118 research outputs found
Drop impact and wettability:From hydrophilic to superhydrophobic surfaces
No full textExperiments to understand the effect of surface wettability on impact characteristics of water drops onto solid dry surfaces were conducted. Various surfaces were used to cover a wide range of contact angles (advancing contact angle from 48° to 166°, and contact angle hysteresis from 5° to 56°). Several different impact conditions were analyzed (12 impact velocities on 9 different surfaces, among which 2 were superhydrophobic). Results from impact tests with millimetric drops show that two different regimes can be identified: a moderate Weber number regime (30 < We < 200), in which wettability affects both drop maximum spreading and spreading characteristic time; and a high Weber number regime (We > 200), in which wettability effect is secondary, because capillary forces are overcome by inertial effects. In particular, results show the role of advancing contact angle and contact angle hysteresis as fundamental wetting parameters to allow understanding of different phases of drop spreading and beginning of recoiling. It is also shown that drop spreading on hydrophilic and superhydrophobic surfaces occurs with different time scales. Finally, if the surface is superhydrophobic, eventual impalement, i.e., transition from Cassie to Wenzel wetting state, which might occur in the vicinity of the drop impact area, does not influence drop maximum spreading.</p
Runback ice formation mechanism on hydrophilic and superhydrophobic surfaces
No full textExperimental results aimed at understanding the different mechanisms of ice accretion on surfaceswith different wettabilities are presented. Ice accretion was studied on a hydrophilic and a superhydrophobic surface of NACA 0021 airfoils, during tests inside an icing wind tunnel. Visualization of ice accretion was performed using an infrared camera, which allows an enhanced view of liquidwater and ice present on the airfoil surface compared to the optical imaging. The use of the infrared camera permitted identification of different ice shapes and ice formation mechanisms on the test articles: on the hydrophilic sample, a compact ice front accreted, whereas on the superhydrophobic sample, small isolated ice islands formed. In addition, the ice on the superhydrophobic sample wasmore susceptible to be shed fromthe surface, as shown by shedding of several ice islands due to aerodynamic drag during tests. Combined analysis of infrared camera images and optical images confirmed that the fraction of the airfoil covered by ice decreases for the superhydrophobic sample, for a two minute test, for all the input heat power tested
Investigation of ice shedding properties of superhydrophobic coatings on helicopter blades
No full textThe state-of-the-art of icing protection systems for helicopter rotor blades is based on active thermal de-icing systems that require large amounts of power. This work focused on assessing the potential icephobicity of superhydrophobic coatings as an alternative passive strategy. Ice shedding tests were conducted in a helicopter blade icing chamber, to simulate atmospheric icing conditions. Ice accretion and shedding were tested on four different materials, including two common metals and two superhydrophobic materials, with the objective of evaluating icephobic potential for anti-icing purposes. Coating test results showed a strong influence of temperature and surface roughness on the ice adhesion: the strength increased when temperature decreased and roughness increased. Ice regime was independent of the type of surface used, but superhydrophobic surfaces resulted in a thinner ice shape in comparison with common metals, which resulted in a shorter shedding time, especially in rime ice conditions. The relationship between ice regime and adhesion load showed that ice adhesion load substantially increases in rime ice conditions, demonstrating that ice regime is an important parameter in the ice adhesion process. Additional results showed that superhydrophobic surfaces were associated with a decrease in the adhesion load with respect to the baseline materials ranging from the 16% to the 70% in the best case; but this reduction may not be revealing for practical applications as ice reduction mechanisms need to be first understood.</p
Drop rebound after impact:The role of the receding contact angle
No full textData from the literature suggest that the rebound of a drop from a surface can be achieved when the wettability is low, i.e., when contact angles, measured at the triple line (solid-liquid-air), are high. However, no clear criterion exists to predict when a drop will rebound from a surface and which is the key wetting parameter to govern drop rebound (e.g., the "equilibrium" contact angle, θeq, the advancing and the receding contact angles, θA and θR, respectively, the contact angle hysteresis, Δθ, or any combination of these parameters). To clarify the conditions for drop rebound, we conducted experimental tests on different dry solid surfaces with variable wettability, from hydrophobic to superhydrophobic surfaces, with advancing contact angles 108 < θA < 169 and receding contact angles 89 < θR < 161. It was found that the receding contact angle is the key wetting parameter that influences drop rebound, along with surface hydrophobicity: for the investigated impact conditions (drop diameter 2.4 < D0 < 2.6 mm, impact speed 0.8 < V < 4.1 m/s, Weber number 25 < We < 585), rebound was observed only on surfaces with receding contact angles higher than 100. Also, the drop rebound time decreased by increasing the receding contact angle. It was also shown that in general care must be taken when using statically defined wetting parameters (such as advancing and receding contact angles) to predict the dynamic behavior of a liquid on a solid surface because the dynamics of the phenomenon may affect surface wetting close to the impact point (e.g., as a result of the transition from the Cassie-Baxter to Wenzel state in the case of the so-called superhydrophobic surfaces) and thus affect the drop rebound.</p
Drop impact onto a cantilever beam:behavior of the lamella and force measurement
No full textIn this work, the process of drop impact onto an elastic surface (a cantilever beam) was studied. Different from previous studies which typically focused on the behavior of the elastic surface (e.g., deformation and oscillation), the focus of this work is to examine the behavior of the resulting lamella during the impact. It was found that the maximum contact diameter of the lamella in an elastic impact compared to impact onto a ridged surface is significantly smaller (e.g., 17% for impact at 2 m/s). The results were explained through an analysis of impact energy and the stored elastic energy in the beam. In this work, we also demonstrated how to use a cantilever beam to measure maximum drop impact force. It was found that a large natural frequency of the cantilever beam is needed for the maximum force measurement to produce acceptable values.</p
Shedding of Multiple Sessile Droplets
No full textA droplet which is placed on a surface and is exposed to an airflow, can be shed, if the drag force overcomes the droplets adhesion force. Presence of other sessile droplets, in proximity, changes the drag force, so the minimum airflow velocity required to shed the droplets (Ucr) can vary. In this thesis, an experimental study on shedding of the multiple sessile droplets was performed on both hydrophilic and hydrophobic surfaces. The effects of the droplets arrangement type, and the spacing on Ucr were elucidate. For a pair of sessile droplets, a model was proposed to predict the Ucr based on droplets size, spacing, arrangement, and surface wettability. For three, or four sessile droplets arranged in triangle, square, reversed triangle, and diamond configurations, the effects of the droplets interaction on variation of the Ucr, was clarified. A critical value for spacing was determined beyond which multiple sessile droplets shed independently
Experimental and Analytical Investigation of an Array of Sessile Droplets Behaviour on Heated and Unheated Substrates
No full text**Summary (194 words):**
This thesis investigates the evaporation and interaction dynamics of sessile droplets, crucial for applications in thermal management, microfluidics, and surface wetting. A novel and computationally efficient **Point Source Model (PSM)** was developed to predict droplet evaporation on isothermal and heated surfaces. By simplifying complex mass transfer processes, the PSM provides accurate results with minimal computational cost.
Initially, the PSM modeled quasi-steady, **purely diffusive evaporation** of two droplets, capturing the effects of **separation distance** and **evaporation modes**—Constant Contact Angle (CCA) and Constant Contact Radius (CCR)—with deviations under 9% compared to experiments. A **critical separation ratio (L/d ≥ 10)** marked the onset of independent behavior.
To address heated substrates, the model incorporated **natural convection** via a new empirical correlation. For **Ra·L/d 2400**, convection stabilizes.
Further experiments explored **vapor-mediated interactions** between water and propylene glycol-water droplets at 24–135°C. Significant increases in droplet velocity and fragmentation were observed, especially at **20% PG**, driven by surface tension gradients.
Finally, the PSM was extended to a **three-droplet array**, revealing enhanced **vapor shielding** and a new isolation threshold (**L/d > 20**), emphasizing the impact of droplet geometry on evaporation behavior
Behavior of Liquid Bridges between Nonparallel Surfaces
No full textFormation of liquid bridges between two solid surfaces is frequently observed in industry and nature, e.g. printing. When the two solid surfaces are not parallel (with dihedral angle between them), two significant phenomena emerge in the bridge behavior: First, if exceed a critical angle (_c), the bridge is no longer stable and propel itself horizontally towards the cusp of the surfaces. Second, if a stable bridge is squeezed and stretched, a horizontal bulk motion of the bridge along the surfaces can be observed. Through both experimental and numerical studies, we demonstrated that _c can be increased by increasing advancing contact angle (_a), and Contact Angle Hysteresis (CAH) of the surfaces. We also demonstrated that the magnitude of the bulk motion can be increased by increasing , the amount of compressing and stretching, and/or by decreasing _a and CAH of the surfaces
Drop Impact on an Inclined and a Moving Surface
No full textThis thesis has made progress in two different areas related to drop impact onto a surface. Firstly, a systematic experimental study has been performed to understand asymmetric spreading of low and high surface tension liquids on a moving surface. A new time evolution model for droplet spreading on a moving surface was developed. This model regardless the value of surface tension of the liquid can predict the spreading of low viscous (1-4cSt) liquids on a moving surface. Secondly, liquids with viscosity (1-5cSt) and surface tension (17.4-72.8mNm) were used to study the drop impact on moving and inclined surface. Experiments performed with similar normal (0.9-2.9m/s) and tangential (0.8-2.9m/s) velocities on both surfaces to test our hypothesis that spreading/splashing for these two surface conditions should be same. Results indicates that our hypothesis is true, except for some special conditions when, normal and tangential velocities are greater than the range of our analysis
Droplet Impact onto a Spherical Particle in Mid-Air
No full textCollision between a droplet and a particle has a wide range of applications in chemical and petrochemical industries, polyethylene synthesis, and particle coating. Various studies in the literature indicate that the collision products are very different depending on the size and velocity of the particle and droplet, particle wettability and roughness, and physical properties of the liquid and the surrounding gas. The collision outcome is a liquid film (i.e. lamella) and the objective of this thesis is to identify various impact products in different conditions and to study how each category of the above mentioned parameters or a combination of them affect the lamella formation. Investigation of the droplet impact was divided into two parts: drop impact onto a still particle, and droplet impact onto a moving particle in mid-air. Contribution of this thesis to the field can be summarized as following. First, studying the impact phenomenon in a wider range of both Weber number (0.1<We<1146) and droplet-to-particle diameter ratio (1.4<Dr<5.0) compared to what already exists in the literature. Both experimental and numerical tools were developed and used to study the head-on impact between a droplet and a particle. Second, studying the effect of impact velocity, particle wettability, and the amount that each of these parameter contributes on collision outcomes. The required conditions for a lamella to be formed was also studied, and how the lamella geometry changes in case the impact velocity is changed, or hydrophilic/hydrophobic types of particles are used. Third, investigation of the effect of liquid viscosity on lamella formation; what the dynamics of the liquid is inside the film, and how the fluid field inside the lamella is affected by the viscosity changes. Fourth, identifying the role of ambient gas in lamella formation and how each of the drag and lift forces contribute in creating the liquid film. Fifth, developing a pneumatic droplet generator capable of producing single drops with various droplet sizes. The breakup phenomenon in the nozzle and droplet velocity upon pinch-off were also investigated in detail
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