Active Research

I am actively involved in research in a number of different areas including: the dynamics of complex fluids; laminar and turbulent drag redution; the development and utilization of superhydrophobic surfaces; shear and extensional rheology of a number of different complex fluids; non-Newtonian fluid dynamics; microfluidics; nanotechnology; non-isothermal flows; hydrodynamic stability; and polymer processing. Below you will find a number of short examples of the active research in my group along with links to the corresponding publications and graduate students responsible for the work.


Extensional Rheology of Wormlike Micelle Solutions

We are currently investigating the behavior of complex fluids such as entangled wormlike micelle solutions in transient homogeneous uniaxial extensional flows. In our experiments, a filament stretching rheometer is used to follow the evolution of the tensile stress and the flow induced birefringence of a series of CTAB/NaSal and CPyCl/NaSal wormlike micelle solutions in an extensional flow. The wormlike micelle solutions demonstrate significant strain hardening and a failure of the stress-optical law. These fluid filaments have also been found to exhibit a very interesting failure mechanism. At a critical stress, nearly independent of strain rate, the wormlike micelle solution filaments fail through a dramatic rupture near the axial midplane, shown here through a series of high speed images. This filament failure is not the result of elastocapillary thinning, but appears similar to the ductile failure of an elastic solid. We believe that this filament failure stems from the local scission of individual wormlike micelle chains resulting in a dramatic failure of the entangled micelle network. Our recent work has shown that branching in wormlike micelle solutions eliminates extensional thickening.

To view a high speed movie showing the failure of the fluid filament please download the following avi file: ctab05_1s.avi (3.8MB)

Relevant articles can be found on our publications page.

 

Formation of Shear Band structure in Wormlike Micellar Solutions

In conjuction with extensional experiments, we are also exploring the interesting shear banding behavior in solutions of wormlike micelles. CPyCl/NaSal wormlike micelle solutions of various concentration have been explored in shear rheology, and those in the semi-dilute regime are being studied further in a specially designed large Couette cell. This Couette was constructed with clear optical access for PIV and FIB measurements in situ. Velocity and birefringince measurements indicate the simultaneous co-existence of bands of fluid at both high and low shear levels. This phenomenon is confirmed by the existence of a plateau in the shear stress over at least an order of magnitude in shear rate.

To view a movie of a startup flow (shear rate = 8s^-1), download the following avi file: CPyCl-100mM-8startup.avi (6.5MB)

Relevant articles can be found on our publications page.

 
Flow Around a Sphere of a Wormlike Micellar Solution


Many of the flows experienced by wormlike micellar solutions are complex, containing regions of both shear and strong extensional flows. We focus the flow of a wormlike micellar solution past a sedimenting sphere, a prototypical complex flow. The test fluid is a 0.05M CTAB/ 0.05M NaSal solution which is characterized both in shear and transient homogeneous uniaxial extension. A single sphere-to-tube aspect ratio (a/R = 0.0625) is investigated over a wide range of Deborah numbers. As the Deborah number is increased, the drag correction factor, K(a/R, De), is initially found to decrease due to the effects of shear thinning near the sphere walls. At a Deborah number of approximately De = 1.0, the drag correction factor reaches a minimum and begins to increase with increasing Deborah number as a result of the strong extensional flow in the wake of the sphere. At a critical Deborah number, the sedimentation of the sphere becomes unsteady.


Flow visualization, particle image velocity (PIV), Birefringence measurements are used to analyze flow fields for both the steady and unsteady sedimentation of the sphere. An attempt is made to correlate the unsteady behavior of the sphere with the extensional rheology measurements. Currently, we are very interested in the instability when the Tungsten ball sediments.

Relevant articles can be found on our publications page.

                                                 
Flow of a Wormlike Micelle Solutions through Porous Media

       

 

We are currently investigating the behavior of complex fluids (specifically wormlike micelle solutions) as they flow though a porous medium. This type of flow field is very common in many day to day situations: how does shampoo flow through your hair? Additionally, these flows have important industrial applications such as oil well drilling and recovery and groundwater remediation. In order to study the kinetics and kinematics of such flows, we use an idealized porous media: a periodic array of cylinders. By seeding the test fluid with microscopic reflective spheres, using a laser for illumination, capturing the fluid motion with a high speed camera, and using a computer algorithm, we can generate full field velocity profiles and streamline images. Additionally, the test fluids we use are birefringent under flow. Accordingly, we can use polarized light and a camera to capture and process images highlighting areas of high stress within the fluid. By combining these techniques with pressure drop measurements, we are able to explore the behavior of complex fluids under controllable conditions. Our measurements show a reduction in pressure drop as the fluid shear thins and enhancement in the pressure drop as the extensional flow begins to dominate the kinematics. At large rates, the flow becomes unstable for some, but not all of the wormlike micelle solutions tested.

Relevant articles can be found on our publications page.

Laminar Drag Reduction and Enhanced Mixing Using Superhydrophobic Surfaces

 

Superhydrophobic surfaces contain micron or nanoscale hydrophobic surface structures which result in very large contact angles.. These surfaces are fabricated with photolithography technology to ensure the precision of the surface pattern’s geometries and arrangements. In channel flow, a shear-free air-water interface is formed between the surface structures which provide some considerable slip velocity on the hydrophobic surface. By putting these surfaces into micro channels and mixing cells in different sizes, drag reduction and mixing enhancement were achieved in laminar flows. Experiment results demonstrated up to 40% of pressure drop reduction and 20 micron slip length with the superhydrophobic surface on the bottom of the channel. The drag reduction effect was increased by optimize the surface patterns’ designs, and the effect is more obvious in smaller channels. The slip velocity along the superhydrophobic surface has been directly measured with micro-PIV. The maximum slip velocity at the center of the shear-free interface was increased to 60% of the average velocity by increase the air-water interfaces.

The superhydrophobic surfaces with microridges designs were tested as a passive mixing enhancement technology in the y-shape mixing cells. The off axial slip velocity formed secondary flows which stretched and folded the two species met in the microchannel. Confocal microscopy was used to image the wholescale flow behavior and the non-dimensionlized intensity was used to represent the degree of mixing. In the case of Pe>300, mixing length has been dramatically reduced compare with the smooth surface. The mixing effects of different pattern designs, such as the microridges with incremental spacing and angles to the flow direction, were studied and compared with full scale numerical simulation models.

Relevant articles can be found on our publications page.
     
Turbulent Drag Reduction Using Superhydrophobic Surfaces

Superhydrophobic surfaces, consisting of micro or nanoscale features on a hydrophobic substrate, have received considerable attention for their ability to reduce drag in laminar flows by trapping air between peaks in the surface topology, producing a shear free interface over a considerable portion of the surface (See past work of Jia Ou). In the present research we demonstrate that engineered, micropatterned superhydrophobic surfaces produce the same effect in turbulent flows. Particle image velocimetry, a direct velocity measurement technique that correlates successive images of tracer particles in the flow, is used to measure velocity profiles over conventional and superhydrophobic surfaces in turbulent channel flow. Pressure drop measurements are also conducted to directly measure the drag in the channel. The scope of the project includes designing and constructing the surfaces and apparatus used in the experiments. A variety of photolithography, etching and embossing techniques have been utilized. PIV measurements have measured slip velocities up to 40% of the mean flow with shear stress reductions up to 60% on a single wall and slip lengths of 230µm over several microridge geometries. Drag reduction is found to increase with increasing feature size and spacing, as in laminar flows, and with increasing Reynolds number. These experimental results were subsequently confirmed and extended to a host of new superhydrophobic geometries through Direct Numerical Simulations (DNS). Successful implementation and commercialization of superhydrophobic surfaces would result in a significant drag reduction and subsequent fuel savings for marine vessels.

Relevant articles can be found on our publications page.

 

  

Effect of Slip on Flows Past Superhydrophobic Cylinders and Hydrofoils

 

In addition to experiments in turbulent flows through superhydrophobic channels. We are currently studying the effect of superhyrophobic surfaces and slip on the flow past bluff bodies like cylinders, hydrofoils and boats. Superhydrophobic surfaces are found to have a major impact on both the lift and drag of hydrofoils and on the vortex shedding dynamics from a cylinders.

Relevant articles can be found on our publications page.

 

 

  
Novel Techniques for Creating Superhydrophobic Surfaces - Sanding Teflon  
We have found that by sanding Teflon with different grit sand papers we can produce surfaces with a range of different wetting behavior. These include high advancing contact angle and high hysteresis surfaces (petal effect -Wenzel state), high advancing contact angle low hysteresis surfaces (lotus effect - Cassie state) and everything in between. The SEM to the left includes shows the results from sanding Teflon with increasing sandpaper grit number.
  

 

  

Digital Microfluidics Using Superhydrophobic Surfaces

 

Digital (one drop at a time) microfluidics is largely understood in enclosed devices. A simpler approach would be to perform task in an enclosed device on an open surface, specifically drop positioning, movement, mixing, and sorting. By utilizing the water-repellent properties of superhydrophobic surfaces, we are investigating the ability to perform all of those tasks on a flat surface. We have shown that by sanding Teflon, it is possible to create superhydrobic surfaces with a range of advancing contacts angles with water and also contact angle hysteresis. In maintaining a constant advancing contact angle, we can characterize the effect of contact angle hysteresis on the dynamics of drop coalescence via high speed photography. We can also use differences of contact angle hysteresis in various orientations to deflect drops, and also sort drops in terms of Weber number. Sorting has been achieved at an optimal Weber number and angle of contact angle hysteresis change of over 1 drop diameter (see image to the right).

Relevant articles can be found on our publications page.

Capillary Force Lithography
Capillary actions of melted polymers can be used to drive large-area printing of smallscale features. This low-cost, high efficiency, high resolution fabrication technique has great potential in micro- and nano- scale fabrication of electronic, optical, and mechanical devices.

Our understandings of capillary action and more generally equilibrium in surface-tension-dominated regime is far from thorough. When looking at the meniscii formed in oddly shaped cavities, such as a capillary with triangular or star-shaped cross-sections, one is likely to be fascinated by the richness of geometries, and more importantly, the physics that leads to them. Here we use an open source software, the Surface Evolver (SE), to simulate the shape of the equilibrium meniscii formed in various capillaries. This study has applications in modern micro-nano contact lithography.

We are also interested in the condensation of liquid droplets on micro-posts. The efficiency of a condensation device is largely related to the amount of liquid surface tension can support, which is in turn deternmined by the three-dimensional geometry of the post. Simulations based on SE is also performed to investigate what types of goemetries (e.g., a square pyramide frustum) are benefitial to maximizing the suported volume of the liquid droplet.

Relevant articles can be found on our publications page.


  

 

Fabrication of Hierarchical Structured Surfaces

 

Hierarchical structured surfaces observed in nature have greatly inspried scientists and engineers in resent years. In this project, our goal is to better understand the physics behind many seemingly peculiar hydrodynamical behaviors associated with patterned surfaces, based on which we can develop novel devices with various desirable properties. Currently, dual-scale elcetron beam lithography (EBL) is used to fabricate hierarchical superhydrophobic surfaces with well-defined micro-structures and regular nano-features. A typical hierarchical structure consists of an array of 10 micron square blocks with 10 micron spacing as the primary structure and 100~200nm rigdes or posts with 200~500nm spacing covering the entire area as the secondary structure. Important geometric parameters including the size, shape, spacing, and aspect ratio of the primary as well as the secondary features can be control relatively independantly over a rather large range. We are also very interested in studying droplet wicking on a hierachical superhydrophilic surfaces consist of an array of micron-size ridges as primar features decorated with micron size bands with nano-size patterns perpendicular to the primary ridges. The fabrication techniques developed in this project is potentially applicable to many other reseach fields as well.

Relevant articles can be found on our publications page.
                                               
Drop Formation and Deformation in Microfluidic Devices

Micro and nanofluidics are finding increasing use in the toolbox of nanotechnology because these systems are driven by the effects of surface tension, diffusion and viscosity allowing us to probe new phenomena. Microfluidic devices can be used to create a variety of differently shaped interfaces between two immiscible fluids, such as jets, drops and planar/parallel interfaces.


Microfluidics droplet production is a well understood process with several methods for droplet creation commonly used. In most recent work, the co-flowing stream method of droplet generation was used to create droplets of deionized water and four aqueous surfactant solutions in oil. The morphological developments of the drops in extensional flow at various extension rates were studied using a hyperbolic contraction. This novel approach to droplet deformation within a microfluidic device allowed for the probing of droplets within a nearly uniform extensional flow. The focus of this work was to study the effect of confinement on droplet deformation and breakup in extensional flows. Droplet deformation was found to increase with both increasing capillary number and increasing confinement, with the effect of confinement being quite dramatic. The addition of surfactant to the droplets resulted in the production of tails, which streamed from the rear of the droplets and produced daughter droplets much smaller than the parent droplet. In the low confinement limit where the flow was purely extensional, a single tail was formed at the center of the droplets trailing edge. With enhanced confinement, shear effects from the wall became important, the droplets were observed to take on a bullet like shape and two tails formed at the trailing edge of the droplet. The critical value of the capillary number and confinement needed for the formation of tails varied with the surfactant used.

Relevant articles can be found on our publications page.

    

Extensional Flow Induced Crystallization of Polymer Melts Using a Filament Stretching Rheometer

We are investigating the effect of uniaxial flow on the crystallization of a number of different polymers. An extensional rheometer with a custom-built oven is used to stretch molten samples at various extension rates and strains. The filaments are then allowed to crystallize fully before being removed.

Differential Scanning Calorimetry (DSC),is used to measure the crystallinity of the samples and compare to an unstretched reference. The crystallization of the polymer is then characterized as a function of extension rate and strain. Small Angle X-ray Scattering (SAXS) and microscopy through crossed polarizers is used to qualitatively examine the change in crystal size, quantity, and structure.

We find that a minimum extension rate is required for flow induced crystallization to occur and that there is an extension rate for which flow induced crystallization is maximized. This trend is similar across multiple different polymers studied in the past. A maximum increase in crystallization of 19% from the quiescent case was observed for polypropylene. The increase in crystallinity is likely due to the flow induced orientation and alignment of tubes of constrained polymer chains in the flow direction.

Relevant articles can be found on our publications page.


  

Sharkskin Instability in Polymer Extrusion

Sharkskin is a surface roughness caused during the extrusion of several polymers such as linear low-density polyehtylene (LLDPE) and polybutadiene (PBD). First reported in the 1960's, this instability limits the speed at which the polymer can be extruded, thereby increasing the cost and energy required to make a viable product. The image to the right is a typical extrudate strand showing the sharkskin surface instability.

  

Although this phenomenon has been studied thoroughly in the past 40 years, the focus of our research is to further identify and observe the mechanism that causes this instability and subsequently reduce or delay its onset. Current industrial processes use fluoropolymer polymer processing additives (PPA) to minimize the effects of sharkskin. This is a very successful method, however in several applications, the addition of fluoropolymer is undesirable in the final product. Our research hopes to address this issue by more thoroughly analyzing the instability in terms of basic rheological properties of the polymer.

For the experimental work, we have fabricated a primitive, but purpose-built extruder that uses pressurized nitrogen to force the polymer through interchangeable capillary dies. A range of instrumentation including pressure transducers and thermocouples, along with visual obersvation using a CCD camera will be used to monitor the extrudate.

Relevant articles can be found on our publications page.

 
                                               

Drop Impacts on a Thin-Film of Surfactant Solution      

We are studying the impact dynamics of water drops on thin films of viscoelastic wormlike micelle solutions composed of a surfactant, cetyltrimethylammonium bromide (CTAB), and a salt, sodium salicylate (NaSal), in deionized water. We are modifying the composition and thickness of the thin film is modified to investigate the effect of fluid rheology on the evolution of crown growth, the formation of satellite droplets and the formation of the Worthington jet. Some pictures of a typical drop impact are shown above and movies can be accessed below. The size, velocity, composition and number of the impacting drops are varied to study the relative importance of Weber, Ohnesorger and Deborah numbers and multiple impact on the resulting dynamis dynamics. The addition of elasticity to the thin film fluid is found to suppress the crown growth and the formation of satellite drops with the largest effects observed at small film thicknesses. Additionally, a plateau is observed in the growth of the maximum height of the Worthington jet height with increasing impact velocity. It is postulated that the complex behavior of the Worthington jet growth is the result of a dissipative mechanism stemming from the scission of wormlike micelles.

Relevant articles can be found on our publications page.

    

 
                                                 
Nanoparticle Encapsolation of Viscelastic Jets
   

We have developed a number of ways to form long-lived cylindrical jets of a viscoelastic fluid using hydrodynamic focusing in a microfluidic device. A solution of polyacrylamide in water is driven coaxially with immiscible oil and subjected to strong extensional flow. At high flow rates, the aqueous phase forms jets that are 4 to 90 microns in diameter and several centimeters long. The liquid surfaces of these jets are then used as templates for assembly of microspheres into novel rigid and hollow cylinders.

Relevant articles can be found on our publications page.
Fluid Webs: Tearing of Viscoelastic Films

 

We have performed a detailed series of experiments investigating the atomization viscoelastic fluid films formed by commercial nozzles and impinging jets. Images using still and high speed photography were obtained and analyzed for a wide range of flow strengths and fluid rheology. The elasticity of the fluid delayed the atomization of the sheet and the break-up of the rim resulting in the formation of a new flow structure we call 'fluid webs.'

Download our poster from APS (2MB):

Watch a high speed movie of the fluid web formation (4MB)

Relevant articles can be found on our publications page.

 

 

  
                                                 

The Axisymmetric Abrupt Contraction-Expansion


 



The image to the left is a pseudo streak image of the extensional flow of a highly elastic solution through an axisymmetric 4:1:4 contraction-expansion. The fluid is a 0.025wt% high molecular weight monodisperse polystyrene dissolved in an oligomeric polystyrene. A pressure drop up to five times larger than the pressure drop for a similar Newtonian fluid is observed.

  
    • A movie showing the growth of the enhanced upstream vortex structure and the onset of an elastic instability, click on the avi file:  4_1_4.avi (8MB)
    • A movie showing the lip vortex growth structure for the 2:1:2 contraction-expansion, click on the avi file:  2_1_2.avi (5MB)
    • A movie showing a dramatic jetting instability just above the entrance to the contraction plane, click on the avi file:  jetting_instability.avi (7MB)

Relevant articles can be found on our publications page.

 

Thermal Modification of Elastic Instabilities

The image on the right shows the secondary flow developed between a rotating cone-and-plate rheometer after the onset of a purely elastic flow instability.

In the research described in the 2001 Physics of Fluids paper found on the publication page, viscous heating is employed to modify the stability of the flow of a high molecular weight polystyrene solution.

When the characteristic timescale for viscous heating is much longer than the relaxation time of the test fluid (Na^0.5/De << 1) the critical conditions for the onset of the elastic instability are in good agreement with the predictions of isothermal linear stability analyses. As the thermoelastic number approaches a critical value, the strong temperature gradients induced by viscous heating reduce the elasticity of the test fluid and delay the onset of the instability. At even larger values of the thermoelastic parameter, viscous heating stabilizes the flow completely.

To see a movie of the secondary flow within the gap between a rotating cone-and-plate rheometer click on the avi flile, cone-and-plate.avi (6MB).

Relevant articles can be found on our publications page.
                                                 
 

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Site last updated on December 22, 2010