Hao Huang's Research
1. Optical Coherence Tomography (OCT)-Gravimetry-Video method for studying the drying process of waterborne latex
Latex, a colloidal system of polymer particles suspended in water, is consumed in large quantities each year as waterborne coatings on various material substrates. During drying of latex, particles distribute inhomogeneously in space, compromising the final quality of the coating. To understand the mechanism of drying inhomogeneity, we integrate Optical Coherence Tomography (OCT) with gravimetric and video analysis (called “OCT-Gravimetry-Video” method) to monitor the internal structure, the water evaporation rate and the appearance of a latex coating simultaneously. Hard polystyrene particles with two different particle sizes (135 nm and 53 nm in diameter) were used as model latexes. Cracks and drying fronts were recorded by video. Packing of particles, debonding of film and shear bands were observed in reflection image and speckle variance image of OCT. Drying rate was constant and close to that of deionized water before cracking or debonding. The effects of particle size on the packing and cracking phenomena were profound and unexpected. This method provides a non-invasive and non-destructive approach to study the drying process of latex, especially with the OCT imaging on the cross-section of coating that traditional methods can hardly achieve.
2. Skin Layer Formation during Drying of Latex Films
Waterborne latex coatings for protection and waterproofing purposes require film thicknesses in the millimeter range. During drying of such thick latex films, the top surface of the film can get dried early and form a transparent polymer skin while beneath it is still wet and opaque. Skin layer impedes water evaporation and thus extends the film drying time considerably. To study the mechanism of skin layer formation, optical coherence tomography (OCT), analytical balance and video camera were integrated together (called “OCT-Gravimetry-Video” method) to simultaneously monitor the film’s thickness and internal structure, the weight loss by water evaporation, and the visual appearance of film as a function of drying time. Three drying stages for skin layer formation were observed: I) packing of particles, II) compaction of particles and III) formation of skin. In Stage I, particles accumulate and become packed from the top to the bottom of the film, and the packing layer thickness increases linearly with time; the drying rate during this stage is equal to that of pure water. In Stage II, packed particles become further consolidated or compressed while the drying rate is still close to that of pure water. In Stage III, particles on the top of the film become deformed and coalesced (polymer chains in adjacent particles inter-diffused) into a transparent skin, whose thickness increases with about square root of time until the film is totally dried; the drying rate during this stage is dramatically reduced and determined by the permeability and thickness of skin. In coating applications, it is anticipated to lose as much water as possible before the skin impedes the drying rate. Some water-soluble additives (such as surfactants, that were mixed with latex) were found to lower the water content trapped in film beneath the skin. OCT also showed that surfactant additive can accelerate growth of skin thickness and the whole film becomes transparent earlier. We propose that: an additive, which can retard coalescence between deformed particles, makes the skin permeable in the initial short time period of skin formation. This allows more water evaporation before deformed particles become fully coalesced. This study is expected to provide a guideline in additive selections to solve the drying problem of thick latex films.
3. A novel dielectrophoresis potential spectroscopy for colloidal nanoparticles (DOI: 10.1002/elps.201700049)
Dielectrophoresis (DEP) has been widely used to manipulate nanoparticles in microfluidic applications. However, determination of the DEP force of nanoparticles by theoretical models is not easy due to complications caused by the polarization of electrical double layer. Additionally, there is a lack of suitable experimental techniques to quantify the DEP force of nanoparticles. This article reports a statistical mechanics-based experimental method to determine the DEP potential energy of a single particle by measuring the equilibrium number density of particles in a DEP force field. Results show that at high frequencies, the measured potentials agree with the Maxwell-Wagner-O’Konski (MWO) theory. At frequencies lower than the crossover frequency, the measured potential values are larger than MWO theory’s predictions. When an effective particle radius (particle radius plus Debye length) is used to replace the particle radius, MWO theory fits better with the measured potentials on both sides of crossover frequency. Also, the measured crossover frequency was found inversely proportional to the effective particle radius, which agrees with MWO theory. The new DEP potential spectroscopy is not limited to the size or shape of particles, opening doors to investigate the DEP response functions of quantum dots and proteins in an alternating current electric field.
