Neural-Network-based Viscosity Closure for Non-Newtonian Multiphase Flows
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The pith
A neural network trained on rheometer data serves as the viscosity closure inside a Cahn-Hilliard-Navier-Stokes solver and reproduces experimental droplet rise velocities.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
A neural network trained on experimental rheometry data and exported via ONNX produces simulated rise velocities that fall within the experimentally measured spread and a steady-state droplet shape that agrees with observations for the characterized silicone inks when used as the viscosity closure inside a Cahn-Hilliard-Navier-Stokes finite element solver.
What carries the argument
The ONNX-exported neural network queried at runtime by the CHNS solver to supply local viscosity from the instantaneous shear rate, with Lipschitz regularization applied during training to keep the viscosity field smooth.
If this is right
- A single solver binary can handle Newtonian and non-Newtonian materials by swapping only the closure call.
- New ink formulations require only rheometer data collection and network retraining, not new code inside the flow solver.
- The adaptive mesh refinement already concentrates degrees of freedom where shear rates and interfaces are steepest.
- The separation of rheology characterization from solver development allows experimental groups to supply closures without finite-element expertise.
Where Pith is reading between the lines
- The same ONNX workflow could be applied to other local constitutive relations such as viscoelastic stress tensors if suitable experimental data exist.
- Because the network is queried pointwise, the method extends immediately to other finite-element or finite-volume codes that already support external function calls.
- If shear-rate history or temperature dependence becomes important, the network input vector can be enlarged without changing the solver interface.
Load-bearing premise
Viscosity measured as a function of shear rate alone on a rheometer is sufficient to determine the local viscosity field inside the dynamic, interface-containing flow solved by the CHNS equations.
What would settle it
A simulation of one of the characterized silicone inks in which the predicted rise velocity lies outside the experimental measurement spread while the network still reproduces the original rheometer curve to high accuracy.
Figures
read the original abstract
Materials used in polymer-based additive manufacturing processes, such as Digital Light Processing (DLP) and direct ink writing (DIW), typically exhibit non-Newtonian rheology. Carreau--Yasuda and power-law models describe basic shear-thinning and shear-thickening behavior well, but applying them to a new material requires choosing a functional form, deriving it, and re-implementing it inside the flow solver. We present a deployment workflow in which a neural network trained on experimental rheometry data serves as the viscosity closure inside a Cahn--Hilliard--Navier--Stokes (CHNS) finite element solver. Lipschitz regularization during training produces smooth viscosity predictions, and the trained network is exported in the Open Neural Network Exchange (ONNX) format and queried by the solver at runtime via the ONNX runtime, without solver modification or network reimplementation. The framework is built on a parallel octree-based adaptive mesh refinement infrastructure that concentrates resolution at the fluid interface. We validate the CHNS solver against benchmark shear-thinning bubble-rise cases from the literature, reproducing reported bubble shapes across varying power-law indices and Weber numbers. We characterized two silicone ink formulations, recorded their rise dynamics in perfluorodecalin on high-speed video, and used the resulting data to test the full workflow. Simulated rise velocities fall within the experimentally measured spread, and the simulated steady-state droplet shape agrees with the observed one. This work contributes to a growing body of literature on integrating neural constitutive closures into multiphysics simulations, and demonstrates a practical path for deploying experimentally trained rheological surrogates inside finite element solvers.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces a workflow for using a neural network trained on experimental rheometry data as a viscosity closure (via ONNX export) inside a parallel adaptive-mesh Cahn-Hilliard-Navier-Stokes solver for non-Newtonian multiphase flows. It validates the solver on literature shear-thinning bubble-rise benchmarks and then applies the full pipeline to two characterized silicone inks, reporting that simulated rise velocities lie within the experimental spread and that steady-state shapes match high-speed video observations.
Significance. If the central claim holds, the work supplies a reproducible, solver-agnostic route for embedding experimentally trained rheological surrogates into multiphysics codes without manual reimplementation of constitutive models. The combination of Lipschitz-regularized training, ONNX deployment, and octree AMR infrastructure is a concrete engineering contribution to data-driven non-Newtonian simulation.
major comments (2)
- [Validation against experimental data] Validation section (experimental droplet-rise comparison): the reported agreement with rise velocity and shape tests the integrated workflow but does not isolate whether the single-variable NN closure η(γ̇) remains adequate once the flow becomes unsteady and spatially inhomogeneous near a moving interface. No discussion is provided of possible thixotropy, viscoelastic normal stresses, or interfacial rheology in the silicone inks that would violate the steady-homogeneous rheometer assumption.
- [Neural-network viscosity closure and CHNS coupling] Workflow description: the claim that the NN can be queried at runtime with the instantaneous local shear rate computed from the velocity gradient inside the CHNS solver rests on the untested premise that rheometer-derived η(γ̇) suffices everywhere, including in regions of high interface curvature and transient shear. No auxiliary test (e.g., comparison against a known viscoelastic or thixotropic model) is shown to bound the error introduced by this assumption.
minor comments (1)
- [Training procedure] The abstract states that Lipschitz regularization produces smooth viscosity predictions, yet the manuscript does not quantify the resulting Lipschitz constant or demonstrate that the ONNX-evaluated field remains sufficiently smooth for the finite-element discretization.
Simulated Author's Rebuttal
We thank the referee for the constructive and detailed comments. We address each major comment below and indicate where revisions will be made to the manuscript.
read point-by-point responses
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Referee: [Validation against experimental data] Validation section (experimental droplet-rise comparison): the reported agreement with rise velocity and shape tests the integrated workflow but does not isolate whether the single-variable NN closure η(γ̇) remains adequate once the flow becomes unsteady and spatially inhomogeneous near a moving interface. No discussion is provided of possible thixotropy, viscoelastic normal stresses, or interfacial rheology in the silicone inks that would violate the steady-homogeneous rheometer assumption.
Authors: We agree that the experimental comparisons validate the end-to-end workflow rather than isolating the performance of the NN closure under unsteady or spatially varying conditions. The inks were characterized exclusively with steady shear rheometry, and no separate measurements of thixotropy, viscoelasticity, or interfacial effects were conducted. The observed agreement in rise velocity (within experimental spread) and steady shape provides indirect support for the closure in these cases, but does not bound errors from the steady-homogeneous assumption. We will add an explicit limitations paragraph in the discussion section acknowledging these assumptions and their potential violation for other materials. revision: partial
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Referee: [Neural-network viscosity closure and CHNS coupling] Workflow description: the claim that the NN can be queried at runtime with the instantaneous local shear rate computed from the velocity gradient inside the CHNS solver rests on the untested premise that rheometer-derived η(γ̇) suffices everywhere, including in regions of high interface curvature and transient shear. No auxiliary test (e.g., comparison against a known viscoelastic or thixotropic model) is shown to bound the error introduced by this assumption.
Authors: The paper's primary contribution is the solver-agnostic deployment workflow (Lipschitz-regularized training, ONNX export, runtime querying) rather than exhaustive validation of the single-variable closure against all possible rheological complexities. Benchmark cases use known power-law and Carreau-Yasuda models, while the experimental cases use the measured inks directly. No auxiliary comparison to viscoelastic or thixotropic constitutive models was performed, as this lies outside the stated scope of demonstrating practical integration of experimental rheometry data. We will revise the methods and conclusions sections to state the modeling assumption more explicitly and list the absence of such auxiliary tests as a limitation and avenue for future work. revision: partial
Circularity Check
No circularity: NN closure trained on independent rheometry, validated on separate video and literature benchmarks
full rationale
The paper trains an NN on experimental rheometry data (shear-rate vs viscosity curves) and deploys it via ONNX inside a CHNS solver. Validation uses separate high-speed video of droplet rise (rise velocity and shape) plus literature benchmark cases for shear-thinning bubbles. No equation or step reduces the reported agreement to quantities already fitted inside the training set. The workflow is self-contained against external data; the central claim does not reduce by construction to its inputs. Minor self-citations on the solver infrastructure or prior NN work are not load-bearing for the reported validation results.
Axiom & Free-Parameter Ledger
free parameters (1)
- Neural network weights and biases
axioms (2)
- domain assumption Viscosity depends only on the local shear rate (no dependence on pressure, temperature, or history).
- domain assumption The underlying CHNS finite-element solver correctly transports the interface and computes the flow field once supplied with an accurate local viscosity.
Reference graph
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