Sam Dehaeck

Vapor-Based Interferometric Measurement of Local Evaporation Rate and Interfacial Temperature of Evaporating Droplets

The local evaporation rate and interfacial temperature are two quintessential characteristics for the study of evaporating droplets. Here, it is shown how one can extract these quantities by measuring the vapor concentration field around the droplet with digital holographic interferometry. As a concrete example, an evaporating freely receding pending droplet of 3M Novec HFE-7000 is analyzed at ambient conditions. The measured vapor cloud is shown to deviate significantly from a pure-diffusion regime calculation, but it compares favorably to a new boundary-layer theory accounting for a buoyancy-induced convection in the gas and the influence upon it of a thermal Marangoni flow. By integration of the measured local evaporation rate over the interface, the global evaporation rate is obtained and validated by a side-view measurement of the droplet shape. Advective effects are found to boost the global evaporation rate by a factor of 4 as compared to the diffusion-limited theory.

Analyzing closed-fringe images using two-dimensional Fan wavelets

In this paper, it will be shown how the use of two 2D Fan wavelets to analyze closed-fringe images can lead to a relatively fast and exceptionally noise-resistant algorithm capable of extracting not only local phase but also local frequency information. Our algorithm is up to 10 times faster than the current state-of-the-art in wavelet processing techniques and even up to 30 times faster than windowed Fourier transform programs, which achieve similar noise-resiliency figures. This improvement is mainly achieved by the use of Fan wavelets instead of Morlet wavelets, but a more efficient scale-space discretization strategy is also described, and three different alternatives are suggested capable of solving the phase sign-ambiguity problem in a quick and efficient manner. Finally, the application of the algorithm to real and numerically generated images shows that a precision of 1/30th of a fringe is achievable for noise levels going up to 1/5th of the input contrast.

3D-printed vision-based micro-force sensor dedicated to in situ SEM measurements

Working at micro-scale efficiently requires accurate and integrated force feedback implemented with a sensor adapted to the scale. This paper presents a 3D-printed vision-based micro-force sensor intended to be used inside the chamber of a Scanning Electron Microscope (SEM). The combination of 3D printed elastic structures with a highly effective vision based measurement method allows to design integrated sensors at the cutting edge of the state of the art. Moreover the presented design respects the Abbes alignment principle. The paper presents the general design, manufacturing and experimental characterization in SEM environment of the proposed sensor. Images of periodical patterns are used to measure the differential displacement between the two parts of the compliant structure. By the knowledge of its stiffness, the force applied on it is measured. The stiffness of the elastic structure has been measured to be 15.3 N.m−1, leading to a force range of 25 µN.

Zero-overlap stitching of microlens arrays with two-photon polymerisation

When printing microlens arrays with a Nanoscribe Photonics Professional machine, the object is typically split in many smaller hexagonal or square fields, which are written with high-precision galvo-scanning mirrors (or with a piezo-stage). Displacements between these sub-fields are performed with a mechanical stage, which has a lower precision (±1μm). This results in misalignments in the XY-plane. However, also differences in the Z-plane are present due to limitations of the interface finding method. If such a stitching seam goes through a microlens, its resolving power is affected. In the present contribution, we will present an adaptive stitching algorithm that automatically places the writing blocks in such a way so as to avoid cutting through the individual microlenses completely.

Magnetomigration of Rare-Earth Ions Triggered by Concentration Gradients

Mach-Zehnder interferometry was applied to explore the effects of inhomogeneous magnetic fields on the mobility of rare-earth ions in aqueous solutions. No migration of ions was observed in a thermodynamically closed system when a homogeneous solution was subjected to a magnetic field gradient alone. However, magnetomigration could be triggered by a concentration gradient of the rare-earth ions in the solution. When a concentration gradient was introduced in the sample by solvent evaporation, consistent migration of paramagnetic Dy3+ ions from the bulk solution to regions with stronger magnetic fields was observed. By contrast, no movement was detected for diamagnetic Y3+ ions in the presence of a concentration gradient.

Evaporation dynamics of completely wetting drops on geometrically textured surfaces

This study deals with the evaporation dynamics of completely wetting and highly volatile drops deposited on geometrically textured but chemically homogeneous surfaces. The texturation consists in a cylindrical pillars array with a square pitch. The triple line dynamics and the drop shape are characterized by an interferometric method. A parametric study is realized by varying the radius and the height of the pillars (at fixed interpillar distance), allowing to distinguish three types of dynamics: i) an evaporation-dominated regime with a receding triple line; ii) a spreading-dominated regime with an initially advancing triple line; iii) a cross-over region with strong pinning effects. The overall picture is in qualitative agreement with a mathematical model showing that the selected regime mostly depends on the value of a dimensionless parameter comparing the time scales for evaporation and spreading into the substrate texture.