Author Archive

Four Academic Generations

February 25, 2014 Leave a comment

Four Academic Generations

Bob Bird, Bob Armstrong, Gareth McKinley, and Shelley Anna at the Bob Bird 90th Birthday Symposium in the Department of Chemical and Biological Engineering, University of Wisconsin-Madison. January 30-31, 2014.

Chris Nelson won Best Teaching Assistant Award!

May 12, 2013 Leave a comment

Congratulations!  Chris Nelson won the Best Teaching Assistant Award for Mechanical Engineering!  The award is for Spring 2013, for Chris’s work in 24-231, Fluid Mechanics.  Chris will be receiving the award at the Mechanical Engineering Commencement Ceremony.

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Flow Focusing Paper is in the top 50 cited papers in 50 years of Applied Physics Letters!

September 18, 2012 Leave a comment
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Anna Receives Grant to Study Oil Dispersants-Mechanical Engineering – Carnegie Mellon University

August 23, 2012 Leave a comment

We’ve been highlighted on the CMU Website.  See here for more info and a link to the University press release:

Anna Receives Grant to Study Oil Dispersants-Mechanical Engineering – Carnegie Mellon University.

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Predicting conditions for tipstreaming

August 2, 2012 Leave a comment

Our paper entitled “Predicting Conditions for Microscale Surfactant-Mediated Tipstreaming,” by T.M. Moyle, L.M. Walker, and S.L. Anna, has been accepted for publication in Physics of Fluids.  UPDATE: The paper is now published and can be accessed at this link, and cited as: Phys. Fluids 24, 082110 (2012), DOI: 10.1063/1.4746253

Predicted operating diagram for Microscale Tipstreaming

Abstract: Microscale tipstreaming is a unique method to overcome the limiting length scale in microfluidics allowing for production of submicron sized droplets.  Tipstreaming is the ejection of small drops from a liquid thread formed by interfacial tension gradients and convective transport of surfactant.  Controlling and understanding this process is essential for successful application in areas such as synthesis of nano-scale particles, manipulation of biomolecules, enzyme activity studies and others.   However, models that predict operating conditions for microscale tipstreaming do not currently exist.  In this work, we develop a semi-analytical model aimed at capturing the essential physics of the tipstreaming mechanism.  The model relies on interfacial shape observations indicative of microscale tipstreaming to simplify the fluid flow and surfactant transport equations.  The result is an interfacial mass balance of surfactant.  Conditions where the mass balance can be satisfied define the operating conditions for microscale tipstreaming.  Results from the model are compared with our own experimental results. Good agreement is found between model predictions and experiments.  Scaling of each boundary that controls the feasible tipstreaming region is given.  Finally, the model is able to guide selection of device geometry and surfactant properties to shift or expand the feasible region where microscale tipstreaming is expected.

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Adsorption of colloidal particles to bubbles in microchannels

August 2, 2012 Leave a comment

Our paper entitled “Probing timescales for colloidal particle adsorption using slug bubbles in rectangular microchannels,” by A.P. Kotula and S.L. Anna, has been published in Soft Matter (DOI:10.1039/C2SM25970B).

The flow of long bubbles in microchannels containing a surface-active particle suspension leads to unique bubble dynamics that arise due to particle adsorption at the gas-liquid interface.

Abstract: The adsorption of particles to fluid-fluid interfaces is a key step in the generation of colloidosomes and particle-stabilized emulsions. Microfluidic channels are a promising tool for generating particle-stabilized drops and bubbles with independent control over the bubble size and the concentration of particles adsorbed at the fluid interface. In this paper, we present experimental observations of the adsorption of a nanoparticle-surfactant suspension to confined bubbles translating along a microchannel. Long bubbles exhibit a unique two-lobed shape that is linked to the adsorption of surface-active particles to the interface at a timescale comparable to the residence time in the channel. An accompanying decrease in the bubble velocity results from the added viscous drag at the bubble interface. We develop a transport model to describe the rate of particle adsorption to the interface and find good agreement between the model estimates of bubble shape changes and experimental observation. The formation of the two-lobed shape is due to a difference in the velocity between the front and rear of the bubble, which can promote bubble break-up.

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Depletion of surfactants in finite volumes

June 21, 2012 Leave a comment

Our paper entitled  “A Criterion to Assess the Impact of Confined Volumes on Surfactant Transport to Liquid-Fluid Interfaces,” by N.J. Alvarez, L.M. Walker, and S.L. Anna, has been accepted for publication in  Soft Matter. UPDATE: The article has been published online as of July 23, 2012.

When surfactants adsorb to interfaces in finite geometries, equilibrium surface properties and transport timescales are strongly influenced by the geometry.

Abstract: When dissolved surfactant adsorbs at an interface, the bulk concentration decreases. If the initial concentration is low or the interfacial area large, the concentration decrease can be significant, and the solution depleted. Although depletion is not a new phenomenon, properly accounting for it requires a global mass conservation constraint in addition to a mass transport model. The emergence of new applications involving adsorption in finite volumes and with large surface areas, including micro- and nanoscale droplet formation, has introduced new scenarios in which depletion can be significant but complex to analyze. The purpose of this paper is to develop simple criteria to allow practitioners in these applications to rapidly and easily assess the potential impact of depletion. We use a global mass balance to show that two dimensionless parameters fully describe the role of depletion in both equilibrium surface properties and timescales to reach equilibrium. The dimensionless parameters represent the potential mass lost to the interface, denoted ζ, and the surface activity of the surfactant, denoted f. Characteristic transport timescales are shown to be a function of the finite geometry. A scaling analysis is developed for the case of surfactant dissolved inside a spherical drop, and compared with that of a finite spherical shell. The analyses developed here lead to simple criteria that are useful even when the surfactant properties are not well characterized or a full transport analysis is difficult. The criteria can be generalized to adsorption at solid surfaces.

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