Effect of hydrophobicity on the interfacial rheological behaviors of nanoparticles at decane-water interface

Interfacial Rheological Investigation of Modified Silica Nanoparticles with Different Alkyl Chain Lengths at the n-Octane/Water Interface

The interfacial dilational rheology of silica nanoparticles (NPs) directly reflects the relationship between surface structure and interfacial behaviors in NPs, which has attracted significant attention in various industrial fields. In this work, modified silica nanoparticles (MNPs) with various alkyl chain lengths were synthesized and systematically characterized using Fourier transform infrared spectra, Zeta potential, and water contact angle measurements. It was found that the MNPs were successfully fabricated with similar degrees of modification. Subsequently, the interfacial behaviors of the MNPs in an n-octane/water system were investigated through interfacial dilational rheological experiments. The length of the modified alkyl chain dominated the hydrophilic-lipophile balance and the interfacial activity of the MNPs, evaluated by the equilibrium interfacial tension (IFT) variation and dilational elasticity modulus. In the large amplitude compression experiment, the balance between the electrostatic repulsion and interfacial activity in the MNPs was responsible for their ordered interfacial arrangement. The MNPs with the hexyl alkyl chain (M6C) presented the optimal amphipathy and could partly overcome the repulsion, causing a dramatic change in surface pressure. This was further confirmed by the variations in IFT and dilational elasticity during the compression path. The study provides novel insights into the interfacial rheology and interactions of functionally modified NPs.

Interfacial Rheological and Emulsion Properties of Self-Assembled Cyclodextrin-Oil Inclusion Complexes

To investigate the effect of the molecular size of alkanes and the cavity size of cyclodextrins (CDs) on the formation of interfacial host-guest inclusion complexes, the interfacial tension (IFT) of CD (α-CD, β-CD, γ-CD) solutions against oils (hexadecane, dodecylbenzene) was determined by interfacial dilational rheology measurements. The results show that the "space compatibility" between CDs and oil molecules is crucial for the formation of interface host-guest inclusion complexes. Hexadecane with a smaller molecular size can form host-guest inclusion complexes with small cavities of α-CD and β-CD, dodecylbenzene with a larger molecular size can form interfacial aggregates with the medium-sized cavity of β-CD easily, and the polycyclic aromatic hydrocarbon molecules in kerosene can form inclusion complexes with the large cavity of γ-CD. The formation of interfacial inclusion complexes leads to lower IFT values, higher interfacial dilational modulus, nonlinear IFT responses to the interface area oscillating, and skin-like films at the oil-water interface. What's more, the phase behavior of Pickering emulsions formed by CDs with different oils is explored, and the phenomena in alkane-CD emulsions are in line with the results in dilatation rheology. The interfacial active host-guest structure in the kerosene-γ-CD system improves the stability of the Pickering emulsion, which results in smaller emulsion droplets. This unique space compatibility characteristic is of great significance for the application of CDs in selective host-guest recognition, sensors, enhanced oil recovery, food industries, and local drug delivery.

Water droplet can mitigate dust from hydrophobized micro-post array surfaces

Water droplet rolling motion over the hydrophobized and optically transparent micro-post array surfaces is examined towards dust removal pertinent to self-cleaning applications. Micro-post arrays are replicated over the optically transparent polydimethylsiloxane (PDMS) surfaces. The influence of micro-post array spacing on droplet rolling dynamics is explored for clean and dusty surfaces. The droplet motions over clean and dusty micro-post array surfaces are monitored and quantified. Flow inside the rolling droplet is simulated adopting the experimental conditions. Findings reveal that micro-post gap spacing significantly influences droplet velocity on clean and dusty hydrophobized surfaces. Air trapped within the micro-post gaps acts like a cushion reducing the three-phase contact line and interfacial contact area of the rolling droplet. This gives rise to increased droplet velocity over the micro-post array surface. Droplet kinetic energy dissipation remains large for plain and micro-post arrays with small gap spacings. A Rolling droplet can pick up dust particles from micro-post array gaps; however, few dust residues are observed for large gap spacings. Nevertheless, dust residues are small in quantity over hydrophobized micro-post array surfaces.