Physics-Informed Neural Networks (PINN)

• PINNs are machine learning models that embed differential equations and physical laws into neural network training for consistent, mesh-free predictions.
• They employ composite loss functions that weight both data misfit and PDE residuals, effectively handling sparse or noisy observations.
• Advanced strategies like adaptive weighting, domain decomposition, and Bayesian uncertainty capture extend PINNs to complex, multi-scale, and inverse problem settings.

Physics-Informed Neural Network (PINN) is a class of scientific machine learning models that approximate solutions to differential equations by embedding known physical laws directly into the training of neural networks. Unlike traditional purely data-driven approaches, PINNs enforce physical consistency by penalizing the residuals of the governing equations—such as partial differential equations (PDEs), boundary, and initial conditions—in the loss function. This approach yields a mesh-free, flexible solver capable of integrating sparse or noisy data with first-principles models, making PINNs adaptable to both forward and inverse problems across a variety of scientific and engineering domains.

1. Mathematical Formulation and Core Principles

In a typical PINN setup, one seeks to approximate the solution $u(x, t)$ of a PDE given by $F(u(x, t)) = 0$ (possibly with unknown coefficients), along with auxiliary boundary and initial conditions. The neural network $u(x, t; \theta)$, with parameters $\theta$, is trained via a composite loss: $$\mathcal{L}(\theta) = \mathcal{L}_\text{data} + \mathcal{L}_\text{PDE}$$ where $\mathcal{L}_\text{data}$ encodes data misfit at observations and boundary/initial points, and $$\mathcal{L}_\text{PDE} = \sum_{(x_k, t_k)} |F(u(x_k, t_k; \theta))|^2$$ penalizes discrepancies with the physics at collocation points. Gradients of $u$ required by $F$ are computed via automatic differentiation, allowing meshless computation on arbitrary domains (Raissi et al., 29 Aug 2024).

PINNs can accommodate forward (predicting solution fields) and inverse (inferring unknown physical parameters) problems, and even data-driven discovery of dynamics, as demonstrated in nonlinear biochemical oscillator modeling utilizing multistep time-integration and symbolic regression to reconstruct explicit governing equations (Raissi et al., 29 Aug 2024).

2. Practical Extensions: Adaptive Weighting, Domain Decomposition, and Long-Time Integration

Recent extensions to the PINN framework have sought to address challenges in training stability, scalability, and accuracy, particularly for large, multiscale, and time-dependent problems:

• Adaptive Weights: Adaptive weighting strategies—such as those based on the Neural Tangent Kernel (NTK) eigenvalues or residual-based, pointwise losses—are used to dynamically adjust the importance of different components (boundary, PDE, data residuals), preventing imbalance and enhancing training efficacy (Raissi et al., 29 Aug 2024).
• Domain Decomposition: The combination of PINNs with domain decomposition (e.g., cPINN, XPINN, hp-VPINN) enables partitioning of the computational domain into subdomains, each handled by a dedicated network. Solution and flux continuity are imposed via interface terms in the loss, increasing scalability and parallelization efficiency. These methods are particularly effective in multi-scale or high-dimensional settings (Figueres et al., 26 Apr 2025, Raissi et al., 29 Aug 2024).
• Long-Time and Multi-Scale Integration: Novel training regimes—such as time-sweeping collocation or transfer learning—help mitigate difficulties encountered over long temporal integration windows and in multi-scale PDEs, where conventional PINNs may fail to propagate accurate solutions (Mustajab et al., 5 Jan 2024, Raissi et al., 29 Aug 2024). Transfer learning, in particular, allows networks pretrained on lower-frequency (or less stiff) problems to be rapidly adapted to more challenging high-frequency or high-velocity cases, reducing computational requirements (Mustajab et al., 5 Jan 2024).

3. Bayesian and Uncertainty-Aware Formulations

The uncertainty quantification capability of PINNs is critical in scientific computing. Bayesian PINNs (BPINN) extend the standard PINN formulation by associating probability distributions with the network weights, propagating epistemic and aleatoric uncertainties through to the predictions (Graczyk et al., 2023, Figueres et al., 26 Apr 2025). This is typically realized via:

• Gaussian priors on the weights, updated using Laplace approximations or sampling-based techniques (e.g., Hamiltonian Monte Carlo).
• Posterior predictive uncertainty estimates computed via the inverse Hessian of the loss at optimum.
• Bayesian evidence maximization used for hyperparameter learning and model comparison.
• Explicit integration of measurement/data uncertainties directly into the probabilistic loss (Figueres et al., 26 Apr 2025).

Combining BPINNs with domain decomposition, the $PINN framework [Editor's term for "dollar-PINN", (Figueres et al., 26 Apr 2025)] allows efficient local uncertainty estimation in each subdomain and robust global uncertainty recovery through interface continuity constraints. This parallelizable approach handles noisy, sparse, or multi-scale data more robustly than monolithic architectures.

4. Advanced Optimization Strategies and Constraint Handling

Standard PINN optimization methods, typically based on first-order gradient descent, can be inadequate for stiff or highly nonlinear problems due to ill-conditioning and complex loss landscapes. Recent research has explored:

• Trust-Region Sequential Quadratic Programming (trSQP-PINN): Reformulates PINN training as a hard-constrained optimization problem. Using quadratic approximations of the objective and constraint linearizations inside a trust region, the method achieves orders-of-magnitude improved accuracy over penalty and augmented Lagrangian approaches, especially in challenging transport/reaction problems (Cheng et al., 16 Sep 2024). Pretraining phases that focus solely on feasibility further boost efficiency and robustness.
• Hard Constraint Satisfaction: Beyond soft-penalty enforcement, methods such as KKT-hPINN use projection layers derived from Karush-Kuhn-Tucker conditions to strictly enforce linear equality constraints, improving prediction fidelity for systems with conservation or operating constraints (e.g., chemical reactors) (Chen et al., 11 Feb 2024).
• Adversarial Training for Alternative Losses: For certain PDEs, such as the high-dimensional nonlinear Hamilton-Jacobi-BeLLMan (HJB) equation, the standard $L^2$ loss is inadequate. Adversarial strategies that focus on minimizing the $L^\infty$ physics-informed loss via a min-max (worst-case) approach yield more robust convergence and higher-fidelity solutions (Wang et al., 2022).

5. PINN Architectures and Neural Hyperparameter Optimization

The performance of PINNs is sensitive to network architecture and hyperparameters:

• Automated Neural Architecture Search (Auto-PINN, NAS-PINN): Automated frameworks leverage Neural Architecture Search (NAS) to systematically explore and optimize choices of activation functions, width/depth, residual connections, and training schedules. These methods decouple and reduce search spaces, improving both training stability and generalization to new PDE classes (Wang et al., 2022, Wang et al., 2023).
• Architectural Insights: Findings indicate that optimal architectures are PDE-specific: for some (e.g., Poisson or advection), wider and shallower networks are superior, whereas for nonlinear or dynamic problems (e.g., Burgers'), deeper networks—even with fewer neurons per layer—are preferable. Residual connections and identity mappings often improve performance in complex or irregular geometries (Wang et al., 2023).

6. Applications Across Scientific and Engineering Domains

PINNs and their extensions have been effectively applied in a wide range of disciplines:

• Multiscale and Multiphase Problems: NH-PINN demonstrates that classical homogenization techniques combined with PINNs enable accurate solution of multiscale PDEs—by decoupling high-frequency (cell) and low-frequency (coarse-grained) components and introducing oversampling strategies for periodic cell problems (Leung et al., 2021).
• Parameter Estimation and Surrogate Modeling: In solid mechanics, PINNs have proven reliable and efficient for dynamic elasticity and inverse material parameter identification, outperforming finite element method (FEM) surrogates when trained even on sparse sensor data (Kag et al., 2023).
• Unsteady and Moving Boundary Systems: Modular architectures—using separate networks for different phases, variables, and interfaces—combined with adaptive loss weighting and temporal causal training, enable PINNs to tackle complex coupled moving boundary problems, such as binary alloy solidification with discontinuous composition fields (Kathane et al., 17 Sep 2024).
• Aerodynamics and Coupled Multiphysics: PINNs facilitate parametric surrogate modeling, multi-physics coupling (e.g., fluid-solid heat transfer), and inverse data assimilation (e.g., turbulence closure inference) (Coulaud et al., 26 Mar 2024).
• Compositional and Hybrid Solvers: For stiff subproblems (e.g., Fischer–Tropsch catalyst pellet kinetics), enforcing physically correct asymptotics in PINN outputs is critical; domain knowledge-based transformations in post-processing layers are necessary for maintaining stability in larger workflow integrations (Nikolaienko et al., 15 Nov 2024).

7. Limitations, Open Challenges, and Future Directions

PINNs are inherently flexible and have achieved broad adoption in scientific machine learning. However, open challenges remain:

• Achieving accuracy and convergence on par with established high-order solvers, especially for forward problems and over long-time horizons (Raissi et al., 29 Aug 2024).
• Scalability issues in high dimensions or for highly multiscale systems, motivating further development of domain decomposition and transfer learning strategies.
• Rigorous theoretical understanding of convergence, generalization, and optimal hyperparameter selection is still evolving.
• Enhanced robustness is needed under high noise, non-Gaussian disturbances, and extrapolation outside the training domain (Pilar et al., 2022, Nikolaienko et al., 15 Nov 2024).
• Extensions to advanced constraints (inequality, nonlinear), uncertainty quantification, and operator learning constitute active research frontiers (Figueres et al., 26 Apr 2025, Chen et al., 11 Feb 2024, Cheng et al., 16 Sep 2024).
• Exploitation of parallel and distributed computing architectures for training subdomain networks is encouraged by the modularity of current frameworks (Figueres et al., 26 Apr 2025).

A plausible implication is that as adaptive optimization, domain decomposition, and robust Bayesian approaches continue to mature, PINNs and their extensions will underpin the next generation of scientific computing platforms, enabling seamless integration of data, physical principles, and uncertainty quantification in large-scale multiphysics applications.