Durable Superhydrophobic Titanium Surfaces via Laser Texturing : Toward Long-lasting Self-cleaning Performance

다국어 초록 (Multilingual Abstract)

This study presents a method to simultaneously achieve superhydrophobicity and mechanical durability by forming hierarchical micro/nano-composite structures on titanium alloy substrates using a nanosecond pulsed laser （wavelength 355 nm, output 1 W）. Laser texturing was performed in a cross-hatch pattern with a dual-axis galvanometer scanner and an F-theta lens at an average output of 1 W, a pulse repetition rate of 80 kHz, an off-time of 2.5 μs, and a focused spot diameter of ~20 μm （pulse fluence ~4 J/cm²）. Conventional sulfuric-acid etching suffers from toxic-waste generation and poor reproducibility, [1] making laser texturing—a non-contact, solvent-free, and highly reproducible process—a promising alternative. [2] Two process variables were independently controlled: scan repetition count （1, 3, 5, 7） and scan line spacing （50–1,000 μm）. Mechanical durability was evaluated with a rubber- roller reciprocating wear tester （2 kgf, 9.04 cm/s） simulating human-finger contact （~4 MPa）. The static contact angle （SCA） was measured up to 250,000 cycles, and the sliding angle （SA） up to 2,000 cycles in the repetition-count comparison and up to 250,000 cycles in the laser-versus-etching comparison, thereby assessing both short- and long-term durability. The point at which a droplet no longer slides at a 90° tilt （SA ≥ 90°） was defined as the end-of-life （EOL） of surface functionality—a binary lifetime indicator marking the complete transition from the Cassie-Baxter to the Wenzel state. [3] In the repetition-count comparison （100 μm fixed）, the 5-pass specimen met the superhydrophobicity criterion with balanced durability, reaching SA ≥ 90° at ~1,800 cycles within the 2,000-cycle window. The 3-pass specimen recorded SA = 71.51° at 700 cycles and reached EOL around 900 cycles, while the 1-pass specimen failed much earlier. The residual SCA after 1,000 cycles increased from 122.99° （1 pass） to 151.33° （5 passes）, peaking at 5 passes; the life extension with repetition count was thus quantitatively confirmed, and 5 passes was selected as the optimum. In the spacing experiment, applying the SA ≥ 90° criterion excluded conditions ≥ 550 μm, which failed from the initial measurement. The 100 μm condition was optimal across SCA, SA, and wear resistance; spacings up to 250 μm satisfied both criteria, but beyond 400 μm the SA rose sharply and the functional lifetime shortened markedly. Compared with sulfuric-acid-etched specimens, the optimal laser-treated specimen （5 passes·100 μm） maintained SCA ≥ 110° throughout 250,000 cycles, whereas the etched specimen reached the SA EOL criterion （90°） at ~50,000 cycles, demonstrating a functional-lifetime improvement of at least five-fold. This superiority arises because the laser-formed hierarchical micro-frame acts as a protective barrier that physically shields the nanostructure, as verified by comparing SAM-coating residual distribution and structural damage via SEM·EDS analysis before and after wear. A single criterion, SA ≥ 90°, thus serves two roles: ① determining the functional EOL under rolling wear and ② enabling quantitative durability comparison between laser-treated and etched surfaces. Overall, this study provides quantitative design guidelines linking laser process parameters （passes·line spacing） to the functional lifetime of superhydrophobic surfaces, and demonstrates that eco-friendly, highly reproducible laser texturing is a practical alternative to conventional wet etching.

목차 (Table of Contents)

• CHAPTER 1. Introduction 1
• 1.1 Background and Objectives 1
• 1.1.1 Functional Characteristics and Industrial Need for Superhydrophobic Surfaces 1
• 1.1.2 The Need for Titanium Surface Modification 2
• 1.1.3 Limitations of Conventional Wet-Etching Processes 5
• 1.2 Research Objectives and Scope 6
• 1.2.1 A Laser-Texturing Approach to Superhydrophobic Fabrication 6
• 1.2.2 Key Contributions and Thesis Organization 8
• CHAPTER 2. Theoretical Background 10
• 2.1 Surface Wettability Theory 10
• 2.1.1 Young’s Contact-Angle Equation 11
• 2.1.2 The Wenzel Model 14
• 2.1.3 The Cassie-Baxter Model 17
• 2.1.4 The Cassie–Wenzel Transition Mechanism 18
• 2.2 Mechanical Durability of Superhydrophobic Surfaces 21
• 2.2.1 Mechanical Fragility of Nanostructures 22
• 2.2.2 Barrier Effect of Hierarchical Micro/Nano-Composite Structures 23
• 2.2.3 Sliding Angle as a Functional-Lifetime Indicator 25
• 2.3 Principles of Laser Surface Texturing 26
• 2.3.1 Interaction of Nanosecond Pulsed Lasers with Metal Surfaces 26
• 2.3.2 Role of Laser Process Parameters （Scan Number and Line Spacing） 29
• 2.3.3 Superhydrophobicity via Self-Assembled Monolayer （SAM） Coating 30
• 2.4 Surface Properties of Titanium and Prior Research Trends 31
• 2.4.1 Physical and Chemical Properties of Titanium and Its Alloys 31
• 2.4.2 Prior Studies on Laser-Textured Superhydrophobic Ti 33
• 2.4.3 Limitations of Prior Work and Novelty of This Study 34
• CHAPTER 3. Experimental Methods 36
• 3.1 Experimental Design 36
• 3.1.1 Rationale for Control-Variable Selection （Scan Number and Line Spacing） 37
• 3.1.2 Specimen Condition Configuration 38
• 3.2 Specimen Fabrication 39
• 3.2.1 Substrate Cleaning and Pretreatment 40
• 3.2.2 Laser Texturing Process （355 nm, 1 W Nanosecond Pulsed Laser） 41
• 3.2.3 Sulfuric-Acid Etching Process （Comparison Specimens） 43
• 3.2.4 PFOTS Self-Assembled Monolayer Coating 44
• 3.3 Mechanical Durability Evaluation — Rolling Wear Test 44
• 3.3.1 Test Apparatus and Principle 44
• 3.3.2 Simulated Human-Finger Contact Design （~4 MPa） 45
• 3.3.3 Test Conditions （2 kgf, 9.04 cm/s, up to 250,000 cycles） 47
• 3.3.4 Definition of the Functional End-of-Life Criterion （SA ≥ 90°） 47
• 3.4 Wettability Measurement 48
• 3.4.1 Static Contact Angle Measurement System 48
• 3.4.2 Sliding Angle Measurement Method 49
• 3.4.3 Measurement-Interval Design 50
• 3.5 Surface Analysis and Statistical Processing 51
• 3.5.1 Scanning Electron Microscopy （SEM） and Energy-Dispersive X-ray Spectroscopy （EDS） 51
• 3.5.2 Statistical Significance Testing （Student’s t-test, p 〈 0.05） 52
• CHAPTER 4. Results and Discussion 53
• 4.1 Initial Characterization of Superhydrophobic Surfaces. 53
• 4.1.1 Surface Morphology of Laser-Textured Specimens （SEM） 53
• 4.1.2 Initial Static Contact Angle and Sliding Angle Results 55
• 4.2 Process Parameter ① — Effect of Laser Scan Number 57
• 4.2.1 Change in Initial Surface Properties with Scan Number 57
• 4.2.2 Wear-Durability Comparison （by Cycles to SA ≥ 90°） 58
• 4.2.3 Residual SCA after 1,000 Wear Cycles （122.99° → 151.33°） 60
• 4.2.4 Selection of the Optimal Scan Number 62
• 4.3 Process Parameter ② — Effect of Scan Line Spacing 64
• 4.3.1 Initial Static Contact Angle by Spacing （50–1,000 µm） 64
• 4.3.2 Exclusion of Conditions Not Meeting the Superhydrophobic Criterion （SCA ≥ 150°） 65
• 4.3.3 Optimal Spacing from Initial Static Contact Angle and Wear Resistance 67
• 4.3.4 Optimal Spacing Selection （100 µm） and Design Map 70
• 4.4 Durability Comparison versus Sulfuric-Acid Etching 70
• 4.4.1 Initial Static Contact Angle （Laser 164.96° vs. Etching 156.84°） 71
• 4.4.2 Short-Term Wear Behavior （0–2,000 cycles） 71
• 4.4.3 Mid-Term Wear Behavior （0–60,000 cycles） 73
• 4.4.4 Long-Term Wear Behavior （0–250,000 cycles） — Laser SCA ≥ 110°. 73
• 4.4.5 Comparison of Cycles to SA ≥ 90° and Functional-Lifetime Quantification 76
• 4.4.6 Wear-Mechanism Analysis — Barrier Effect of the Hierarchical Frame （SEM·EDS） 78
• CHAPTER 5. Conclusions 79
• 5.1 Summary of Findings 79
• 5.2 Academic and Practical Contributions 81
• 5.3 Future Research Directions 82
• References 85
• 국문초록 89