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Pinch-off of a viscous suspension thread
Published online by Cambridge University Press: 03 August 2018
Abstract
The pinch-off of a capillary thread is studied at large Ohnesorge number for non-Brownian, neutrally buoyant, mono-disperse, rigid, spherical particles suspended in a Newtonian liquid with viscosity $\unicode[STIX]{x1D702}_{0}$ and surface tension $\unicode[STIX]{x1D70E}$. Reproducible pinch-off dynamics is obtained by letting a drop coalesce with a bath. The bridge shape and time evolution of the neck diameter, $h_{\mathit{min}}$, are studied for varied particle size $d$, volume fraction $\unicode[STIX]{x1D719}$ and liquid contact angle $\unicode[STIX]{x1D703}$. Two successive regimes are identified: (i) a first effective-viscous-fluid regime which only depends upon $\unicode[STIX]{x1D719}$ and (ii) a subsequent discrete regime, depending both on $d$ and $\unicode[STIX]{x1D719}$, in which the thinning localises at the neck and accelerates continuously. In the first regime, the suspension behaves as an effective viscous fluid and the dynamics is solely characterised by the effective viscosity of the suspension, $\unicode[STIX]{x1D702}_{e}\sim -\unicode[STIX]{x1D70E}/{\dot{h}}_{\mathit{min}}$, which agrees closely with the steady shear viscosity measured in a conventional rheometer and diverges as $(\unicode[STIX]{x1D719}_{c}-\unicode[STIX]{x1D719})^{-2}$ at the same critical particle volume fraction, $\unicode[STIX]{x1D719}_{c}$. For $\unicode[STIX]{x1D719}\gtrsim 35\,\%$, the thinning rate is found to increase by a factor of order one when the flow becomes purely extensional, suggesting non-Newtonian effects. The discrete regime is observed from a transition neck diameter, $h_{\mathit{min}}\equiv h^{\ast }\sim d\,(\unicode[STIX]{x1D719}_{c}-\unicode[STIX]{x1D719})^{-1/3}$, down to $h_{\mathit{min}}\approx d$, where the thinning rate recovers the value obtained for the pure interstitial fluid, $\unicode[STIX]{x1D70E}/\unicode[STIX]{x1D702}_{0}$, and lasts $t^{\ast }\sim \unicode[STIX]{x1D702}_{e}h^{\ast }/\unicode[STIX]{x1D70E}$.
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- © 2018 Cambridge University Press
References
Château et al. supplementary movie 1
Pinch-off of a capillary bridge of a suspension of particles with a diameter d = 10 micrometers and a particle volume fraction phi = 48%, suspended in pure PEG (close to 2500 times as viscous as water). In order to compensate for the continuously increasing rates of deformation at the bridge neck, the time in the movies is increasingly slowed down as pinching proceeds. The duration, t_0 - t, remaining before the pinch-off is indicated at the top. The width of the image is 4.45 mm.
Château et al. supplementary movie 2
Pinch-off of a capillary bridge of a suspension of particles with a diameter d = 135 micrometers and a particle volume fraction phi = 50%, suspended in pure PEG (close to 2500 times as viscous as water). In order to compensate for the continuously increasing rates of deformation at the bridge neck, the time in the movies is increasingly slowed down as pinching proceeds. The duration, t_0 - t, remaining before the pinch-off is indicated at the top. The width of the image is 4.45 mm.
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