Qi Ni

Studying of Contact Angle Friction and Contact Angle Hysteresis (CAH) Though Force Measurements

A novel method of measuring contact line friction and contact angle hysteresis is described. In this method, a droplet is constrained between two surfaces while the surface of interest initiates motion. The results are compared to conventional characterization methods such as measuring the angle of inclined plane for droplet motion and measuring advancing and receding contact angles by infusing/withdrawing liquid from the substrate. At slow speeds, the proposed method provides a measure of the hysteresis but can also capture information about the contact line friction and viscous affects. Droplet force dependence on droplet size (height/width) is also investigated.Copyright

Micro-Scale Part Manipulation on a Liquid Interface Through Interface Curvature Effects

Microscale assembly has many factors that limit assembly rates [1]. At this scale, capillary interactions between particles and nearby substrates are significant, and can be utilized for controlling assembly processes [2,3]. Typically these assembly processes involve direct capillary bonding, but lateral capillary forces can also be applied to floating parts by changing the local curvature of the fluid interface [4]. In this work, we introduce some basic concepts of a microscale component integration system that utilizes local changes in the fluid interface curvature to manipulate floating prismatic parts. Two approaches for achieving fluidic micro-integration, on a water-oil interface, are proposed. The first technique is intended to individually acquire, re-position and release floating parts. It has the capability of short distance part translation/orientation. The second technique provides long-distance part conveying.Copyright

Ultrasonic Excitation Induced Wenzel to Cassie Transition

Wetting on textured solids has gained much attention in the past decade due to increasing interest in artificial superhydrophobic surfaces. (Bahadur & Garimella, 2007; Boreyko & Chen, 2009; Forsberg, Nikolajeff, & Karlsson, 2011; Heikenfeld & Dhindsa, 2008) On textured surfaces, the wetting liquid can be in either the Cassie–Baxter state, which the liquid does not fill the surface texture; or the Wenzel state, which the liquid completely wets the surface and fills the recesses. For a hydrophobic micro-scale rough surface, the Cassie state is usually a more favorable state since it requires less energy. However, due to contact angle hysteresis, the Wenzel state can also be meta-stable. By controlling the roughness of the texture and initial droplet position, both Cassie and Wenzel states can be stable simultaneously. (Koishi, Yasuoka, Fujikawa, Ebisuzaki, & Xiao, 2009) However, with the proper energy input, the droplets can be induced to transition between states. While multiple methods have been developed to switch from Cassie to Wenzel states (Bormashenko, Pogreb, Whyman, & Erlich, 2007; Krupenkin et al., 2007; Kumari & Garimella, 2011; Ran, Ding, Liu, Deng, & Hou, 2008), it is much more difficult to switch from the Wenzel state to the Cassie state. Wenzel-Cassie transitions have been achieved by changing the surface structure to destabilize the Wenzel state (Krupenkin et al., 2007)(Ran et al., 2008) or by changing the ambient fluid. (Dhindsa et al., 2006)Copyright

Controlling Normal Stiffness in Droplet-Based Linear Bearings

While capillary forces are negligible relative to gravity at the macroscale, they provide adequate force to effectively manipulate millimeter to micro meter objects. The fluidic actuation can be accomplished using droplets that also act as bearings. While rotary droplet bearings have been previously demonstrated, this paper addresses the positioning accuracy of a droplet-based bearing consisting of a droplet between a moving plate and a stationary substrate with constrained wetting region under a normal load. Key wetting cases are analyzed using both closed form analytical approximations and numerical simulations. The vertical force and stiffness characteristics are analyzed in relation to the wetting boundaries of the supporting surface. Case studies of different wetting boundaries are presented and summarized. Design strategies are presented for maximizing load carrying capability and stiffness. These results show that controlled wetting and opposing droplet configurations can create much higher stiffness fluidic bearings than simple droplets.