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Papers by yihan liu

Langmuir, 2010

The conventional treatment of catastrophic inversion is based on a two-phase model of oil-in-wate... more The conventional treatment of catastrophic inversion is based on a two-phase model of oil-in-water (O/W) or waterin-oil (W/O). The present investigation takes a closer look at the process of inversion with focus on its relation to the detailed phase changes in the system. It is found that phase behavior inserts a decisive call for when the inversion starts and completes, even for an inversion seemingly brought by a simple change of water-to-oil ratio. The phases involved also play a critical role in the fine details of the emulsion structure, during both emulsification and evaporation. The presence of liquid crystal is instrumental in the inversion process as substantiated by the observation that its presence coincides with the presence of the intermediate multiple emulsions during emulsification. Multiple emulsions also appear during evaporation, though the mechanism of their formation is different from that during emulsification. The temporary stability of the multiple emulsions during both emulsification and evaporation is affected by the presence of the liquid crystal. It had been well established that the phase behavior plays a decisive role in transitional inversions and that the transformation to the inverse state is a gradual one. This is apparently also the case with the catastrophic inversion investigated here.

Langmuir, 1996

Frictional properties of monolayers formed from 11 double-chain quaternary ammonium surfactants s... more Frictional properties of monolayers formed from 11 double-chain quaternary ammonium surfactants self-assembled onto mica were measured using lateral force microscopy. Frictional forces differ by orders of magnitude and can increase or decrease with increasing humidity, and frictional force vs velocity curves in some cases display maximum. These differences correlate at a molecular level with variations in the surfactant's chemical composition, degree of unsaturation, chain length, and ω functional group. These differences can also be directly related to the monolayer's structure, phase transition temperature, compressibility and surface hydrophobicity as determined by X-ray diffraction, scanning calorimetry, contact angle, and simulation data. Comparison between lateral force measurements using a standard AFM tip and measurements modified by replacing the tip with a sphere provide a relationship between nanoscale and microscale frictional properties. These observations establish a relation between frictional properties and molecular properties of thin films which are important in many applications including lubrication and tribology.

Journal of Colloid and Interface Science, 2002

The surface force profiles between an atomic force microscopy (AFM) tip and a self-assembled mono... more The surface force profiles between an atomic force microscopy (AFM) tip and a self-assembled monolayer of noctadecyltrichlorosilane on oxidized silicon were measured in an ABA triblock series of silicone and Pluronic surfactant solutions. Changes in steric force barrier thickness and height were examined as a function of hydrophobic and hydrophilic block size and ethanol to water ratio in mixed solvents. In water, all the surface force profiles exhibited repulsive forces except for the Pluronic surfactants with short polyoxyethylene blocks. The steric barrier thickness increased with increasing hydrophilic block size, with an estimated force decay length proportional to (number of oxyethylene units) 0.62. However, the same data yielded no clear correlation between the hydrophobic chain block size and the steric barrier. The silicone surfactants provided steric repulsion up to 80% ethanol in solution, while the Pluronic surfactants lost their surface activity at approximately 40% ethanol in solution. The difference between the Pluronic surfactants and the silicone surfactants is attributed to the polysiloxane block being hydrophobic as well as oleophobic. In addition, the solubility and surface activity of silicone surfactants in alcoholic media are tunable by the ratio of oxyethylene to oxypropylene units.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2012

The catastrophic inversion process from an oil-continuous to a polar phase-continuous emulsion wa... more The catastrophic inversion process from an oil-continuous to a polar phase-continuous emulsion was investigated with a system consisting of silicone oil, water and a nonionic surfactant. The polar phase involved an aqueous micellar solution, an inverse micellar solution, a hexagonal liquid crystal and a lamellar liquid crystal. Results indicated that a suitable polar phase structure with a positive spontaneous curvature was required for the formation of multiple emulsions leading to inversion at a relatively low dispersed phase fraction. Differences observed in the inversion pattern and the resultant drop size were explained in terms of the rheology of the polar phase and interfacial tension. The inverted emulsions were gel emulsions exhibiting similar viscoelastic behavior regardless of the specific polar phase structure. The same viscoelastic behavior was also exhibited in a lamellar liquid crystal (but not in a hexagonal liquid crystal) suggesting a common response mechanism to shear.