Pinch-off of a viscous suspension thread

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 eta(0) and surface tension sigma. 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(min), are studied for varied particle size d, volume fraction empty set and liquid contact angle theta. Two successive regimes are identified: (i) a first effective-viscous-fluid regime which only depends upon empty set and (ii) a subsequent discrete regime, depending both on d and empty set 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, eta(e)similar to-sigma(/)h(min), which agrees closely with the steady shear viscosity measured in a conventional rheometer and diverges as (empty set(c) - empty set(c))(-2) at the same critical particle volume fraction, empty set. For empty (sic) 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(min) equivalent to h* similar to d (empty set(c) - empty set)(-1/3), down to h(min) approximate to d, where the thinning rate recovers the value obtained for the pure interstitial fluid, empty set/eta(0) and lasts t* similar to eta(e)h*/sigma.