As engineered systems below the micron scale become more viable for biological and chemical analysis and detection, the boundary condition at the liquid-solid interface plays an increasingly important role in fluid flow and transport of nanoparticles. Using quantum dot (QD) imaging as a tool for interrogation of flow and nanoparticle transport and diffusion within nano channels, we utilize a combined experimental and computational approach based on molecular dynamics (MD) simulations to investigate the fundamental principles of liquid flow, diffusion, and confinement in nano channels. Specifically, the aim is to interrogate the solid-liquid slip boundary condition based on the diffusive motion of QD nano-particles and at the same time answer the following questions: (1) How does confinement affect the thermal diffusive motion of a nanoparticle in equilibrium and in the presence of shear flow? (2) How do surface characteristics (nanoroughness and wetting) affect the slip flow in a confined geometry and the local mobility of a nanoparticle? Answers to these questions are important to designing chemo- and bio-sensing technologies that involve micro- and nano-fluidics.
Intellectual Merit: The proposed research is focused on addressing the fundamental questions regarding the solid-liquid boundary condition by using the measurement of the thermal motion of nanoparticle in a nano channel as a sensitive method of probing the degree of fluid slip at the surface. The quantum dot imaging method proposed here introduces a powerful approach for obtaining quantitative information about flow and transport within nano channels. The integration of experiments and molecular dynamics simulations is unique, in that the simulations are used to help interpret the experimental results by identifying and isolating the various physical effects that influence the fluid flow and the dynamics of QD nanoparticles.
Broader Impacts: The experimental and computational methods developed in this proposal will be beneficial to a broad range of researchers in physics and engineering who investigate and wish to control fluid transport properties at micro and nano scales. In many chemo- and bio-sensing applications in microfluidics, the detection of a chemo/bio agent is limited by its transport towards the sensor located at the channel wall. The fundamental knowledge gained from the proposed work will directly impact the design of methods to address these detection limits. An important impact of this work will be the education of two PhD students, along with the early involvement of undergraduate students in research. The technical focus of the PIs and their approaches to computational and experimental nano-scale research will educate our students by providing a broad interdisciplinary exposure to nano-scale flow physics, modern experimentation, large-scale computations, and optical diagnostics. In addition to educating graduate students as future educators and researchers, our program will include undergraduate student researchers each summer. The PIs plan to give a two-day workshop on their nano-scale research to mid-Michigan high school science teachers enrolled in MSU?s teacher certification program, to provide them with ideas for stimulating their own high-schoolers in the current thrusts of modern research and how they relate to basic science and engineering. The quantum dot imaging methods developed in this research program will add educational material to our existing graduate level course in experimental methods in fluid dynamics. A new graduate course will be designed to educate our students in computational methods based on Monte Carlo and molecular dynamics. The scientific progress of the proposed research will be disseminated through technical conferences and journal publications. In addition, we plan to develop a website for this project with simple simulations and explanation of nano-scale flows.
Motion of nanoparticles under confinement is of particular interest for biological and chemical analysis and detection at small scales where transport of fluid mixtures is severely hindered. In such flows, the boundary condition at the liquid-solid interface plays an important role on the overall transport of fluid or nanoparticles in solution. The purpose of this project was to study the liquid-solid boundary condition and assess the impact of different surface conditions on the mobility of nanoparticles. In this research nanoparticles (about 100 nm in diameter) were imaged directly inside a confined space with a known gap height ranging from 200 nm to about 2 micron. By tracking individual nanoparticles over a sufficiently long time scales, the corresponding diffusion coefficient was measured. The results show a significant reduction in diffusivity of the particles as the gap size shrinks. The diffusion of nanoparticles was almost undindered when the gap size was 20 times larger than the particle diameter but it was reduced to about 70% of free diffusion value when the gap height was about twice the particle size. The effect of surface hydrophobicity was also a factor that was studied as it has impications on the slip boundary condition at the surface. By conducting the nanoparticle imaging measurements under confinement with similar gap heights but different surface hydrophobicity, it was observed that particle diffusivity is less hindered when the surface is hydrophobic. Using the ratio of diffusion measured over hydrophobic and hydrophilic surfaces, a slip length on the order of particle size (50 nm in this case) was estimated. This means that the boundary over hydrophobic surface is expected to have this additional slippage over a hydrophilic one. Using molecular dynamics simulations, we also studied interfacial diffusion and slip flows at surfaces with anisotropic textures of different wettability. It was found that the angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the stripe width is larger than several molecular diameters. Perhaps most interestingly, we find that the directional diffusion coefficient of fluid molecules in contact with patterned substrate correlates well with the effective slip length as a function of the shear flow direction with respect to the texture orientation. These findings lend support for the microscopic justification of the tensor formulation of the effective slip boundary conditions for noninertial flows of Newtonian fluids over smooth surfaces with nanoscale anisotropic textures. Findings of this research were presented at the annual meetings of the American Physical Society and are being prepared for journal publications.
