The small geometric size and complex spatial distribution of gas film holes in the hot-end components of aircraft engines make it essential to use digital image correlation (DIC) methods to investigate deformation localization and its effects on mechanical behavior. This process requires the preparation of uniformly fine speckle patterns at the micrometer level to enhance spatial resolution and strain measurement accuracy. Among the available techniques, the spraying method has become a preferred approach for speckle preparation due to its cost-effectiveness and adaptability. However, optimizing spray parameters to achieve small, uniformly distributed speckles is often hindered by limitations related to experimental conditions and atomization devices. The aerodynamic atomization characteristics of fluids reveal that droplet size depends on atomization parameters (e.g., air-liquid flow ratio, air pressure) and liquid properties (e.g., viscosity, surface tension).To address the challenges of refining and homogenizing speckle size using the spraying method, this study modifies the viscosity and surface tension of the premixed solution. These adjustments improve the speckle size and enhance its distribution uniformity, ensuring sufficient resolution and precision for characterizing deformation around the holes.

To address the challenges of refining speckle size and achieving uniformity in speckle preparation via the spraying method, this study prepared smaller and more uniformly distributed speckles by modulating the liquid properties—specifically viscosity and surface tension—of the premixed solution based on optimized spray process parameters. The viscosity and surface tension of acrylic paint-water solutions at various ratios were measured using a viscometer and a plate tensiometer, respectively. Pneumatic atomization tests were subsequently conducted using a twin-fluid air-assisted airbrush under different spray parameters, with droplet size distribution analyzed using a laser diffraction analyzer. A correlation model was developed to link droplet characteristic diameters (D_v0.5 and D_v0.9) with parameters such as the air-liquid flow ratio (ALR), Reynolds number, and Weber number. The model demonstrated the influence of liquid properties, including the viscosity and surface tension of the premixed solution, and spray process parameters, such as the air-liquid flow ratio, on speckle size. This provided a framework for optimizing speckle size for improved uniformity.

Under specific air-liquid flow ratios (ALR values of 0.5, 0.7, and 1.0), an increase in air flow rate enhances the aerodynamic forces that facilitate droplet breakup, resulting in a notable decrease in D_v0.5 and D_v0.9, as shown in Figs. 4(a) and 4(b). When the air flow rate (AFR) increases to a critical value (3.0 L/min), the refinement effect on droplet size diminishes, causing D_v0.5 and D_v0.9 to approach an asymptote. Reducing liquid viscosity (η) contributes to a decrease in the viscous forces that maintain droplet cohesion. As illustrated in Figs. 4(c) and 4(d), under higher viscosity conditions, reducing liquid viscosity gradually decreases D_v0.5 and D_v0.9. For lower viscosity conditions, the influence of viscosity on droplet size becomes more pronounced, leading to a significant reduction in D_v0.5 and D_v0.9 with decreasing liquid viscosity. The influence of liquid properties and spray parameters on droplet size can be predicted by constructing functional relationships using dimensionless numbers. The ALR reflects spray parameter influence, with higher ALR values reducing droplet size. The Reynolds number (Re) and Weber number (We) reflect the effects of liquid viscosity and surface tension, respectively. For high-viscosity liquids, such as glycerol-water mixtures (η=80 mPa·s), the exponent of Re is considerably smaller than that of We, indicating that surface tension exerts a greater influence on droplet size. In contrast, for low-viscosity liquids (η≈10 mPa·s), such as the acrylic paint-water mixture used in this study, liquid viscosity plays a more significant role in determining droplet size.

This study achieved refinement and homogenization of speckle size by regulating the viscosity and surface tension of the premixed solution and optimizing spray parameters. Increasing the air flow rate and reducing the liquid flow rate to enhance the air-liquid flow ratio resulted in a decrease in the average droplet size. However, the refinement effect on droplet size was also influenced by the Reynolds number and Weber number of the fluid. For speckle solutions with a viscosity of approximately 10 mPa·s and a surface tension of around 30 mN/m, the average droplet size exhibited a linear relationship with the 0.386th power of the surface tension coefficient, as influenced by the fluid Weber number. Similarly, due to the Reynolds number, the average droplet size displayed a linear relationship with the 0.699th power of the liquid viscosity, demonstrating that reducing liquid viscosity significantly decreases droplet size. Additionally, droplet size in pneumatic atomization was found to be closely correlated with speckle size, where smaller droplet sizes yielded smaller speckles. The statistical mean and variance of speckle sizes were directly proportional to the droplet characteristic sizes D_v0.5 and D_v0.9-D_v0.5, respectively, underscoring the critical role of droplet size in speckle refinement.