Accelerating 3D printing for surface wettability research

The wettability of a surface is affected by its physical and chemical properties, but it can be modulated by patterning it. Researchers use many different techniques for surface patterning, each one with different trade-offs in terms of cost, flexibility, convenience and realizable geometries. Very high-resolution 3D printing technologies (such as stereolithography by two-photon absorption) have the potential to greatly increase the range of realizable surface geometries, but they are currently not in wide use because they are too slow for printing the relative large surface areas required for wetting experiments. To enable the use of these 3D techniques, we are developing new slicing algorithms able to speed up 3D-printing technologies.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Is superhydrophobicity robust with respect to disorder?

We consider theoretically the Cassie-Baxter and Wenzel states describing the wetting contact angles for rough substrates. More precisely, we consider different types of periodic geometries such as square protrusions and disks in 2D, grooves and nanoparticles in 3D and derive explicitly the contact angle formulas. We also show how to introduce the concept of surface disorder within the problem and, inspired by biomimetism, study its effect on superhydrophobicity. Our results, quite generally, prove that introducing disorder, at fixed given roughness, will lower the contact angle: a disordered substrate will have a lower contact angle than a corresponding periodic substrate. We also show that there are some choices of disorder for which the loss of superhydrophobicity can be made small, making superhydrophobicity robust

Multiresolution Layered Manufacturing

PURPOSE: Two-photon polymerization (TPP) has become one of the most popular techniques for stereolithography at very high resolutions. When printing relatively large structures at high resolutions, one of the main limiting factors is the printing time. The goal of this work is to present a new slicing algorithm to minimize printing times. DESIGN/METHODOLOGY/APPROACH: Typically, slicing algorithms used for TPP do not take into account the fact that TPP can print at a range of resolutions (i.e. with different heights and diameters) by varying parameters such as exposure time, laser power, photoresist properties, and optical arrangements. This work presents Multiresolution Layered Manufacturing (MLM), a novel slicing algorithm that processes 3D structures to separate parts manufacturable at low resolution from those that require a higher resolution. FINDINGS: MLM can significantly reduce the printing time of 3D structures at high resolutions. The maximum theoretical speed-up depends on the range of printing resolutions, but the effective speed-up also depends on the geometry of each 3D structure. RESEARCH LIMITATIONS/IMPLICATIONS: MLM opens the possibility to significantly decrease printing times, potentially opening the use of TPP to new applications in many disciplines such as microfluidics, metamaterial research or wettability. ORIGINALITY/VALUE: There are many instances of previous research on printing at several resolutions. However, in most cases, the toolpaths have to be manually arranged. In some cases, previous research also automates the generation of toolpaths, but they are limited in various ways. MLM is the first algorithm to comprehensively solve this problem for a wide range of true 3D structures.NANO3D (a BEWARE Fellowship from the Walloon Region, Belgium, part of the Marie Curie Programme of the ERC). IAP 7/38 MicroMAST (Interuniversity Attraction Poles Programme from the Belgian Science Policy Office, the Walloon Region and the FNRS)

On the Statistical Mechanics and Surface Tensions of Binary Mixtures

Within a lattice model describing a binary mixture with fixed concentrations of the species we discuss the relation-ship between the surface tension of the mixture and the concentrations

Wetting transitions and contact angles

peer reviewe

How wettability controls nanoprinting

Using large scale molecular dynamics, we study in detail the impact of nanometer droplets of low viscosity on substrates and the effect of the wettability between the liquid and the plate. We show the maximal contact diameter during the nanodroplet impact (Dmax) as well as the time required to reach it (tmax) are in agreement with experimental data at the macroscale showing similarities between droplet impacts at the nano and the macro scales. The comparison between the MD simulations and different models reveals that most of these models do not take into acvount all the effects we observe at the nanoscale. Moreover, most of their predictions for the impact at the nanoscale do not correspond to the simulation results. Because of this, we have developed a simple model for Dmax which is in agreement not only with the simulation data but also the experimental observations and it also takes into account the effects of the liquid-solid wettability. We also propose a new scaling for tmax with respect to the impact velocity which is also in agreement with the experimental observations. With the new model for Dmax plus the scaling found for tmax, we present a new way to collapse in a master curve the evolution of the micro to nanometer drop contact diameter during impact for different wettabilities and different impact velocities. We believe our results may help to design better nanoprinters since they provide an estimation of the maximum impact velocities required to obtain a smooth and homogeneous coverage of the surfaces without dry spots

Surface wettability to improve heat transfer

Heat pipe characteristics are linked to the surface properties of the diabatic surfaces, and, in the evaporator, surface properties influence both the onset boiling temperature (TONB) and the critical heat flux (CHF). In this work the effect of surface wettability in pool boiling heat transfer is studied in order to understand if there could be a path to increment heat pipe thermal performance. This work analyses the effects of surface wettability on boiling (tested fluid is pure water) and proposes a new super-hydrophobic polymeric coating which can have a very important effect in improving the heat pipe start-up power load and increasing the thermal performance of heat pipes when the flux is lower than the critical heat flux. The polymeric coating is able to reduce the TONB (-11% from 117°C to about 104°C) compared with the uncoated surfaces, as it inhibits the formation of a vapour film on the solid-liquid interface, avoiding CHT conditions up to maximum wall temperature (125°C). This is realized by the creation of a heterogeneous surface with SHS zones dispersed on top of a hydrophilic surface (stainless steel surface). The proposed coating has an outstanding thermal resistance: No degradation of SH properties of the coating has been observed after more than 500 thermal cycles

S.O.S. approximants for Potts crystal shapes

peer reviewe

A new model to predict the influence of surface temperature on contact angle

Abstract The measurement of the equilibrium contact angle (ECA) of a weakly evaporating sessile drop becomes very challenging when the temperatures are higher than ambient temperature. Since the ECA is a critical input parameter for numerical simulations of diabatic processes, it is relevant to know the variation of the ECA with the fluid and wall temperatures. Several research groups have studied the effect of temperature on ECA either experimentally, with direct measures, or numerically, using molecular dynamic simulations. However, there is some disagreement between the authors. In this paper two possible theoretical models are presented, describing how the ECA varies with the surface temperature. These two models (called Decreasing Trend Model and Unsymmetrical Trend Model, respectively) are compared with experimental measurements. Within the experimental errors, the equilibrium contact angle shows a decrease with increasing surface temperatures on the hydrophilic surface. Conversely the ECA appears approximately constant on hydrophobic surfaces for increasing wall temperatures. The two conclusions for practical applications for weakly evaporating conditions are that (i) the higher the ECA, the smaller is the effect of the surface temperature, (ii) a good evaluation of the decrease of the ECA with the surface temperature can be obtained by the proposed DTM approach