparticles and polymer additives

We investigate the impact dynamics of droplets containing silica nanoparticles and/or poly(ethylene oxide) (PEO) additives by using a high speed camera, and relate the impact behavior to the rheological properties of liquids. For a droplet with both particles and polymer additives, the rebound is damped much faster and the instability behavior is suppressed. Interestingly, the rebound can be inhibited even when impact is at high velocity (1.88 m s−1). The transition from “rebound” to “stick” by enhancing the impact velocity is mainly due to the increase of the friction force of the nanoparticles and polymer aggregates with the substrate. This is confirmed by the increase of the sliding angle with impact velocity.

Switchable opening and closing of a liquid marble via ultrasonic levitation

Liquid marbles have promising applications in the field of microreactors, where the opening and closing of their surfaces plays a central role. We have levitated liquid water marbles using an acoustic levitator and, thereby, achieved the manipulation of the particle shell in a controlled manner. Upon increasing the sound intensity, the stable levitated liquid marble changes from a quasi-sphere to a flattened ellipsoid. Interestingly, a cavity on the particle shell can be produced on the polar areas, which can be completely healed when decreasing the sound intensity, allowing it to serve as a microreactor. The integral of the acoustic radiation pressure on the part of the particle surface protruding into air is responsible for particle migration from the center of the liquid marble to the edge. Our results demonstrate that the opening and closing of the liquid marble particle shell can be conveniently achieved via acoustic levitation, opening up a new possibility to manipulate liquid marbles coated with non-ferromagnetic particles.

Influence of the contact angle of silica nanoparticles at the air–water interface on the mechanical properties of the layers composed of these particles

We have studied the properties (surface pressure, compression and shear moduli, texture) of silica nanoparticle layers at the air–water interface. Particle hydrophobicity or, equivalently, the contact angle between particles, air and water, is the main factor that influences surface organization and surface elastic moduli. The surface layers are denser for particles of higher hydrophobicity. The compression and shear moduli, as well as the yield and melt strains, present a maximum for contact angles around 90°. The dependence of the mechanical properties on particle hydrophobicity is closely related to the foamability and stability of the foams made from dispersions.

Effect of particle hydrophobicity on the properties of liquid water marbles

We study both the static properties and dynamic behavior of liquid water marbles coated with silica nanoparticles of varied hydrophobicity. The static properties are characterized by the variation of marble height and diameter with increasing marble volume, such that the effective surface tension γeff of the marble can be obtained. The dynamic behavior of liquid marbles includes their impingement on a solid surface and their compression between two parallel glass plates. Marbles coated with particles of intermediate hydrophobicity exhibit maximum γeff values and enhanced mechanical robustness. Due to particle detachment from or particle rearrangement at the air–water interface caused by the impact, the dynamic surface tensions γd of liquid water marbles are different in magnitude to those of γeff. In fact, γd plays an important role in determining the contact time and oscillation period during the impact and rebound processes. Our results show that both the static effective and dynamic surface tension depend on the hydrophobicity of the particles coating the marble surfaces.

Aggregation behavior of polystyrene-b-poly(acrylic acid) at the air–water interface

The aggregation behavior of amphiphilic block copolymer polystyrene-b-poly(acrylic acid) (PS-b-PAA) at the air–water interface was investigated through surface pressure measurements (isotherms, compression–expansion hysteresis and compression relaxation experiments), Brewster Angle Microscopy (BAM) imaging and Atomic Force Microscopy (AFM) imaging. It is found that PS-b-PAA (Mn = 11490 g mol−1, PAA wt%∼62%) forms a stable Langmuir monolayer on the water surface (pH = 7) by using N,N-dimethylformamide (DMF) as the spreading solvent. The aggregation of block copolymer is induced by an initial diffusion of DMF into water from the interface. Upon compression of the film, the pseudoplateau observed in the Langmuir isotherm corresponds to a “pancake-to-brush” transition with the PAA chains gradually dissolving in the water subphase and stretching underneath the PS cores. Based on the isotherms and the BAM images, it is suggested that the polymer chain dynamics in spreading solutions with different concentrations at the time of solvent diffusion influence the interfacial behavior of block copolymers significantly. The Langmuir–Blodgett (LB) films prepared at different surface pressures from different spreading solution concentrations were scanned by AFM. A variety of morphologies such as wormlike, porous and reticulate structures, and dots were observed. The isotherms and AFM images show the spreading solution concentration and surface pressure dependence of the aggregation behavior of PS-b-PAA copolymer at the air–water interface.

Morphology and wettability control of honeycomb porous films of amphiphilic fluorinated pentablock copolymers via breath figure method

Amphiphilic pentablock copolymers of poly (trifluoroethyl methacrylate)-b-poly (methyl methacrylate)-b-poly (ethylene glycol)-b-poly (methyl methacrylate)-b-poly (trifluoroethyl methacrylate) (PTFEMA-b-PMMA-b-PEG-b-PMMA-b-PTFEMA) with three different block ratios as well as molecular weights were used to fabricate honeycomb structured porous films through the breath figure technique. Several critical influencing factors such as macromolecular structures, solvent properties, copolymer concentrations and the substrates were investigated to control the morphology of the pores. The results showed that chloroform utilized as a solvent with an appropriate concentration of 45 mg mL−1 on the substrate of silicon wafer offered the optimum condition. It could be evidenced that the pore sizes of the honeycomb films were increased by enhancing the molecular weight of the copolymer. In addition, an increase in the copolymer concentration leads to a decrease in the pore size, and even causes a few porous structures to disappear. The viscosity of the liquid substrates also affects the pore size, and it was found that a larger pore size was formed with an increase in the viscosity. The porous films possessed better hydrophobic and oleophobic properties than the flat films. In addition, the pincushion structure films exhibited the best hydrophobic and oleophobic properties and an excellent water-adhesion ability in the reverse state. This work may facilitate our understanding of the breath figure process and assist us in preparing films under different conditions, which show different perspectives as micro-suckers, hydrophobic and oleophobic membranes.

Blood drop patterns: Formation and applications

The drying of a drop of blood or plasma on a solid substrate leads to the formation of interesting and complex patterns. Inter- and intra-cellular and macromolecular interactions in the drying plasma or blood drop are responsible for the final morphologies of the dried patterns. Changes in these cellular and macromolecular components in blood caused by diseases have been suspected to cause changes in the dried drop patterns of plasma and whole blood, which could be used as simple diagnostic tools to identify the health of humans and livestock. However, complex physicochemical driving forces involved in the pattern formation are not fully understood. This review focuses on the scientific development in microscopic observations and pattern interpretation of dried plasma and whole blood samples, as well as the diagnostic applications of pattern analysis. Dried drop patterns of plasma consist of intricate visible cracks in the outer region and fine structures in the central region, which are mainly influenced by the presence and concentration of inorganic salts and proteins during drying. The shrinkage of macromolecular gel and its adhesion to the substrate surface have been thought to be responsible for the formation of the cracks. Dried drop patterns of whole blood have three characteristic zones; their formation as functions of drying time has been reported in the literature. Some research works have applied engineering treatment to the evaporation process of whole blood samples. The sensitivities of the resultant patterns to the relative humidity of the environment, the wettability of the substrates, and the size of the drop have been reported. These research works shed light on the mechanisms of spreading, evaporation, gelation, and crack formation of the blood drops on solid substrates, as well as on the potential applications of dried drop patterns of plasma and whole blood in diagnosis.

Surface wrinkling and cracking dynamics in the drying of colloidal droplets.

Abstract.The cracking behavior accompanied with the drying of colloidal droplets containing polytetrafluoroethylene (PTFE) nanoparticles was studied. During evaporation, due to the stretching effect of the liquid zone, the receding wet front leads to the formation of radialized surface wrinkling in the gel zone. This indicates the building of a macroscopic stress field with a similar distribution. As a result, the cracks in the deposited films are in a radial arrangement. The propagation velocity of the cracks depends on the thickness of the film, ∼ H3/5 . In addition, sodium dodecylsulfate (SDS) additives can be used to tune crack behavior by causing a reduction of the capillary force between particles. The results highlight the significance of the receding wet front in building the drying deposition stress field and may be helpful in other fields related to drying and cracking processes.Graphical abstract

Tuning the wettability of an aluminum surface via a chemically deposited fractal dendrite structure

We have developed a straightforward method to tune the wettability of an aluminum substrate within a contact angle (CA) range from 2° to 170° by chemical deposition in CuCl2 solution and fluoroalkylsilane (FAS) modification. The CA of the as-deposited surface decreases with deposition time due to the growth of fractal copper dendrites, which enhance the surface roughness significantly. After subsequent modification with FAS, a superhydrophobic surface with CA 170° and sliding angle less than 5° has been obtained. With the increase of CA, the maximum spreading of water droplets is reduced. A bouncing behavior is observed for droplets impinging on the superhydrophobic substrate, suggesting its potential application as a self-cleaning surface.Graphical abstract