The formation of electrospun nanofibers involves complex multiscale phenomena, spanning from the macroscopic behavior of the polymer jet to the microscopic dynamics of polymer chains and solvent evaporation. Hybrid molecular dynamics-continuum simulations provide a powerful framework to bridge these scales, enabling detailed investigation of the electrospinning process. By coupling jet dynamics with polymer chain entanglement and solvent evaporation, these simulations offer insights into the interplay of process parameters and resulting fiber properties.

At the macroscopic level, continuum models describe the electrospinning jet as a viscoelastic fluid subjected to electric forces, surface tension, and viscous stresses. The jet undergoes stretching and whipping instabilities due to electrostatic repulsion and aerodynamic forces. The thinning of the jet is governed by the balance between these forces and the rheological properties of the polymer solution. Continuum simulations solve the Navier-Stokes equations with additional terms for electric field interactions, providing predictions of jet trajectory and thinning behavior.

At the nanoscale, molecular dynamics simulations capture the behavior of individual polymer chains and solvent molecules. The entanglement of polymer chains plays a critical role in determining the extensional viscosity of the solution, which influences jet stability. As the jet thins, solvent evaporation occurs, leading to increased polymer concentration and chain entanglement. Molecular dynamics models track the diffusion of solvent molecules and the resulting changes in polymer chain conformation. The coupling between continuum and molecular dynamics simulations is achieved through iterative exchange of parameters such as viscosity, polymer concentration, and electric field distribution.

Process parameters significantly influence fiber formation. Applied voltage affects the initial jet formation and the onset of whipping instabilities. Higher voltages typically lead to greater jet stretching, resulting in thinner fibers. Simulations show that increasing the voltage from 10 kV to 20 kV can reduce fiber diameter by approximately 30 to 50 percent, depending on polymer concentration. Flow rate determines the mass of polymer delivered to the jet, with lower flow rates favoring finer fibers due to reduced jet diameter. Humidity impacts solvent evaporation rates, with higher humidity slowing down solidification and potentially increasing fiber diameter. Simulations incorporating humidity effects predict a 10 to 20 percent increase in fiber diameter when relative humidity rises from 30 to 70 percent.

The hybrid modeling approach provides detailed predictions of fiber morphology. Fiber diameter is primarily determined by the balance between jet stretching and polymer chain entanglement. Simulations reveal that higher molecular weight polymers produce thicker fibers due to increased entanglement density. Porosity arises from phase separation during solvent evaporation, with faster evaporation rates leading to higher porosity. Alignment of fibers within nonwoven mats is influenced by the whipping instability and collector speed. Simulations predict that higher collector speeds can improve alignment by 15 to 25 percent, as quantified by orientation order parameters.

Comparisons with experimental data validate the accuracy of hybrid simulations. Scanning electron microscopy measurements of fiber diameter show good agreement with simulated predictions, typically within 10 to 15 percent deviation. Mechanical characterization of electrospun mats, including tensile strength and Young's modulus, aligns with simulation outputs when accounting for fiber-fiber interactions and mat density. For example, simulations of polycaprolactone nanofibers predict a Young's modulus of 120 to 150 MPa, matching experimental nanoindentation results within 8 percent error.

The hybrid modeling framework also elucidates the role of polymer-solvent interactions. Simulations demonstrate that solvents with higher vapor pressure lead to faster solidification, producing fibers with smoother surfaces. In contrast, slower evaporating solvents result in phase separation and porous fiber morphologies. The choice of solvent affects chain entanglement during jet stretching, with good solvents promoting extended chain conformations and poor solvents leading to collapsed configurations. These molecular-scale insights guide the selection of solvent systems for desired fiber properties.

Future developments in hybrid simulations could incorporate additional physical phenomena, such as crystallization dynamics in semicrystalline polymers or the effects of additives on jet stability. Enhanced computational efficiency will enable larger-scale simulations, capturing the formation of entire nonwoven mats rather than individual fibers. Such advancements will further strengthen the predictive power of these models, facilitating the rational design of electrospun materials for specific applications.

The integration of molecular dynamics and continuum simulations represents a significant advancement in understanding electrospinning. By capturing both macroscopic jet behavior and nanoscale polymer dynamics, these models provide a comprehensive view of the process-structure-property relationships in electrospun nanofibers. The quantitative agreement with experimental data underscores the reliability of this approach for process optimization and material design. As computational power increases and models become more sophisticated, hybrid simulations will play an increasingly vital role in advancing nanofiber technology.