And Yet it Moves! Microfluidics Without Channels and Troughs

Abstract

A simple, versatile, rapid, and inexpensive procedure based on the immersion method is developed to fabricate chemical gradients on chemically activated Si/SiO2 surfaces by a trichloro (1H,1H,2H,2H-perfluorooctyl) silane self-assembly monolayer (SAM). Contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy data based on the intensity of the signals of C1s and F1s, which progressively increase, indicate that the surface is characterized by the presence of increasing amounts of the SAM along the gradient direction. Experimental conditions are optimized by maximizing the variation of the contact angle of water drops at the starting and the ending points of the gradient. The application of the chemical gradient to droplet motion is demonstrated. The results are rationalized by dissipative particle dynamics simulations that well match the observed contact angles and the velocities of the drops. The simulations also show that the intrinsic nature of the gradient affects the velocity of the motion

Directed Autonomic Flow: Functional Motility Fluidics

“…Where the relative position of the movement was mainly dependent on the angular aperture, speed and distance were found to be influenced by other parameters as well such as the absolute difference between hydrophilicity and hydrophobicity, gradient steepness, and linearity of wettability development. These findings are in line with recently performed modeling studies by Lugli et al revisiting the original Whitesides system and indeed via parameter manipulation identified that speed and droplet movement progression greatly depend on the profile of gradient . Their modeling results coincide with droplet speed and movement progression dependent on the wettability gradient development as found here (Section SI7, Supporting Information).…”

“…The movement was induced by a chemical surface gradient and was capable of moving a water droplet even against gravity on a slope of 15°. This work initiated detailed analysis including surface characterization and theoretical calculations to explain this phenomenon . Additionally, mechanical gradients and switchable light responsive surfaces were found to drive liquid motion, including movement on graphene, but displayed shorter and slower movement than the pioneering work of Whitesides or needed additional energy input such as heat to improve the movement process …”

Self-Running Liquid Metal Drops that Delaminate Metal Films at Record Velocities

“…The gradients may arise from thermocapillarity [13][14][15][16] , gradients in chemical species [17][18][19][20][21][22] , reactive wetting [23][24][25][26][27][28][29] , physical surface structures (e.g., asymmetric topography on a surface) [30][31][32][33] , or using external triggers [34][35][36][37][38] (including voltage 34,39 ). In general, these approaches often utilize asymmetric contact angles on both sides of the droplet to induce motion 40,41 , which often requires vibrations 3 to reduce pinning and drag of the droplets 42,43 .…”

Unidirectional Liquid Manipulation Via an Integrated Mesh with Orthogonal Anisotropic Slippery Tracks

“…With respect to the transporting logic, the unidirectional wetting phenomena can be divided into two strategies. One strategy is the on‐surface liquid spreading or motion in one specific direction, which depends on the asymmetrical microstructure or the well‐designed surface wettability . The other strategy is the one‐way fluid penetration through an asymmetric membrane with specific modification.…”