Additive manufacturing (AM) techniques and so-called 2D materials have undergone an explosive growth in the past decade. The former opens multiple possibilities in the manufacturing of multifunctional complex structures, and the latter on a wide range of applications from energy to water purification. Extrusion-based 3D printing, also known as Direct Ink Writing (DIW), robocasting, and often simply 3D printing, provides a unique approach to introduce advanced and high-added-value materials with limited availability into lab-scale manufacturing. On the other hand, 2D colloids of graphene oxide (GO) exhibit a fascinating rheology and can aid the processing of different materials to develop ‘printable’ formulations. This work provides an in-depth rheological study of GO suspensions with a wide range of behaviours from Newtonian-like to viscoelastic ‘printable’ soft solids. The combination of extensional and shear rheology reveals the network formation process as GO concentration increases from <0.1 vol% to 3 vol%. Our results also demonstrate that the quantification of ‘printability’ can be based on three rheology parameters: the stiffness of the network via the storage modulus (G′), the solid-to-liquid transition or flow stress (σ_f), and the flow transition index, which relates the flow and yield stresses (FTI = σ_f/σ_y).

Pressure-driven laminar and turbulent flow in a horizontal partially filled pipe was investigated using stereoscopic particle imaging velocimetry (S-PIV) in the cross-stream plane. Laminar flow velocity measurements are in excellent agreement with a recent theoretical solution in the literature. For turbulent flow, the flow depth was varied independently of a nominally constant Reynolds number (based on hydraulic diameter, $D_{H}$ ; bulk velocity, $U_{b}$ and kinematic viscosity $\unicode[STIX]{x1D708}$ ) of $Re_{H}=U_{b}D_{H}/\unicode[STIX]{x1D708}\approx 30\,000\pm 5\,\%$ . When running partially full, the inferred friction factor is no longer a simple function of Reynolds number, but also depends on the Froude number $Fr=U_{b}/\sqrt{gD_{m}}$ where $g$ is gravitational acceleration and $D_{m}$ is hydraulic mean depth. S-PIV measurements in turbulent flow reveal the presence of secondary currents which causes the maximum streamwise velocity to occur below the free surface consistent with results reported in the literature for rectangular cross-section open channel flows. Unlike square duct and rectangular open channel flow the mean secondary motion observed here manifests only as a single pair of vortices mirrored about the vertical bisector and these rollers, which fill the half-width of the pipe, remain at a constant distance from the free surface even with decreasing flow depth for the range of depths tested. Spatial distributions of streamwise Reynolds normal stress and turbulent kinetic energy exhibit preferential arrangement rather than having the same profile around the azimuth of the pipe as in a full pipe flow. Instantaneous fields reveal the signatures of elements of canonical wall-bounded turbulent flows near the pipe wall such as large-scale and very-large-scale motions and associated hairpin packets whilst near the free surface, the signatures of free surface turbulence in the absence of imposed mean shear such as ‘upwellings’, ‘downdrafts’ and ‘whirlpools’ are present. Two-point spatio-temporal correlations of streamwise velocity fluctuation suggest that the large-scale coherent motions present in full pipe flow persist in partially filled pipes but are compressed and distorted by the presence of the free surface and mean secondary motion.

Aqueous solutions of graphene oxide (GO) with a very large average flake aspect ratio, AR = O ( 10 4 ), are probed using capillary breakup experiments and shear rheological measurements for concentrations up to ϕ = 0.4 vol . %. GO, a chemically oxidized sheet of graphene that we obtained from graphite exfoliation, is suspended in distilled water to produce solutions with volume concentrations spanning 0.02 ≤ ϕ ≤ 0.4 vol . %. For concentrations between 0.1 ≤ ϕ ≤ 0.4 vol . %, the apparent shear yield stress, τ y (determined from creep tests), scales with concentration following a power-law relationship with exponent ∼ 2.2. Using capillary breakup extensional rheometry (CaBER), we demonstrate that for concentrations ϕ ≳ 0.1 vol . %, aqueous GO solutions exhibit power-law fluid behavior in uniaxial elongation. The deformation-rate-dependent transient Trouton ratio over the range where the deformation rates match in shear and capillary breakup experiments is independent of the deformation rate, but inversely proportional to the volume concentration with all values exceeding 3. An attempt is made to estimate the apparent elongational yield stress (for ϕ ≥ 0.3 vol . %) by using the Laplace pressure within the fluid filament as its surrogate, and we find that in the strain rate regime that we can observe, the apparent elongational yield stress, τ y , e, is not 3 times the shear yield stress, i.e., for our aqueous GO solutions, we are not able to observe agreement with the von Mises plasticity criterion. GO samples at a given concentration probed in capillary breakup extensional rheology experiments under different protocols, i.e., slow-retraction opposed to step-stretch, exhibited different responses confirming that GO is highly sensitive to the deformation history; however, they still exhibit power-law fluid behavior even given the different initial conditions. Although slightly limited in our ability to quantify the extension-thinning yield-stress-like behavior of aqueous GO in uniaxial elongational flow by the fact that we cannot prescribe the strain rates in CaBER experiments, we demonstrated that even a very small amount of GO flakes can significantly alter the shear rheology and the pinch-off dynamics of a liquid bridge in comparison to Newtonian fluids of equivalent shear viscosity.Aqueous solutions of graphene oxide (GO) with a very large average flake aspect ratio, AR = O ( 10 4 ), are probed using capillary breakup experiments and shear rheological measurements for concentrations up to ϕ = 0.4 vol . %. GO, a chemically oxidized sheet of graphene that we obtained from graphite exfoliation, is suspended in distilled water to produce solutions with volume concentrations spanning 0.02 ≤ ϕ ≤ 0.4 vol . %. For concentrations between 0.1 ≤ ϕ ≤ 0.4 vol . %, the apparent shear yield stress, τ y (determined from creep tests), scales with concentration following a power-law relationship with exponent ∼ 2.2. Using capillary breakup extensional rheometry (CaBER), we demonstrate that for concentrations ϕ ≳ 0.1 vol . %, aqueous GO solutions exhibit power-law fluid behavior in uniaxial elongation. The deformation-rate-dependent transient Trouton ratio over the range where the deformation rates match in shear and capillary breakup experiments is independent of the d...

Recent direct numerical simulations (DNS) and experiments in turbulent channel flow have found intermittent low- and high-drag events in Newtonian fluid flows, at $R{e}_{\tau }=u_{\tau }h/\nu$ between 70 and 100, where $u_{\tau }$, h and $\nu$ are the friction velocity, channel half-height and kinematic viscosity, respectively. These intervals of low-drag and high-drag have been termed “hibernating” and “hyperactive”, respectively, and in this paper, a further investigation of these intermittent events is conducted using experimental and numerical techniques. For experiments, simultaneous measurements of wall shear stress and velocity are carried out in a channel flow facility using hot-film anemometry (HFA) and laser Doppler velocimetry (LDV), respectively, for $R{e}_{\tau }$ between 70 and 250. For numerical simulations, DNS of a channel flow is performed in an extended domain at $R{e}_{\tau }$ = 70 and 85. These intermittent events are selected by carrying out conditional sampling of the wall shear stress data based on a combined threshold magnitude and time-duration criteria. The use of three different scalings (so-called outer, inner and mixed) for the time-duration criterion for the conditional events is explored. It is found that if the time-duration criterion is kept constant in inner units, the frequency of occurrence of these conditional events remain insensitive to Reynolds number. There exists an exponential distribution of frequency of occurrence of the conditional events with respect to their duration, implying a potentially memoryless process. An explanation for the presence of a spike (or dip) in the ensemble-averaged wall shear stress data before and after the low-drag (or high-drag) events is investigated. During the low-drag events, the conditionally-averaged streamwise velocities get closer to Virk’s maximum drag reduction (MDR) asymptote, near the wall, for all Reynolds numbers studied. Reynolds shear stress (RSS) characteristics during these conditional events are investigated for $R{e}_{\tau }$ = 70 and 85. Except very close to the wall, the conditionally-averaged RSS is higher than the time-averaged value during the low-drag events.