Enhancing Thermal Diffusivity of Al₂O₃ Nanofluids Through Modulated Laser‑Induced Photothermal Fragmentation

Abstract

Modulated continuous‑wave (CW) lasers trigger a photothermal response that rapidly absorbs light and generates thermal waves around irradiated nanostructures. In this study, we explore how such laser irradiation influences particle fragmentation and consequently improves the thermal diffusivity of Al₂O₃ nanofluids. A cost‑effective diode laser (532 nm, 200 mW) was used to break up agglomerated Al₂O₃ nanoparticles dispersed in deionized water. The laser‑generated thermal waves, governed by modulation frequency and the optical/thermal characteristics of the nanofluid, were briefly summarized. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) quantified the effects of irradiation time on particle size and distribution, while the photopyroelectric (PPE) method measured thermal diffusivity. Results show that partial fragmentation reduces agglomerate size toward the intrinsic 11 nm diameter, yielding a narrower size distribution. Correspondingly, thermal diffusivity increased from 1.444 × 10⁻³ to 1.498 × 10⁻³ cm² s⁻¹ after 30 min of irradiation, demonstrating the potential of photothermal fragmentation to tailor nanofluid heat transfer properties.

Background

Metal‑oxide nanofluids, especially Al₂O₃, are prized for their superior thermal conductivity and diffusivity compared to base fluids such as water or oil. Their performance, however, is highly sensitive to particle size, shape, and dispersion state. Agglomeration quickly degrades thermal properties, necessitating effective dispersion strategies. Laser‑based fabrication has emerged as a versatile tool for generating and modifying nanoparticles directly within a liquid medium. By adjusting laser parameters—wavelength, pulse duration, energy, and repetition rate—one can influence particle size and distribution.

While pulsed lasers have traditionally been used for ablation and fragmentation, continuous‑wave lasers offer advantages in cost, size, and portability. Modulated CW lasers, in particular, generate pronounced photothermal (PT) effects and thermal waves (TWs) that can break up particle clusters. Despite extensive studies on laser‑induced nanoparticle synthesis, a detailed investigation of the PT effect of modulated CW lasers on Al₂O₃ nanofluid thermal diffusivity remains lacking. This work fills that gap by systematically studying the fragmentation dynamics and the resulting heat transfer enhancement.

Methods

Nanofluid Preparation

Al₂O₃ nanoparticles (11 nm, Nanostructured and Amorphous Materials, Inc.) were dispersed at 0.05 g per 25 mL deionized water, with 1 % v/v polyvinylpyrrolidone (PVP, MW ≈ 29 kDa) to stabilize the suspension. After 1 h stirring and 30 min probe sonication (VCX 500, 25 kHz, 500 W), the suspension achieved a homogeneous distribution, verified by DLS.

Laser Fragmentation

Fragmentation was carried out in a 2 mL quartz cuvette placed on a magnetic stirrer. A 532 nm, 200 mW diode laser (MGL 150(10)) was focused to a 0.1 mm spot (≈2.5 kW cm⁻²) and modulated at 10 Hz using an optical chopper (SR540). Irradiation times of 10 and 30 min were applied. Post‑irradiation, TEM (Hitachi H‑7100) and DLS (Sympatec Nanophox) characterized morphology and size distribution.

Thermal Diffusivity Measurement

The photopyroelectric (PPE) technique, renowned for its high precision, measured the thermal diffusivity of the nanofluids. A copper foil (50 µm) acted as a pyroelectric generator, while a 52 µm PVDF film served as the detector. The laser‑induced thermal wave propagated through the sample, generating a voltage proportional to the thermal diffusivity. A lock‑in amplifier (SR 530) recorded amplitude and phase at 6.7 Hz, and data were fitted using the cavity‑scan method (Equations 6–8) to extract α.

Results and Discussion

Thermal Wave Generation

Modulation frequency critically affects the amplitude of thermal waves. Simulations (Fig. 2) indicate an optimal frequency around 10 Hz, balancing signal amplitude and noise. Optical absorption, driven by particle size, shape, and concentration, directly influences thermal wave strength. Al₂O₃/water nanofluids exhibit favorable absorption, with up to 13 % of incident light absorbed—enhanced further by higher nanoparticle loading.

Fragmentation Dynamics

TEM images (Fig. 5) show a progressive reduction in particle size: from 16.4 ± 7.8 nm (pre‑irradiation) to 14.2 ± 5.4 nm after 10 min, and 12.0 ± 3.5 nm after 30 min. DLS data (Fig. 6) confirm a shift toward a narrow, monodisperse distribution, with hydrodynamic diameters decreasing from ~90 nm to ~91 nm after irradiation. Fragmentation saturates near 30 min, as particles reach a critical size where further absorption‑induced ablation diminishes.

Thermal Diffusivity Enhancement

Calibration against distilled water yielded α = 1.446 × 10⁻³ cm² s⁻¹ (within 1 % of literature). Nanofluid α increased from 1.444 ± 0.008 × 10⁻³ cm² s⁻¹ (no irradiation) to 1.498 ± 0.012 × 10⁻³ cm² s⁻¹ after 30 min (Fig. 7). The 3–6 % rise reflects enhanced Brownian motion and reduced agglomerate size, which promote efficient heat transfer from particles to the base fluid. The improved thermal diffusivity underscores the effectiveness of modulated CW laser fragmentation as a practical strategy for nanofluid optimization.

Conclusions

Modulated CW laser irradiation efficiently fragments Al₂O₃ agglomerates, producing a narrow size distribution and boosting thermal diffusivity by up to 6 % within 30 min. This low‑cost, scalable approach demonstrates the viability of photothermal fragmentation for tailoring nanofluid heat transfer performance, with implications for industrial heat exchangers, electronics cooling, and medical therapies.