Hans C. Mayer

Microscale tipstreaming in a microfluidic flow focusing device

A microfluidic flow-focusing device is used to explore the use of surfactant-mediated tipstreaming to synthesize micrometer-scale and smaller droplets. By controlling the surfactant bulk concentration of a soluble nonionic surfactant in the neighborhood of the critical micelle concentration, along with the capillary number and the ratio of the internal and external flow rates, we observe several distinct modes of droplet breakup. For the most part, droplet breakup in microfluidic devices results in highly monodisperse droplets in the range of tens of micrometers in size. However, we observe a new mode of breakup called “thread formation” that resembles tipstreaming and yields tiny droplets in the range of a few micrometers in size or smaller. In this work, we characterize the growth of the thread and its maximum length as a function of flow variables and surfactant content, and we also characterize the period of droplet breakup as a function of these variables. Our results suggest possible methods for controlling the process. Using a simple flow visualization experiment as the basis, we report on preliminary efforts to model the thread formation process.

Landau-Levich flow visualization: Revealing the flow topology responsible for the film thickening phenomena

An extensive body of experimental work has proven the validity of the analysis of Landau and Levich, who were the first to determine theoretically the thickness of the film deposited by the withdrawal of a flat substrate from a bath of liquid with a clean interface. However, there are a number of experimental investigations that have shown that surfactants in the liquid may result in a thickening of the deposited film. Marangoni phenomena have usually been considered responsible for this effect. However, some careful experiments and numerical simulations reported in the literature seemed to rule out this view as the cause of the observed behavior. Despite all these studies and the number of reports of film thickening, an experimental study of the flow field close to the coated substrate in the presence of surfactants has never been undertaken. In this paper we will present a set of flow visualization experiments on coating of a planar substrate in the range of capillary numbers 10−4 ≲ Ca ≲ 10−3 for sodium dodecyl sulfate solutions with bulk concentrations of 0.25 CMC ⩽ C ⩽ 5.0 CMC (critical micelle concentration). It was evident during experiments that the flow field near the meniscus region exhibits patterns that can only be explained with a stagnation point residing in the bulk and not at the interface. As opposed to patterns with an interfacial stagnation point, the observed flow fields allow for the increase in film thickness due to the presence of surfactants compared to the clean interface case.

Singular structures on liquid rims

This experimental note is concerned with a new effect we discovered in the course of studying water hammering phenomena. Namely, the ejecta originating from the solid plate impact on a water surface brings about a liquid rim at its edge with the fluid flowing towards the rim center and forming a singular structure resembling a “pancake.” Here, we present the experimental observations and a qualitative physical explanation for the effect, which proves to be fundamental to the situation when the size and speed of the impacting body are such that the capillary effects become important.

The nature of chemical reaction-driven tip-streaming

The discovery of chemical reaction-driven tip-streaming (also known as “an amazing drop”) was made about a decade ago during measurements of the dynamic interfacial tension of a water-alkali pendant droplet immersed in oil-linoleic acid. A plausible explanation for this self-sustained ejection of micron sized droplets from the tip of the macroscopic pendant drop was offered at that time and attributed to Marangoni stresses driving the reaction-produced surfactant along the interface. Later, asymptotic theory based on the analysis of a complete fluid dynamical formulation supported this hypothesis. As this discovery promised a way of microdroplet generation without the need for complex microchannel geometries or externally imposed flow or electric fields, we were recently motivated to study the influence of the reagent concentrations and reaction rate on the droplet generation. However, in an attempt to recreate the original experiments, we revealed that the cause for tip-streaming is not what it originall...

Liquid film dewetting induced by impulsive Joule heating

Motivated by the need for understanding the boiling processes in three-phase microscopic systems, the present work aims to uncover the physics of forced dewetting of a liquid film initially attached to a metal wire frame, which is heated with a rate up to O(108) K s−1 by discharging a capacitor impulsively. Depending on the corresponding heat flux O(1011) J m−2, there are several key dewetting regimes—no detachment, nonuniform detachment, and uniform detachment of a film—differentiated by boiling transitions in the film Plateau borders. Transitions between these regimes prove to occur, for various wire diameters and frame sizes, around the same values of the capacitor energy per unit wire volume. Also, an intrinsic transverse instability manifesting itself in the formation of fingers along the detached liquid film rim is discovered and analyzed in detail.

A Microfluidic Tensiometer

Recent theoretical predictions indicate that a shift in surfactant transport mechanism from diffusion controlled to kinetically controlled occurs at highly curved interfaces where the length scale is on the same order as feature sizes in microfluidic devices. At present, experimental evidence of this shift in transport mechanism is lacking and this is due to the limitations on degree of interface curvature imposed by traditional methods of surface tension measurement. We show that the measurement of dynamic surface tension is possible at highly curved interfaces using a microfluidic tensiometer that utilizes glass micropipettes to control curvature dimension. Comparison of dynamic surface tension data from our microfluidic tensiometer with data obtained from traditional techniques will validate the theoretical arguments reported, and will improve understanding of two phase flows in microfluidic devices.Copyright