(6ep) Control of Slip at the Fluid-Surface Interface Using Molecular Additives

The fluid behavior in a confined geometry is dictated by the boundary conditions at the fluid-wall interface. From the macroscopic point of view, fluid is immobile at a solid wall that is known as the no-slip boundary conditions. This assumption shows shortcomings at microscopic length scales, where the interaction between the wall and the fluid plays an important role in determining the fluid motion. At a rough wall, fluid velocity is the same as the wall velocity (i.e. stick condition), while at a smooth wall, the fluid velocity is larger than the wall velocity that is called slip condition. The slip between fluid/surface and fluid/fluid interfaces occurs in various applications such as polymer processing, and microfluidic and nanofluidic devices. To create wall slip, short hydrocarbons with functional groups, such as fatty amides, are used as additives in polypropylene processing. It is been reported in the literature that a very low concentration (500 ppm) of these molecules is required for the surface modification. A possible scenario for this phenomenon is that these small molecules migrate to the vicinity of the wall, and form an interphase (boundary layer) that has a low cohesive energy with the polymer phase. The underlying mechanisms of this process are obscure, and I intend to focus on the experimental and computational design of molecular additives that can alter the slippage of large molecules, such as polymers, in confined geometry. I will be using atomistic and coarse-grained simulations techniques, and experiments to characterize the mechanisms of slip at the fluid-surface and fluid-fluid interphases of polymeric systems that are discussed briefly:

Molecular Design of slip modifiers: In order to understand the mechanisms of the slip and effect of different chemical groups in the slip modifiers (additives), atomistically-detailed molecular dynamics (MD) simulations will be used. MD simulations will provide the details of microstructure and local dynamics of different components in the interphase of the polymer and boundary layer.

Coarse-grained model for polymer–boundary layer–wall: Given that the timescale of the atomistic simulations is very limited (<0.1 μs), the shear rates in the atomistic simulations is above 10⁷ s^-1, which is extremely larger than accessible shear rates observed in experiments. To tackle this problem, a coarse-grained model will be developed to simulate the rheology of polymer melts in the presence of small molecules in a confined geometry. The interaction between small molecules and polymer melt will be estimated from the atomistic simulations. Results will be used to determine a model for the slip velocity as a function of the shear stress in these systems.

Interaction between polymer and additive molecules: To validate the coarse-grained model results, a surface force apparatus (SFA) will be used to quantify the cohesive energy between the small molecules and the polymer melt. These results will be used to enhance the accuracy of the atomistic simulations at the interphase of the boundary layer and polymer phase. Furthermore, using rheometry experiments, the rheology of the polymer under shear flow will be determined and compared with the numerical data obtained from the coarse-grained models.

Teaching Interests:

I am excited to teach any core course in the undergraduate curriculum of chemical engineering, and I want to develop graduate elective courses on simulation. I will develop an interactive teaching method for students by using a computer-based teaching method. Using programming languages such as MATLAB or POLYMATH, students will learn how to apply different computational tools to solve not only problems that they face in the class, but also various intricate modeling problems that emerge in chemical engineering.

In particular, I look forward to teaching numerical analysis, process control and design courses that involve mathematical modeling and simulation. An advanced project-based course that covers molecular and Monte-Carlo simulations with a focus on complex fluids, such as ILs and biomolecules will be created as an elective course.

Research Background:

I have been fortunate to work on various research topics during my graduate studies in the Khare research lab at Texas Tech University. I started my graduate research by investigating the structural and rheological properties of dilute polymer solutions of different chain architectures. Later on, I created rigorous atomistically-detailed models for determining the volumetric, structural, and rheological properties of neat and polymer modified asphalts. I showed that master curve can be constructed from simulation values of viscoelastic properties, thus this increases the range of time and frequency scales accessible to simulations by a few decades and facilitates a comparison with experiments. The simulations were able to capture the sub-glass transition temperature relaxation of the model structures that has been reported in experimental studies of glass former liquids. I have also closely collaborated with an experimental group in order to develop novel polymeric materials that can be utilized as the membrane in pervaporation applications. During the latter project, I demonstrated that the dynamics of solvent molecules (water and ethanol) are closely correlated with dynamics of cross-linked polymer regardless of the solvent concentration and this dynamic coupling could be used as an additional factor in designing polymeric materials for pervaporation based separations. In addition, in a collaborative study and, we showed that MD simulations can capture the experimental trend of the rheology of imidazolium-based ILs and is able to connect the macroscopic properties to the microstructure of ILs.

During my post-doctoral appointment in the Bonnecaze research group at the University of Texas (UT) at Austin, I am developing models and simulations of magnetically-driven flows of dilute suspensions of magnetite particles. In this study, we aim to model and simulate the drug delivery to a fully occluded blood vessel using a magneto-rheological fluid that is caused by a rotary magnetic field with a gradient. I also study the rheological properties of soft particle glasses. In particular, I found that the polydispersed soft particles glasses, which are jammed beyond the random close packing volume fraction, can form crystalline structures in the presence of shear flow. In addition, I found that the macroscopic properties, such as shear stress, normal stress differences, and long-time diffusion coefficient of the SPGs can be extracted from the microscopic dynamics of the particles. I have also created atomistic models to study the interface of calcite and water in the presence of oil and surfactant molecules. This project is of interest to oil recovery applications in which the interfacial properties of a reservoir play a crucial role in determining the efficiency of the process. By varying the surfactant coverage, the evolution of surfactant morphologies near the calcite surface is being studied. The current study is being extended to simulate oil-surfactant-calcite interactions to investigate mechanisms of surfactant-induced wettability alteration. Furthermore, in a collaborative MRSEC project at the Center for Dynamics and Control of Materials at UT, we have created two-dimensional network models of germanium nanowire to study the mechanical properties under tension, compression, and shear. In addition, I have been involved to develop a Brownian dynamics simulations tool to study the dynamics and rheology of colloidal gels in shear flow.

In summary, I will apply these skills (atomistic and coarse-grained MD simulations of polymers/ionic liquids, microscale simulations of suspensions, and continuum mechanics) to other problems in the field of engineering to understand the mechanisms underlying different phenomena that occur at nano- and micro-length scales.