arXiv subjects

Lucas Warwaruk

Publications and source records attributed to Lucas Warwaruk.

4 recordsLinked to original sources

arXiv2026

PocketCaBER and PocketDoS: Low-cost open-source tools for teaching and learning advanced topics in fluid mechanics

We describe two open-source, 3D-printable, flexure-based tools for the quantitative measurement of extensional properties of viscoelastic fluids. These low-cost, portable, and scalable devices (which we have termed ``PocketCaBER'' and ``PocketDoS'') are particularly applicable for use in the field and in graduate-level teaching environments due to their low cost, printability on hobby 3D printers, compatibility with cell phone cameras, portability and user-friendly operation. We characterize and benchmark each device's performance against its lab-equivalent counterpart and provide downloadable STL files for rapid fabrication. We discuss experimental limitations of these devices compared with their bench-top counterparts. Finally, we illustrate the use of such tools in facilitating student engagement in polymer science and complex fluids classes---specifically, how progress in learning goals can be uniquely and effectively accelerated by providing the necessary rheological instruments directly to each individual student (especially for advanced modules such as nonlinear extensional rheology). By giving students personal, indefinite access to laboratory-level instrumentation through these open-source frugal science tools, we discuss our efforts to expand participation and engagement within the field of nonlinear rheology.

physics.ed-ph↗

arXiv2025

DoS Dos and Don'ts

Dripping-onto-Substrate (DoS) rheometry is a well-established method for measuring the extensional rheology of low-viscosity liquids. However, clear guidelines on the capabilities and limitations of the technique are lacking. In the present work, we define operational limits for measuring a transient extensional viscosity directly from observation of the rate of filament thinning, as well as model-based bounds on calculating a viscosity $\eta$ and extensional relaxation time $\tau_E$ of a liquid using DoS. Dilute solutions of polyethylene oxide (PEO) and polyacrylamide (PAM) are used to probe the lower limit of measurable $\tau_E$, demonstrating that values as low as 0.1 ms can be resolved, provided (a) the intrinsic Deborah number (based on the ratio of the relaxation time and the Rayleigh breakup time scale) is $De \geq \mathcal{O}(0.1)$ and (b) an instrumental constraint related to spatial and temporal resolution is satisfied. This instrumental constraint is quantified through a new metric we define as the \textit{filament capture rate}, a ``figure of merit'' (expressed in Hz) that can be used to quantify the number of data points within the elasto-capillary regime that are available for extraction of $\tau_E$. We also investigate the sensitivity to other experimental parameters including variations in nozzle radius and Bond number ($Bo$). Across the tested range ($0.2 < Bo < 0.7$), extensional relaxation times for the same fluid vary by less than $\pm16$ \%; however, experiments with low viscosity fluids at $Bo > 0.5$ exhibit damped gravitational oscillations that affect early-time dynamics. Collectively, these results provide a quantitative roadmap for reliable DoS rheometry and affirm its use for measuring sub-millisecond relaxation times in weakly elastic fluids.

cond-mat.soft↗

arXiv2025

Capillary currents and viscous droplet spreading

We present the results of a combined experimental and theoretical study of the spreading of viscous droplets over rigid substrates. First, we experimentally investigate the wetting of a roughened glass surface by a viscous droplet of silicone oil, wide and shallow relative to the capillary length $\ell_c$. The horizontal radius of the droplet grows according to an $R_\mathrm{drop}\sim t^{1/8}$ scaling reminiscent of viscous gravity currents (Lopez et al. 1976). The droplet is preceded by a mesoscopic fluid film that percolates through the rough substrate, its radius increasing according to $R_\mathrm{film}\sim t^{3/8}/(\log t)^{1/2}$. To rationalize these observed scalings, we develop a new 'capillary current' model for the spreading of shallow droplets with arbitrary radius on rough surfaces. Furthermore, on the basis of established similarities between droplet spreading over wetted rough and smooth substrates (Cazabat & Cohen Stuart 1986), we argue its relevance to a broader class of spreading problems. We propose that, throughout their evolution, shallow droplets maintain a quasi-equilibrium balance between hydrostatic and curvature pressure, perturbed only by unbalanced contact line forces arising along the droplet's edge. For drops with horizontal radii small with respect to $\ell_c$, our model converges to the original description of Hervet & de Gennes (1984) and thereby recovers the classic spreading laws of Hoffman (1975), Voinov (1976), and Tanner (1979). For drops wide with respect to $\ell_c$, it rationalizes why millimetric, surface-tension-driven capillary currents exhibit the same spreading behavior as relatively large-scale viscous gravity currents.

physics.flu-dyn↗

arXiv2021

Near-wall lubricating layer in drag-reduced flows of rigid polymers

The current theories on the mechanism for polymer drag-reduction (DR) are generally applicable for long-chain flexible polymers that form viscoelastic solutions. Rigid polymer solutions that generate DR seemingly lack prevalent viscoelastic characteristics. They do, however, demonstrate larger viscosities and a noticeable shear-thinning trend, well approximated by generalized Newtonian models. The following experimental investigation scrutinized the flow statistics of an aqueous xanthan gum solution in a turbulent channel flow, with friction Reynolds numbers ($Re_τ$) between 160 and 680. The amount of DR varied insignificantly between 28% and 33%. The velocity field was measured using planar particle image velocimetry and the steady shear rheology was measured using a torsional rheometer. The results were used to characterize the flow statistics of the polymer drag-reduced flows at different $Re_τ$ and with negligible changes in DR; a parametric study only previously considered by numerical simulations. Changes to the mean velocity and Reynolds stress profiles with increasing $Re_τ$ were similar to the modifications observed in Newtonian turbulence. Specifically, the inner-normalized mean velocity profiles overlapped for different $Re_τ$ and the Reynolds stresses monotonically grew in magnitude with increasing $Re_τ$. Profiles of mean viscosity with respect to the wall-normal position demonstrated a thin layer that consists of a low-viscosity fluid in the immediate vicinity of the wall. Fluid outside of this thin layer had a significantly larger viscosity. We surmise that the demarcation in the shear viscosity between the inner "lubricating" layer and the outer layer cultivates fluid slippage in the buffer layer and an upward shift in the logarithmic layer; a hypothesis akin to DR using wall lubrication and superhydrophobic surfaces.

physics.flu-dyn↗