Author Archive

We attend the 86th ACS Colloid and Surface Science Symposium

June 15, 2012 Leave a comment

A large group of students and faculty from the Center for Complex Fluids Engineering attended the ACS Colloid and Surface Science Symposium at Johns Hopkins this past week.  From the Anna Lab, talks were given by Anthony Kotula, Todd Moyle, Ying Zhang, and Shelley Anna.  Shelley co-chaired the Microfluidics sessions with German Drazer.  Anthony Kotula was selected to talk in the Langmuir Student Awards session.

Denise, Aditya, Lynn, Shelley, Anthony, Matt, and Todd

Categories: AnnaLab Blog

Toroidal Focal Conic Defects in Shear Flow

June 7, 2012 Leave a comment

Our paper entitled “Interaction of Toroidal Focal Conic Defects with Shear Flow,” by S. Chatterjee and S.L. Anna, has been published in Soft Matter, 8 (2012) 6698 – 6705.

Simple shear flow applied to anchored toroidal focal conic defects in smectic liquid crystals leads to the formation of satellite defects with sizes controlled by the size of the main defect.

Abstract: Toroidal focal conic defects form in smectic liquid crystals due to antagonistic surface anchoring conditions. The resulting layer structure, defect size, and the formation of ordered patterns of defects have been studied in detail. Here, we investigate the effect of shear flow on toroidal focal conic defects. The defects are formed and subjected to a steady linear Couette flow in a microscale shear cell. In situ visualization of the evolution of the polarization texture of individual defects reveals the impact of three distinct flow regimes on the layer structure. At low Ericksen numbers, individual defects exhibit an early time elastic regime in which the optical intensity of the layer structure does not change in the presence of the applied shear flow. At increasing applied strains, the defect layer structure deforms, resulting in a monotonically increasing optical intensity within its original footprint. At even larger applied strains, satellite defects are emitted from the anchored defect. Cessation of shear flow at moderate strains, prior to the formation of satellite defects, leads to an exponential decay of the polarization intensity of the defect, suggesting that relaxation of the layer structure occurs with a time constant that is consistent with that predicted by a permeation flow mechanism. This study establishes the relevant length and time scales involved in the interaction of shear stresses and elastic stresses in smectic liquid crystalline samples whose structure deviates significantly from an idealized aligned lamellar configuration.

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Commencement 2012

May 20, 2012 Leave a comment

Commencement 2012

Ozdoganlar, Anna, and Leduc

Categories: Images & Video

Gulf of Mexico Research Initiative Consortium Formed

May 16, 2012 Leave a comment

From the CCFE Website: CFE faculty Anna and Walker have intitated a new project to quantify the interfacial aspects of dispersants used to treat oil spills in marine environments. A three year project will characterize the impact of components of dispersants on model oil-water systems.

The project is funded through the  Consortium for the Molecular Engineering of Dispersant Systems (C-MEDS) awarded to Tulane University.




Categories: AnnaLab Blog

Interfacial dynamics of polymer-grafted nanoparticles

May 1, 2012 Leave a comment

Our paper entitled “Interfacial Dynamics and Rheology of Polymer-Grafted Nanoparticles at Air-Water and Xylene-Water interfaces,” by N.J. Alvarez, S.L. Anna, T. Saigal, R.D. Tilton, and L.M. Walker, was published in Langmuir, 28 (2012) 8052 – 8063.

Dynamics of polymer grafted nanoparticles adsorbing at oil/water and air/water interfaces are characterized along with mechanics of the resulting particle‐laden interfaces.

Abstract: Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene–water and air–water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air–water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.

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Trader Fellowship Reception

April 9, 2012 Leave a comment

Photos of “Team Anna”!

Categories: Images & Video

Tuning bubbly structures in microchannels

April 6, 2012 Leave a comment

Our paper entitled “Tuning Bubbly Structures in Microchannels,” by S.M. Vuong and S.L. Anna, was published in Biomicrofluidics, Special Issue on Multiphase Microfluidics, Guest Ed. Saif Khan, 6 (2012) 022004.

Bubble shapes and structures organized in terms of volume fraction and bubble size

Abstract: Foams have many useful applications that arise from the structure and size distribution of the bubbles within them. Microfluidics allows for the rapid formation of uniform bubbles, where bubble size and volume fraction are functions of the input gas pressure, liquid flow rate, and device geometry. After formation, the microchannel confines the bubbles and determines the resulting foam structure. Bubbly structures can vary from a single row (“dripping”), to multiple rows (“alternating”), to densely packed bubbles (“bamboo” and dry foams). We show that each configuration arises in a distinct region of the operating space defined by bubble volume and volume fraction. We describe the boundaries between these regions using geometric arguments and show that the boundaries are functions of the channel aspect ratio. We compare these geometric arguments with foam structures observed in experiments using flow-focusing, T-junction, and co-flow designs to generate nitrogen bubbles in surfactant-stabilized water and water droplets in surfactant-stabilized oil. The outcome of this work is a set of design parameters that can be used to achieve desired foam structures as a function of device geometry and experimental control parameters.

Categories: AnnaLab Blog