Session T04: Drops: Dynamic Surface Interactions (8:00am - 8:45am CST)

Droplet Mobility and Heat Transfer during Condensation Phase-Change on Hierarchical Lubricant Infused Surfaces

Lubricant infused surfaces (LISs) have drawn increasing attention due to their excellent low-adhesion enabling the occurrence of condensation in a dropwise manner. In this work, we address the condensation performance on LISs paying special attention to the surface structure underneath the condensing water droplets and the oil. We make use of hierarchical micro-/nano-structured and solely nano-structured LISs fabricated as in Maeda \textit{et al.}, \textit{Appl. Therm. Eng.} \textbf{176} (2020). Important differences are here reported when comparing the condensation performance in the presence and absence of micro-structures. On one hand, the presence of micro-structures enhances mobility and droplet shedding performance shifting the droplet size distributions towards greater number of smaller-sized droplets. Whereas on the other hand, the additional thermal resistance exerted by the micro-structures and the oil is responsible for the deterioration of the theoretical heat transfer performance. We conclude that a fine balance between enhancing droplet mobility without penalizing heat transfer must be carefully achieved for the effective rational design of LISs with enhance heat transfer performance.   

Time-dependent mechanical response of ice adhesion on aluminum substrates

Ice adhesion on aerospace-relevant materials is both complex and not well understood. Measuring and understanding the underlying physics requires reliable testing techniques that can yield accurate, multifaceted datasets. Important considerations include surface roughness and its spatial correlation structure, resolution of substrate-induced strain, and direct mechanical adhesion testing. To shed light on ice adhesion on relevant surfaces, we performed time-dependent stress ramps on aluminum specimens in the temperature range -20 to -7$^{\mathrm{o}}$C. Additionally, we characterized the spatial correlation surface roughness maps of the aluminum samples. Our stress-ramp test data using a stress-controlled rheometer showed that the apparent average critical stress depends on both stress-ramp rate and temperature. More specifically, the adhesion strength is higher for steeper stress rates and increases with decreasing temperature. The stress-ramp test appears to represent an upper bound of the time-dependent adhesive behavior of ice. These results take us a step forward to understanding ice adhesion mechanisms.   

The Role of Wettability on the Shedding of Drops

The study explores how a small change in the contact angle (CA) of a drop makes a difference in the shedding from a surface with similar wettability. Three oleophobic surfaces, polystyrene (static CA of 93\textpm 3\textdegree ), tin (104\textpm 4\textdegree ) and glass (118\textpm 4\textdegree ), were chosen. Silicon oil drops (350cSt) of various volumes were shed due to water shear flow. The velocity of shedding of a drop when the adhesion is balanced by the drag provided by the shear-flow, is termed as the critical velocity (Ucr). The results show that Ucr for a given drop on the polystyrene surface is~higher than that of the tin and glass surfaces.~The higher contact length for drops on polystyrene surfaces, makes the adhesion higher. The lower frontal area makes less drag to be applied for the same velocity of the shear-flow. Therefore, higher drag becomes necessary for the shedding. So,~the Ucr increases with the wettability (polystyrene \textless tin \textless glass). The CAs of a drop predominantly found to control the shedding. The Ucr is found to decrease with the increase in the volume for a given surface. Bigger drops provide higher frontal area and makes the drag higher. The flow map is discussed from a modified Weber and Ohnesorge numbers.   

Sedimentation of drops in a viscous liquid on an inclined plane.

We present the dynamics of sedimentation of water drops ($\mu_{w} =8.92\times 10^{-4}Ns/m^{2})$ in silicone oil of viscosities $0.019Ns/m^{2}<\mu_{si} <0.095Ns/m^{2}$ on a flat plate inclined at angles ranging from $10^{o}<\alpha <30^{o}$.The drop motion falls in the Stokes regime since Reynolds Number $Re=\frac{Ud}{\nu_{si} }<1$. The range of Bond Number $(Bo=\frac{\Delta \rho gR^{2}}{\sigma })$ from $0.075 [Preview Abstract]

Moving Droplets on a Wall

In this work, we use the penalty Immersed Boundary Method (pIBM) to simulate the movement of liquid droplets hanging on a vertical wall. We propose a 2D numerical method based on pIBM to tackle the moving contact line problem. Note that the vertical wall is hydrophilic and does not allow slip in most cases, however, in reality we do observe droplets advance on hydrophilic surfaces. We use Lagrangian markers to represent the droplet interface and compute surface tension. The forcing scheme is designed to unify both the surface tension and the unbalanced Young's forces at the contact point into one general equation. We also employ a dynamic re-sampling technique to ensure the uniform distribution of Lagrangian markers.   

Capillary Rise Dynamics of Well-Wetting Liquids on the Outer Surfaces of Cylindrical Nozzles in Low Flow Rate Limits

Parts of well-wetting liquids exiting small-diameter nozzles at low flow rates can rise up along the outer nozzle surfaces. This is problematic for fuel injectors and other devices that incorporate arrays of nozzles to distribute liquids. We will report our experimental and numerical study of this phenomenon for wide ranges of parameters, including the nozzle outer diameter (0.2 to 3.2 mm), flow rate (0.1 to 300 $\mu $L/s), viscosity (1.75 to 970 mPa s), and surface tension (18.7 to 45 mN/m). Our study showed that the interplay of three dimensionless numbers (the Bond number, the Weber number, and the Ohnesorge number) governs the capillary rise dynamics. In general, as the flow rate increases, the capillary rise height at each normalized time becomes smaller. We identified limiting liquid flow rates below which the temporal evolution of the meniscus positions can be well approximated by a quasi-static model based on the Young-Laplace equation. The nozzle diameter and the contact angle at the nozzle surface primarily determine the maximum meniscus height for a particular liquid in the limit of near zero flow rates. For a given contact angle, the capillary rise ceases to occur above a predictable threshold Bond number.