Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q21: Drops: Spreading and Wetting I |
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Chair: Sadegh Dabiri, Purdue University Room: 603 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q21.00001: Wetting dynamics on asymmetric microstructured surfaces Susumu Yada, Shervin Bagheri, Jonas Hansson, Minh Do-Quang, Fredrik Lundell, Wouter Van Der Wijngaart, Gustav Amberg Microstructured surfaces which are able to control the direction of liquid transport are common in nature for fog/water harvesting, surface lubrication, and self-cleaning, and have been inspiring enormous number of man-made structures. However, the spreading of a liquid on such surfaces have been investigated in the slow spreading regime and a fundamental understanding of the early rapid wetting is lacking. In this work, our experimental and numerical investigations on surfaces with periodic patterns of asymmetric microridges provide detailed illustrations of the rapid droplet spreading over complex surface structures. We show that the surface structures are partly wetted as the air-liquid interface above the contact line makes another contact to the structure downstream and creates a new wetting front, leaving some dry surface behind. Furthermore, we elucidate how different physics play roles in different flow directions. In one direction, the spreading is governed by the friction at the moving contact line and the Young's force related to the local dynamic contact angle, whereas in the other direction, it is determined by the contact line pinning and the inertia of the droplet. Based on these physical insights, the effect of different surface geometry is discussed. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q21.00002: Wetting dynamics of a droplet on micro-pillar surfaces with radially varying pitch Rajneesh Bhardwaj, Manish Kumar, Kirti Chandra Sahu We experimentally investigate the wetting dynamics of a droplet placed gently on a square-micropillar surface. These pillars are located with a radially varying pitch described by a parabolic equation. Two sets of surfaces with radially increasing and radially decreasing pitches from the center of the substrate at which the droplet is initially placed have been prepared on silicon wafer using photolithography. Due to the radial variation of the pitch, the droplet experiences a wettability gradient (either increasing or decreasing radially). We observed that on the surface with the radially increasing pitch, the droplet remains in the Cassie state and exhibits higher contact angle than the smooth surface during its spreading stage. On the other hand, in case of the surface with radially decreasing pitch, the droplet goes into the Wenzel state and assumes a lower contact angle as compared to that observed in the smooth surface. The wetted diameter of the droplet is found to be smaller in case of the radially decreasing surface than the radially increasing surface. We also studied the effect of the size of the square pillars and it is found the droplet spreads less in case of smaller size of pillars for both radially increasing and decreasing surfaces. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q21.00003: Cassie-Baxter to Wenzel Transition and a New Phenomenon called ``the Wenzel Deviation'' Arash Azimi, Chae Rohrs, Ping He In general, on rough surfaces, two wetting regimes are possible: (1) the Cassie-Baxter state, in which the droplet sits on top of rough structures, and (2) the Wenzel state, in which the droplet completely sinks into the rough structures. In this talk, we present a numerical study of the Cassie-Baxter to Wenzel transition using a series of 3D simulations for a water droplet on micro-patterned substrates, in which the pillar height and spacing are systematically varied. The contact angles for each case are measured and compared with either the Cassie-Baxter or Wenzel equation. The total surface energy and its time evolution are discussed in detail. Energy barriers for the wetting transition are addressed. Measured contact angles show an excellent agreement for the Cassie-Baxter state, while for the Wenzel state, we find a systematic deviation from the Wenzel equation when the pillar size is large. The critical pillar size, above which the Wenzel deviation is outstanding, is identified based on simulation results and thermodynamic calculations. A modified Wenzel equation is developed to account for the Wenzel deviation. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q21.00004: Walking, Climbing, Bursting, and Shooting : Complex Dynamics in Drops on Vibrated Substrates Lyes Kahouadji, Seungwon Shin, Jalel Chergui, Damir Juric, Richard Craster, Omar Matar We use direct numerical simulations (DNS) to study the phenomena observed in the work of Brunet {\it et al}. (Phys. Rev. Lett., 99, 144501, 2007). Here a drop can climb up an inclined surface when it is subjected to a vertical oscillation in the presence of a gravity. In this talk, we present a detailed study of these climbing phenomena using DNS with a generalized Navier boundary condition in the context of a front-tracking-based multiphase method. Further detailed numerical simulations in the context of vibrated droplet are extended to different vibration configurations (horizontal, vertical, and oblique) in order to explain how these climbing phenomena occur leading to regimes characterised by droplet `walking’, `bursting’, and `shooting’. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q21.00005: Experimental and numerical study of wetting liquids rising up on the outer surface of a nozzle in the dripping regime Erfan Sedighi, Abolfazl Sadeghpour, Hangjie Ji, Claudia Falcon, Y. Sungtaek Ju, Andrea Bertozzi Well-wetting liquids exiting small-diameter nozzles in the dripping regime rise up along the outer nozzle surfaces. This is problematic for certain fuel injectors and direct contact heat and mass exchangers that incorporate a dense array of nozzles to distribute liquids. Such flows along nozzle outer surfaces are governed by the interplay of surface tension, non-uniform pressure within a pendant drop, gravity, and viscous forces. We experimentally and numerically (by solving the full Navier-Stokes equations) study these flows for nozzle outer diameters ranging from 0.7 to 3.2 mm, mass flow rates ranging from 0.0002 to 0.035 grams/second (g/s), and liquid viscosities ranging from 4.6 to 970 mPa.s. The apparent height of a liquid meniscus on the nozzle surface was determined by analyzing video images. We also develop an approximate analytic model to capture the dynamics of meniscus rising. Our results show that, for a single nozzle, while the mass flowrate is relatively low, the rate of initial rise decreases with further decreasing the mass flow rate. On the other hand, for flow rates higher than 0.006 g/s and viscosities less than 100 mPa.s, the rate of initial rise is almost constant. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q21.00006: Contact angle and interface geometry immediately after rapid initiation of the contact line motion Takahiro Ito, Kenji Katoh, Tatsuro Wakimoto The dynamic contact angle on non-smoothed surface generally shows different variation from that expected by the theory (eg, Cox, J. Fluid Mech., 1985). One plausible mechanism for such deviation can be the local stick-slip motion of the contact line. In this study a model is developed to describe interface deformation induced by the rapid initiation of the contact line motion. The model is based on the balance of the normal stress on the surface under the Stokes approximation. The local surface stress is modified from the expression by Huh et al (J Colloid Int Sci, 1971). The contact line velocity is replaced with 'characteristic velocity' in order to take the lag of the time development of the local velocity relative to the contact line speed. The finite speed of the transfer of the interface deformation is also modeled. These modifications are combined with the conventional Hoffman-Tanner-Voinov relation for the dynamic contact angle and the contact line speed. The results obtained with the developed model shows good agreement with the experimental results. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q21.00007: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q21.00008: Numerical Investigation of Droplet Wetting Behavior on Groove-decorated Surface. Zhicheng Yuan, Mitsuhiro Matsumoto, Ryoichi Kurose Super-hydrophobic surfaces are reported as promising candidates for self-cleaning, anti-icing, and dropwise condensation. Therefore, there are some experimental studies and numerical simulations of droplets on hydrophobic walls. However, regarding the durability issues, an alternative technology, improving the surface wettability by grooves, has drawn much attention, whereas the wetting behavior of droplet on groove-decorated substrate has not been fully studied. In this study, a 3-D numerical simulation employing the Coupled Level-Set and Volume of Fluid (CLSVOF) scheme, and the Continuum Surface Force (CSF) method are applied to a liquid drop on rigid substrate, and the validity is investigated by comparing with the experiment. The numerical models are extended to predict the dynamics of a droplet on groove-decorated substrate. The results show that our numerical methods perform well on tracking the move of a droplet on micro-grooved surfaces. In addition, decoration by micro-grooves could be a useful fabrication technology to improve the surface wettability and develop robustness and durability of super-hydrophobic surfaces. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q21.00009: Title: Complex Wetting: Flow profiles close to three-phase contact lines Benedikt B. Straub, Henrik Schmidt, Franziska Henrich, Massimiliano Rossi, Christian J. Kahler, Hans-Jurgen Butt, Gunter K. Auernhammer Wetting and dewetting behavior on solid surfaces is the crucial process underlying many natural phenomena as well as technical applications. We focus on the technically relevant wetting and dewetting behavior of surfactant solutions. In recent studies, focus laid on the influence of surfactants on macroscopic quantities like the contact angle. To explore the origin of the decrease of the contact angle for increasing surfactant concentration and velocity, we focus on the flow close to the contact line. Therefore, we measure three-dimensional flow profiles with an astigmatism particle tracking velocity setup. The results show that surfactants cause a deviation of the flow field in comparison to theoretical predictions for pure liquids. In the case of a receding contact line, a new air-liquid interface is formed at the three phase contact line. The surfactant concentration at the freshly formed interface is, in comparison to the already existing air-liquid interface, not in equilibrium to the bulk surfactant concentration. This causes Marangoni stresses in the direction of the contact line along the interface. This Marangoni stresses oppose the bulk flow close to the air-liquid interface and causes a deviation of the flow field. [Preview Abstract] |
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