Fast Drop Movements Resulting from the Phase Change on a Gradient Surface

Abstract

The movement of liquid drops on a surface with a radial surface tension gradient is described here. When saturated steam passes over a colder hydrophobic substrate, numerous water droplets nucleate and grow by coalescence with the surrounding drops. The merging droplets exhibit two-dimensional random motion somewhat like the Brownian movements of colloidal particles. When a surface tension gradient is designed into the substrate surface, the random movements of droplets are biased toward the more wettable side of the surface. Powered by the energies of coalescence and collimated by the forces of the chemical gradient, small drops (0.1 to 0.3 millimeter) display speeds that are hundreds to thousands of times faster than those of typical Marangoni flows. This effect has implications for passively enhancing heat transfer in heat exchangers and heat pipes.

Local Heating of Discrete Droplets Using Magnetic Porous Silicon-Based Photonic Crystals

“…Because each droplet can be independently controlled, highly integrated, scalable and flexible architectures can be implemented. 10 A number of techniques have been described for the actuation of droplets on solid surfaces including the use of thermocapillary effects, 14 photochemical effects, 15 electrochemical gradients, 16 surface tension gradients, 17 temperature gradients, 18 air pressure, 19 structured surfaces, 20 dielectrophoresis, 21 and electrostatic methods. 8 An extension of this approach is a liquid-liquid microfluidic system for manipulating freely suspended microliter or nanoliter droplets.…”

External‐Field‐Induced Gradient Wetting for Controllable Liquid Transport: From Movement on the Surface to Penetration into the Surface

“…[9][10][11][12][13][14][15][16] The wettability gradient of a surface for a liquid droplet with an asymmetrical contact angle (CA) can produce a driving force for liquid motion, which is generally generated by introducing a chemical or structure gradient. [17][18][19][20][21][22][23][24][25][26][27][28] External-field-responsive liquid transport has received extensive research interest owing to its important applications in microfluidic devices, biological medical, liquid printing, separation, and so forth. To realize different levels of liquid transport on surfaces, the balance of the dynamic competing processes of gradient wetting and dewetting should be controlled to achieve good directionality, confined range, and selectivity of liquid wetting.…”

“…[17][18][19][20][21] The structure gradient can be created by varying the surface micro-/ nanoscale structure. [22][23][24][25][26][27][28] Based on the advancements in engineering for surface fabrication, the liquid-gradient wettingdewetting process and the corresponding liquid film thickness and final wetted shape can be controlled.…”

“…The gradient wetting of a preset surface is fixed, and the liquid motion is very limited because of hysteresis phenomena if there is not a sufficient wettability difference. [18][19][20][21] A more direct way to create a wettability gradient is to adjust the chemical components or micro-/nanoscale structures of the material surface in response to an external field. [29][30][31][32][33][34][35][36][37][38][39] Responsive materials have been extensively studied under external stimuli, such as light illumination, [29,30] magnetic fields, [31][32][33][34] electric fields, [35,36] temperature, [37] gravity, [38] and mechanical stretching.…”

Pressure-driven flow in open fluidic channels

“…These include microcontact printing (3), vapor deposition, and photolithography (4,5). Fluidic motion or guided flow on the substrate can be observed (6) by changing the wetting properties with time. For example, a light induced guided motion of fluids has been reported using a special surface treatment, where the free energy could locally be changed under illumination (7).…”