Dynamics of Magnetic Microdisks in Rotating Magnetic Fields
M. Tan, H. Song, A. Jander, P. Dhagat, T.W. WalkerIn contrast to conventional materials, composites with aligned particles have enhanced mechanical, magnetic, optical, and thermal properties, which can be applied in high-frequency devices, biological tissue scaffolds, and magneto-optics. The anisotropy of the particles allow them to be aligned by applying an external force. With rod-shaped particles, a constant magnetic field can result in a composite material with uniaxial anisotropy. In our study, we have created a planar anisotropy by aligning disk-shaped particles in a rotating magnetic field. To investigate the dynamics of particles in the rotating magnetic field, we have developed a theoretical model, which is based on Stokes flow of a single magnetic oblate ellipsoidal particle in a rotating magnetic field. The model also provides processing conditions that can decrease the alignment time. Under an external magnetic field, magnetic microdisks will also aggregate, creating chains of particles. We are investigating the chaining dynamics and process conditions that can minimize or slow the chaining. To find the timescale associated with alignment and chaining is crucial for designing the materials.
Song et al. IEEE Trans. Magn. Conf. PP, 99 (2015).
Song et al. IEEE Magn. Lett. 6, 5000304 (2015).
Tan et al. Phys. Fluids. 28, 062004 (2016).
Microrheology of Complex Fluids
M. Tan, Y. Mao, T.W. WalkerNanoparticles and microparticles dispersed in fluids can undergo Brownian motion, which is driven by the thermal energy of the fluids, or active motion, which is driven by an external force. The motion of the particles can be associated with the physical properties of the fluids -- the study of this association is called microrheology. The motion of the particles can be observed and recorded by a microscope and a camera and then later be analyzed by particle tracking algorithms to extract information about the trajectories. The generalized Stokes-Einstein relation (GSER) can be used to find the storage and loss moduli of complex fluids when the particles are undergoing Brownian motion. Beside the thermal-driven motion, we have built magnetic tweezers to actively move magnetic particles. The creep compliance of the complex fluids can be studied by applying an impulse force, and the frequency-associated storage and loss moduli can be studied by applying an oscillatory force. Microrheologic studies are especially important for complex fluids that are heterogeneous such as mucus, as conventional rheology can only give averaged information. For example, the microrheology can give us the information of the “easy” spots of the mucus where particles can travel through it easily, while particles with certain surface chemistry and size can “find” those easy spots. This knowledge is valuable for developing drug-delivery vehicles that can cure diseases such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF).
Extensional Rheology of Weakly Viscoelastic Polymer Solutions
K.A. Marshall, A.M. Liedtke, T.W. WalkerWe strive to develop a description of macromolecular dynamics in extensional flow fields for characterization and mod- eling of dilute polymer solutions. The addition of high molecular weight polymer to solvent, even in dilute amounts, can significantly change a fluid’s response to an extensional flow. This behavior is important when formulating models and assessing the ability to print, spray, jet, and spin complex fluids. Generally, macromolecular solutions exhibit large resistance to stretching deformations known as extensional viscosity. For viscous fluids (>20 mPa·s), a capillary breakup extensional rheometer (CaBER) can be used to characterize the extensional relaxation time and transient extensional viscosity of a solution by quantifying the self-thinning of a stretched liquid bridge formed between two parallel plates. Dilute, aqueous solutions, on the other hand, prove challenging to quantify since the time scale of pinch-off is too short for commercial devices, such as the CaBER, to measure. We are exploring a number of novel techniques, as well as evaluating several previous ideas, to provide a detailed description of low-viscosity elasticity.
Marshall, et al. Exp. Fluids. 58, 6 (2017).
Marshall, Walker. J. Rheol. (2017). Submitted.
Pseudomonas aeruginosa Biofilm Rheology
U. Daalkhaijav, T.W. WalkerBacterial biofilms are one of the most intractable problems facing industries ranging from petroleum to the healthcare industry. This matrix of EPS (extracellular polymer substances) provides a diffusion barrier against antimicrobial agents and provides a protective microenvironment where bacterial cultures can thrive. Pseudomonas aeruginosa is an environmental bacteria that is known for its ability to produce alginate incased biofilm. It can cause major problems in the medical field as an opportunistic pathogen causing recurrent infections in cystic fibrosis (CF) patients and acute infections in burn victims. P. aeruginosa is known to be virulent against other pathogens, and in CF patients experiencing respiratory distress, it is usually the dominant strain. The rheology of a P. aeruginosa biofilm can reveal the mechanical properties that give this biofilm its characteristic resistance to environmental stresses such as antibiotics. We find that P. aeruginosa (PAO1) biofilm is viscoelastic, showing predominantly gel-like behavior, which is likely responsible for P. aeruginosa biofilm robustness in the presence of outside stresses such as pH, temperature, DO (dissolved oxygen), and antibiotics. We are interested in characterizing the rheology of clinically relevant strains of P. aeruginosa biofilm and expand to other clinically or industrially relevant biofilm cultures.
Daalkhaijav, Walker. Appl. Rheol. (2017). Submitted.
Role of Platelets Activation in Colon Cancer Metastasis under Coagulation and Shear
J.L. Sylman, Y. Mao, M. Tan, U. Daalkhaijav, T.W. Walker, P. Dhagat, A. Jander, O.J.T. McCartyCancer metastasis is the process whereby cancer cells separate from the primary tumor mass, enter the vascular or lymphatic circulation, exit into a new tissue, and colonize the invaded microenvironment. Although significant progress has been made in deciphering the molecular and genetic features of epithelial cancers, much is still unknown about the behavior and effects of cancer cells in the fluid phase during transit through the circulatory system. Several lines of experimental evidence suggest that the activation of blood platelets and the coagulation system play a crucial role in the metastatic progression of cancer. While some studies have provided insights into the mechanism of platelet-tumor cells interactions under shear flow, they have been largely performed in purified or plasma systems or using anticoagulated blood. As a consequence, the role of platelet activation in regulating platelet-tumor cell interactions in the presence of physiologically-relevant conditions of coagulation and shear remains poorly understood. Dr. Owen McCarty’s group at OHSU has developed an innovative quantitative platform to study cell-cell interactions and fibrin formation under shear. In collaboration with our group, they have recently used this approach to demonstrate the role of coagulation in promoting the recruitment of colon cancer cells to thrombus under shear. Concurrently, we have built a “magnetic tweezers” instrument to probe the transient rheological properties of transparent complex fluids. Together, we are performing a feasibility study on an innovative quantitative platform to study cell-cell interactions and fibrin formation under shear in whole blood. By using active bead microrheology (magnetic tweezers), we have shown that the transient rheological changes in the coagulation cascade of patients with varying coagulation can be characterized when the clotting material is transparent (i.e., plasma). To extend this effort to whole blood, we are teaming with Drs. Pallavi Dhagat and Albrecht Jander at OSU with funding from the OHSU Knight Cancer Institute to develop the instrumentation design necessary to track the microparticles in real time in an opaque environment. Such capability for magnetic particle imaging would allow real-time transient rheology of the coagulation event to be measured in whole blood with minimal external influences, enabling circulation of cancer cells to be understood in physiologically relevant conditions.
Sylman, et al. Ann. Biomed. Eng. 1 (2016).
Rheological Characterization of Cervical Mucus during the Ovulation Cycle
M.J.E. Domingo, H.F. Oldenkamp, L.L. Han, T.W. WalkerAccording to the United Nations Foundation, complications during pregnancy and childbirth are the leading cause of death in women that are in their reproductive years, while estimating that a minimum of 250,000 maternal deaths could be prevented by improving access to family planning and maternal health services. In collaboration with Leo Han, M.D., at OHSU, our group’s overall research goal is to identify and develop innovative solutions to evaluate a woman’s reproductive health by measuring the properties of their cervical mucus (CM). CM is produced by the secretory cells of the endocervix and can vary at different stages of the estrous cycle as hormones are released. The current WHO procedure for evaluating CM requires expensive resources, such as a microscope, and can be time consuming and difficult to interpret. Our current goal is to develop an inexpensive and simple-to-use method that would allow women, or a doctor in the field, to track changes in CM. Our hope is to build a tool that would not only allow women to assess their own reproductive health but would also raise women’s reproductive health awareness and promote better family planning. Direct beneficiaries of the proposed innovation include women and health care providers, specifically doctors working in rural areas. Outcomes of the proposed innovation include the ability for women to assess their own reproductive health when they do not have access to a medical professional, and the ability for doctors to provide more extensive reproductive care in the field. Indirect beneficiaries include all members of the community that place value in the well-being of their women and children. Thus, the innovation has the potential to generate significant social impacts per dollar invested, not only by improving the health of a specific target group but also by impacting the lives of millions of others via improved family planning and reproductive care.
Algal Suspension Rheology
U. Daalkhaijav, T.W. WalkerResearch into algae is taking off recently due to the duel nature of their impact on our society. Nutrient contaminations from farmlands and waste streams can cause toxic blooms in the surrounding lakes and rivers, but algae also has the potential for improving the energy field as a cheap source of biodiesel. During the biodiesel production process, the raw effluent from the photobioreactors or algal raceways are taken and successively concentrated to a level adequate for chemical or mechanical extraction of the algal lipids. Our preliminary results Cyclotella culture reveal that each concentration of the algal sludge changes the rheological response of the sludge. The frequency sweep of the sludge show gel-like behavior dominated by the elastic modulus, while each doubling of the concentration resulted in an order of magnitude increase in modulus. The algal sludge behaves as a shear-thinning fluid, with the viscosities increasing in magnitude with each doubling of concentration. A small yield stress is associated with the algal sludge, which also increased by an order of magnitude with each successive doubling of concentration. These findings highlight the fact that, when processing the algal sludge, we must take into account the increasing interaction between the diatoms resulting in non-Newtonian viscoelastic behavior that will dictate the optimum method that this concentrated sludge should be treated. We are interested in exploring other strains of algae that are more relevant for biofuel work, where their rheological properties can help guide algal sludge processing.
Cosmetic Foundation Rheology
U. Daalkhaijav, T.W. WalkerMany types of foundations are available on the market today to fit any skin type and for any occasion. Most types that endure can credit their success to how the client perceives the product to act once it is applied. Thus, “skin feel” during application and as it sits on the skin is a crucial parameter to consider when evaluating any cosmetic product. Rheological measurements of cosmetic products allow us to translate the qualitative properties of good skin feel to quantitative evaluation of how the material responds to stress and strain. It also allows us to predict formula stability under different storage conditions. The rheological measurements of various popular foundations show gel-like structure that reforms quickly after shearing. All of the foundations had a high viscosity at storage conditions for formula stability, and they all quickly shear thinned for ease of application. Foundations that were marketed as natural or dewy finish had a low yield stress (~ 5 Pa) and contained higher amount of emollients, while those products marketed as matte finish or for oily skin had a high yield stress (~ 20 Pa), caused by fillers like clay and talc. These properties are metrics regularly used by the cosmetics industry to evaluate the value of these products and help predict costumer perception of skin feel. We are handicapped by the fact that we do not know the exact composition of these products. Therefore, we are interested in industry collaborations that can provide that information to better correlate specific additives with rheological properties and how rheological metrics can help predict evaluation of skin feel.
Interfacial Rheology of Eutectic Gallium Indium (EGaIn)
U. Daalkhaijav, Y. Mengüç, T.W. WalkerEutectic gallium indium alloy (EGaIn) is a liquid metal at room temperature that can fill microchannels to act as electrical sensors and conducting wires for small electronic systems. EGaIn has a yield stress that when surpassed allows the liquid to freely flow to fill the channels and forms stable structure, and it is not known to be toxic like mercury would be. This yield stress of the alloy is caused by the gallium oxide skin that forms on the metal extremely quickly when exposed to air. The oxide skin is assumed to be the dominant structure dictating the rheological response of this alloy. Our results show elastically dominated shear-thinning structure with yield stress of ~0.3 N/m. We want to further explore the behavior of this fluid under strain conditions that closer emulates their use in soft sensors.
R. Cashen, J. Conradt, B. Bodily, W.E. Rochefort, T.W. WalkerThe work on rinsing flows is motivated by the desire to remove colloidal, particulate contaminants in the processing of advanced microdevices. Specifically, we are investigating the discovery that polymeric liquids can be effectively used to eliminate contamination particles without damaging delicate surfaces. To investigate the removal, we study different flow types of various rheological fluids to understand the governing physics that allow for removal. We have shown that the presence of a large elongational viscosity is exploited by local extensional flow gradients to overcome the adhesion of the particulate to the surface. We show that an increase in removal effectiveness exists for the siphoning flow over other flow types, while providing a theoretical explanation for the difference.
Walker et al. J. Rheol. 58, 63 (2014).
3D Printing the Future
W.E. Rochefort, T.W. Walker
Modeling the Interfacial Stress Rheometer
S. Fitzgibbon, E.S.G. Shaqfeh, G.G. Fuller, T.W. WalkerThe interfacial stress rheometer or ISR uses a magnetic needle suspended on an interface to probe the rheological characteristics of the interface. By oscillating the rod, the dynamic moduli of thin films on the interface can be measured, but mathematical theories to interpret the device have been slow to develop as a strong coupling between the stresses in the surface and the bulk subphase exists. We simplified the interpretation of this experiment by introducing new lengthscales and the appropriate scaling analysis, reinterpreting the dimensionless numbers. Our numerical and analytical solutions showed good agreement to published data.
Fitzgibbon et al. J. Rheol. 58, 999 (2014).
T.W. Walker, A.N. Logia, G.G. FullerMost recently, I have been investigating the interaction of inhomogeneous droplets and jets that are miscible with their surroundings, such as glycerol drops falling into water. This basic flow problem is central to numerous industrial and natural processes, where mixing of cleansing liquids and creating biocompatible implants for drug delivery are two prominent examples. We conducted experiments to understand several basic multiphase flow problems involving miscible liquids including the free-surface pendant drops resulting from drop impaction and the dissolution of sessile drops in a miscible bath. Using high-speed imaging of the morphological evolution of the flows, we showed that these processes are controlled by interfacial tensions.
Walker et al. Exp. Fluids. 56, 106 (2015).
T.W. Walker, T.T. Hsu, G.G. FullerA significant effort was necessary to gain an understanding of the rinsing flow type. An impinging jet of water was used to rinse coating fluids of varying rheology to investigate the flow structures of the resulting hydraulic jumps qualitatively and quantitatively. We observed the interactions of the two-fluid system during the transient growth of the flow profile, finding rheological dependencies on the magnitude, velocity, and topography. The presence of shear-thinning of the coating liquid influenced the overall velocity of the radial growth, while determining the geometry of the driving front. The dependence of these results on the local rheology was supported by experiments on Newtonian fluids of various shear viscosities. The presence of elasticity was seen to dampen the disturbances of the hydraulic jump, influence the overall jump height, and vary the radial growth of the jump. These observations were supported by experiments of Boger fluids with varying elasticity. Click here for a video.
Hsu et al. Phys. Fluids. 23, 033101 (2011).
Walker et al. Phys. Fluids. 24, 093102 (2012).
Hsu et al. Exp. Fluids. 55, 1645 (2014).