The AnnaLab Website

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.

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.

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.