Session L11: Experimental Techniques: Microscale

Microfluidic Study of Temperature Dependent Phase Changes in Aqueous Aerosol Systems

Atmospheric aerosols are suspensions of small liquid or solid particles suspended in air which play a major role in climate change, either directly by absorption and scattering of solar irradiation or indirectly by cloud formation. They are complex multiphase fluid systems and experience a broad range of relative humidities and subzero temperatures in the atmosphere affecting their phase state i.e. well-mixed liquid, phase separated or crystallized solid. In situ observation of relative humidity and temperature of aerosol droplets are difficult and expensive. In this work, low cost microfluidic devices are used to generate and study phase states of aqueous droplets of real aerosols and chemical mimics in static and flow-through devices at subzero temperatures. The static device dehydrates trapped droplets in quasi-equilibrium to study the effect of relative humidity and water loss on phase state at different temperatures. The high-throughput flow-through microfluidic device measures ice nucleation temperature of aerosols. The results will be used to predict the cloud and ice formation activity of atmospheric aerosols.   

Droplet Nucleation and Condensation on a Hydrophilic Surface.

Dropwise condensation is a ubiquitous phenomenon in nature. Dropwise condensation has the potential to improve the efficiency of condensing surfaces and reduce the maintenance costs of systems. However, efforts to design and fabricate surfaces that can sustain long-term dropwise condensation have not been successful. The main reason is that the nucleation physics, which are key to understanding degradation, behind dropwise condensation are not fully understood; thus, researchers have mostly relied on a trial and error approach for developing new surfaces. In this work, a series of fundamental experiments were done to identify the governing mechanism of dropwise condensation on smooth hydrophilic surfaces by probing the solid-vapor interface during phase-change. The results evaluate the existence and structure of the thin film and initial nuclei that develop during condensation. The adsorption kinetics theory is used to improve understanding of the dropwise condensation mechanism during droplet formation at the onset of condensation.   

Measurement of biofilm stresses in laminar flows by a digital holography interferometry (DHI) and an embedded wrinkle free thin-film polymer mirror

Bacteria are unicellular microorganisms that commonly exist in either planktonic and biofilm lifestyles. Biofilms have viscoelastic nature and are subject to deformation under external disturbances (e.g. fluid sheer stress). Here we developed a technique to perform in-situ measurement of viscous stresses exerted biofilm under different flow velocities. The experiments are performed in a uniquely developed microfluidic platform composed of a flexible thin-film mirror embedded in\textbf{ }Polydimethylsiloxane (PDMS), that performs as a stress sensitive substrate. This 30nm aluminum thin film is sandwiched between two PDMS layers, and is free of wrinkles and cracks in addition to being specular reflective. The microfluidics is attached to a chemostat and two peristaltic pumps that continuously flow bacterial suspensions in close-loop to facilitate biofilm growth and generate flow shear. DHI measures nano-strain of the thin-film and consequently stresses via finite element modeling of thin film.   

Off-axis digital holographic microscopy (DHM) as a method of high temporal and spatial resolution flow visualization

Off-axis digital holographic microscopy is a developing optical modality that is capable of achieving diffraction limited resolution on the sub-micron spatial scale at high temporal resolution and across a large depth of field. By using two coherent beams of light that become recombined at the optical detector, off-axis of each other, the 3D optical information of the sample becomes encoded in the resulting interference pattern (hologram). Numerical processing of the hologram allows the volumetric reconstruction of the sample. The temporal resolution is limited by the frame rate of the camera used to record the holograms and thus dynamic processes such as fluid flows can be imaged and visualized by seeding the flow with appropriately sized particles. Our current implementation of an off-axis DHM is capable of diffraction limited resolution of 780 nm across an interrogation volume of 365x365x900 cubic microns. Hardware and software developments in off-axis DHM show this optical modality to be a promising method of high throughput flow visualization on the microscale.   

Structured-illumination microscopy to improve the spatial resolution of microscale particle velocimetry

The spatial resolution of optical methods in microchannel flows such as microscale particle image velocimetry ($\mu$PIV) is often limited because the entire flow volume is illuminated, and signal from tracers beyond the focal plane affects the measurement. Structured-illumination microscopy (SIM), originally developed for optical sectioning of stationary objects ({\it e.g.} cells) in fluorescence microscopy, is a promising way to acquire planar slices of the flow with a thickness comparable to the depth of field of the imaging system. SIM reconstructs the signal from the focal plane using multiple ``raw" images of the flow illuminated by a sinusoidally varying ({\it i.e.}, structured) intensity distribution at different phases using (spatial) frequency mixing. Initial results are presented here for microscale particle velocimetry using SIM images reconstructed from two raw images of laminar Poiseuille flow seeded with fluorescent polystyrene microparticles through a microchannel. The velocities obtained using SIM-based particle velocimetry are compared with results obtained from ``standard" $\mu$PIV obtained with volume illumination in the same flow, and used to estimate the spatial resolution of this new technique.   

A Novel Transparent Sediment Simulant for Unveiling the Bed Topography and Interstitial Processes

Particle Image Velocimetry (PIV) and Planar Laser-Induced Fluorescence (PLIF) are two commonly used and powerful laboratory experimental methods for whole-field velocity measurements and flow visualization. When they are coupled with refractive index matching (RIM), they can map both velocity fields and porous media architecture. We present a study that utilizes RIM coupled-PIV and RIM coupled-PLIF methods to not only quantify the flow within a packed bed of irregular shaped grains but also to map the internal structure of the porous media. We use a fluoro-carbon polymer with a specific gravity of 1.97 and optical properties similar to that of water as a simulate for sediment grains within a flow cell. These irregular grains, varying in shape and size, provide a structure that may simulate stream bed sediment and other porous media of granular materials. Here, we present the first experiments and discuss the image processing of both PIV and PLIF experiments, the image quality necessary for mapping the grain bed, and its errors and limitations. Our results pave the way for a novel application where both biological (microbial growth) and physical processes can be studied simultaneously.   

Added mass of porous solids oscillating in dense gas

Oscillation frequency of an object submerged in fluid is usually detectably retarded by the mass of co-accelerated fluid. In this study, this added-mass effect is explored for porous solids oscillating in dense gas, using an enclosed spring-mass system at high pressures. Compared with a non-porous solid of the same shape, the added mass of a porous solid is always higher because of pore-residing gas. When the period of oscillation is much greater than the time of viscous relaxation inside the pores, pore-residing gas follows the motion of the porous solid, and its mass can be estimated using the difference in the added mass between porous and non-porous solids. Experiments conducted using Berea sandstone show that masses of pore-residing gas obtained from oscillations were in good agreement with those calculated using pore volume of the sandstone and gas densities. Added mass of porous solids observed from oscillations can therefore serve to give pore volume when gas density is known, or gas density when pore volume is known. Additional added mass was noticed, however, when mesoporous solids (nanoporous silica and rocks) were used with a condensable gas.