Measuring device for the continuous determination of dynamic surface tension according to the principle of the drop weight method

Abstract

Device for the continuous generation of a constant volume flow, wherein defined droplets are formed on a dripping surface, the drip times of which are detected by a light barrier after the droplets have fallen and depend only on the surface tension at constant density and can be used to monitor the surface tension of various liquids, wherein a volume flow (VST) zu ) flows actively in flow or circulation during the measurement period by means of a pump of the corresponding volume delivery range or passively by introducing the liquid from a higher-lying reservoir into a flow cell with two different bottom stages.

Description

[0001] Process water plays a crucial role in a multitude of technical processes across various sectors. In areas such as wastewater treatment and natural water bodies, understanding the properties of water is also of paramount importance. While volumetric properties like pH, conductivity, and temperature are measured, they do not necessarily provide a comprehensive understanding of the water's characteristics. A further, and currently underestimated, parameter is the surface tension of the water. Although known as a quantity, its practical application is limited by the available measurement technologies. Most current techniques operate discontinuously and require significant personnel resources. Processes are subject to time-dependent changes, thus limiting the effectiveness of discontinuous measurements for process characterization.Experiments for the continuous measurement of surface tension are primarily based on the maximum bubble pressure method.

[0002] The drop volume method for determining the surface tension of liquids is generally a discontinuous measurement procedure in which a liquid sample is drawn into a syringe and then, according to various parameters, pressed out of a measuring capillary. Drops form at the tip of the capillary, the volume of which is determined at the moment of deposition, and the surface tension is calculated. σ=V*ρ*g / 2Π*rKap*f σ = Surface tension [mN / m] V = Drop volume [ml] ρ = density [g / ml] g = acceleration due to gravity [cm / s²] 2 ] r Kap = Capillary radius or radius of the drip area [cm] f = correction factor

[0003] Attempts to perform continuous measurements mostly involve automated syringe filling (DE 196 46 925 C1). Even with similar measurement scenarios, the use of syringes contradicts the principle of continuous determination of dynamic surface tension (DE 100 28 520 A1). Other attempts to utilize a continuous, constant liquid flow generate a volumetric flow rate for a very limited time due to a hydrostatic pressure that is only conditionally constant (VB Fainerman, R. Miller / Colloids Surfaces A; Physicochem. Eng. Aspects 97 (1995) 255-262). These variants cannot be implemented in either continuous or recirculating mode. Furthermore, the measurement setup makes automatic start difficult, as the system usually needs to be manually vented before the measurement begins.

[0004] The aim of the present invention was therefore to develop a method that utilizes the relatively simple droplet weight method to continuously measure surface tension over the longest possible period (hours and days) using a robust measurement setup. The main challenge was the precise and continuous supply of the measuring liquid to a defined dripping area to generate drops whose drip times are characteristic of the surface tension present at that time.

[0005] A device ( Fig. A system for the continuous generation of a suitable constant and long-term volume flow to form defined drops on a special dripping surface, whose drip times after falling are recorded by a light barrier and depend only on the surface tension at constant density, was developed.

[0006] By splitting the inflowing liquid volume VST zu into a volume flow VST diverted via a suitable barrier ab and the volume flow rate VSTt generated by the long-term stable low liquid column r , which is used for droplet formation, it is possible to use the hydrophilic droplet surface ( Fig. ) to form defined drops. VSTab>>VSTtr

[0007] The liquid flow can be generated with any pump suitable for the required delivery range and long-term operation, and requires no additional electronic control. The mechanical design ensures long-term stability of the volume flow for droplet formation, allowing measurements to be performed over the course of hours and days.

[0008] The robustness of the method is complemented by the hydrophilic droplet surface used, which on the one hand enables the low hydrostatic pressure and on the other hand ensures the stable, defined droplet formation through the use of the hydrophobic boundary.

[0009] The ability to perform these processes continuously over extended periods opens up a range of applications in industrial process water monitoring, environmental water monitoring (including wastewater, e.g., from sewage treatment plants), and the evaluation of surfactant degradation kinetics. The availability of this measurement technology will undoubtedly lead to the development of new application areas in the future.

Claims

[1] Device for the continuous generation of a constant volume flow, whereby drops are formed on a drip surface, the dripping times of which are recorded by a light barrier after the drops have fallen and, at a constant density, depend only on the surface tension and can be used to monitor the surface tension of various liquids, whereby a volume flow (VST zu ) in flow or circulation actively during the measurement period by means of a pump of the corresponding volume flow range or passively by introducing the liquid from a higher reservoir into a flow cell with two different bottom stages. [2] Device according to claim 1, wherein the main part (VST ab ) of the incoming volume flow (VST zu) is passed over a barrier which creates a liquid column of low height above the drip surface, wherein the barrier is designed to be height-adjustable, the amount of liquid flowing over the barrier (VST ab ) ensures a constant height above the drip surface, so that VST zu = VST ab + VST tr with VST ab >> VST tr and thus the drip volume flow (VST tr ) for drop formation, where the dripping time and the surface tension to be calculated are a function of the drop volume flow (VST tr ) are. [3] Device according to claim 2, wherein the drip surface is hydrophilic, at which the liquid with the drip volume flow (VSt tr) and forms drops on its lower surface with a defined attachment to a circular hydrophilic surface, wherein a hydrophobic surface with a defined inner radius greater than the radius of the hole defines the hydrophilic surface, wherein the hydrophobic surface is formed by means of applied hydrophobic sprays or hydrophobic liquids or by means of glued-on hydrophobic films with different inner radii greater than the radius of the hole or by means of pressed-on hydrophobic thin plates, preferably made of PTFE, which are exchangeable and variable in the size of the inner radius, but larger than the radius of the hole, and are adjustable and exchangeable by means of a defined recess in the drip surface. [4] Device according to one of the preceding claims, wherein the dripping time of the falling drops is detected by means of a light barrier and fed to an evaluation unit, wherein the dripping time and the determined surface tension over time represent a description of the properties or changes of the liquid under consideration and dosing or control tasks can be carried out with long-term data acquisition. [5] Method wherein several devices according to one of the preceding claims are operated with the same or different liquid flows on an evaluation unit. [6] Use of the device according to one of claims 1 to 4 with drip surfaces with one or a plurality of outflow holes for dosing and producing defined drops, for example for the production of emulsions or microparticles.

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