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Research Papers: Fundamental Issues and Canonical Flows

The Role of Contact Line (Pinning) Forces on Bubble Blockage in Microchannels

[+] Author and Article Information
Mahshid Mohammadi

School of Mechanical, Industrial,
and Manufacturing Engineering,
Oregon State University,
204 Rogers Hall,
Corvallis, OR 97331
e-mail: mahshid@lifetime.oregonstate.edu

Kendra V. Sharp

School of Mechanical, Industrial,
and Manufacturing Engineering,
Oregon State University,
204 Rogers Hall,
Corvallis, OR 97331
e-mail: kendra.sharp@oregonstate.edu

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received April 21, 2014; final manuscript received November 4, 2014; published online January 14, 2015. Assoc. Editor: Ali Beskok.

J. Fluids Eng 137(3), 031208 (Mar 01, 2015) (7 pages) Paper No: FE-14-1213; doi: 10.1115/1.4029033 History: Received April 21, 2014; Revised November 04, 2014; Online January 14, 2015

This paper highlights the influence of contact line (pinning) forces on the mobility of dry bubbles in microchannels. Bubbles moving at velocities less than the dewetting velocity of liquid on the surface are essentially dry, meaning that there is no thin liquid film around the bubbles. For these “dry” bubbles, contact line forces and a possible capillary pressure gradient induced by pinning act on the bubbles and resist motion. Without sufficient driving force (e.g., external pressure), a dry bubble is brought to stagnation. For the first time, a bipartite theoretical model that estimates the required pressure difference across the length of stagnant bubbles with concave and convex back interfaces to overcome the contact line forces and stimulate motion is proposed. To validate our theory, the pressure required to move a single dry bubble in square microchannels exhibiting contact angle hysteresis has been measured. The working fluid was de-ionized water. The experiments have been conducted on coated glass channels with different surface hydrophilicities that resulted in concave and convex back interfaces for the bubbles. The experimental results were in agreement with the model's predictions for square channels. The predictions of the concave and convex back models were within 19% and 27% of the experimental measurements, respectively.

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Figures

Grahic Jump Location
Fig. 1

Bubbles with different wetting conditions: (a) dry bubble, (b) consecutive images of a hybrid bubble, and (c) lubricated bubble. Adapted from Ref. [21].

Grahic Jump Location
Fig. 2

Dry stationary bubbles in a polycarbonate microchannel array where clear channels act as a bypass for the flow. Upon stagnation small droplets start to condense and grow on the channel walls inside the saturated bubbles. (a) and (b) were taken upon stagnation and 17 min after, respectively.

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Fig. 3

Pinning forces resisting motion under the action of a piston in a circular tube. Adapted from Ref. [19].

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Fig. 4

(a) Side view of a stationary bubble and forces applied on it by the pressure field and the channel walls, (b) and (c) channel cross section at the left contact line with and without liquid in the corner regions, respectively, note that the dotted border is not a part of the triple contact line but rather a gas–liquid interface, and (d) a moving bubble with a convex back interface

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Fig. 5

Progression of bubble shape with increasing velocity: (a) dry dynamic bubble, (b) thin film forming, (c) lubricated bubble, and (d) bullet-shaped lubricated bubble. Adapted from Ref. [28].

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Fig. 6

Experimental setup for pressure measurements

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Fig. 7

Variation in the contact angles during the motion, (a) and (b) are images of one bubble and (c) and (d) are images of another. (a) concave back interface, (c) convex back interface, and (b) and (d) nearly flat back interfaces and unequal front contact angles at the side walls.

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Fig. 8

Variation in pressure difference across the length of a crawling bubble over time which is due to pinning and variation in the local surface conditions and contact angles

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Fig. 9

Comparison between the model's predictions and the measured pressures for the six sets of experiments. There could be a large difference between the concave and convex back model predictions.

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Fig. 10

Comparison between the measured pressure and the matching models' prediction. There are four irregular occurrences that have a slightly convex back interface but match the concave back model's prediction better.

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Fig. 11

Normalized models' predictions based on the measured pressure

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