Is contact angle a cause or an effect? – A cautionary tale

The most influential parameter on the behavior of two-component flow in porous media is “wettability”. When wettability is being characterized, the most frequently used parameter is the “contact angle”. When a fluid-drop is placed on a solid surface, in the presence of a second, surrounding fluid, the fluid-fluid surface contacts the solid-surface at an angle that is typically measured through the fluid-drop. If this angle is less than 90°, the fluid in the drop is said to “wet” the surface. If this angle is greater than 90°, the surrounding fluid is said to “wet” the surface. This definition is universally accepted and appears to be scientifically justifiable, at least for a static situation where the solid surface is horizontal. Recently, this concept has been extended to characterize wettability in non-static situations using high-resolution, two-dimensional digital images of multi-component systems. Using simple thought experiments and published experimental results, many of them decades old, it will be demonstrated that contact angles are not primary parameters – their values depend on many other parameters. Using these arguments, it will be demonstrated that contact angles are not the cause of wettability behavior but the effect of wettability behavior and other parameters. The result of this is that the contact angle cannot be used as a primary indicator of wettability except in very restricted situations. Furthermore, it will be demonstrated that even for the simple case of a capillary interface in a vertical tube, attempting to use simply a two-dimensional image to determine the contact angle can result in a wide range of measured values. This observation is consistent with some published experimental results. It follows that contact angles measured in two-dimensions cannot be trusted to provide accurate values and these values should not be used to characterize the wettability of the system.

Normal and oblique drop impact of yield stress fluids with thixotropic effects

Normal and oblique drop impact on a solid surface is numerically analysed for yield stress fluids. A rich diversity of results are generated as a consequence of the exploration of the inertial, elastic, plastic and thixotropic features of the process, as well as the inclination of the solid surface. We show that drops of more thixotropic fluids have a higher tendency to bounce in the normal impact, and to roll or to bounce in the case of an oblique drop impact. Concerning elasticity, we found a critical value for the elastic Ohnesorge number above which no bouncing takes place. Experimental findings such as the fact that the stored energy due to the elasticity of the fluid drop plays a role similar to the stored energy of an interfacial nature in inelastic fluid drops are corroborated in the present study.

ANALISIS FAKTOR KEJADIAN PHLEBITIS DENGAN SIMULASI MODEL FISIS ALAT TERAPI INTRAVENA

Intravenous therapy through long-term infusion is at risk for complications such as phlebitis. The influence of medical factors with a history of hypertension and mechanical factors based on the location of the position of infusion is the main study of the causes of phlebitis.One of the causes of phlebitis is the flow of intravenous fluids which is not proportional to the volume of infusion fluid. Intravenous Therapy Devices with the aim of assessing the physical phenomena modeling experiments intravenous therapy with the theory of fluid mechanics and prove the existence of linkage patient's blood pressure and height of intravenous fluid drop rate. The research method is experimental with the physical modeling of intravenous therapeutic devices. Physical model of intravenous therapy devices using a manometer to measure the pressure tube as diastolic pressure and variation on fluid infusion used was NaCl 0.9% and Glucose 5%. The results of this research was obtained diastolic pressure below 80 mmHg produced a drop rate of fluid infusion is almost constant with a maximum height of a standard intravena pole 1meter, while at an altitude above the altitude variation of normal use by 90 mmHg diastolic pressure with height variations of 1.1 to 1.3 meters yield drop rate a linear of infusion liquid to height variations. So to prevent turbulence of intravenous fluids (the cause of phlebitis) by increasing the location standard for infusion

Labyrinthine and secondary wave instabilities of a miscible magnetic fluid drop in a Hele-Shaw cell

A comprehensive experimental study is presented to analyse the instabilities of a magnetic fluid drop surrounded by miscible fluid confined in a Hele-Shaw cell. The experimental conditions include different magnetic fields (by varying the maximum pre-set magnetic field strengths,$H$, and sweep rates,$SR=\text{d}H_{t}/\text{d}t$, where$H_{t}$is the instant magnetic field strength), gap spans,$h$, and magnetic fluid samples, and are further coupled into a modified Péclect number$Pe^{\prime }$to evaluate the instabilities. Two distinct instabilities are induced by the external magnetic fields with different sweep rates: (i) a labyrinthine fingering instability, where small fingerings emerge around the initial circular interface in the early period, and (ii) secondary waves in the later period. Based on 81 sets of experimental conditions, the initial growth rate of the interfacial length,$\unicode[STIX]{x1D6FC}$, of the magnetic drop is found to increase linearly with$Pe^{\prime }$, indicating that$\unicode[STIX]{x1D6FC}$is proportional to the square root of the$SR$and$h^{3/2}$at the onset of the labyrinthine instability. In addition, secondary waves, which are characterised by the dimensionless wavelength$\unicode[STIX]{x1D6EC}=\unicode[STIX]{x1D706}/h$, can only be triggered when the three-dimensional magnetic microconvection is strong enough to make$Pe^{\prime }$exceed a critical value, i.e.$Pe^{\prime }>19\,000$, where$\unicode[STIX]{x1D706}$is the wavelength of the secondary wave. In this flow regime of high$Pe^{\prime }$, the length scale of the secondary wave instability is found to be$\unicode[STIX]{x1D6EC}=7\pm 1$, corresponding to the Stokes regime; meanwhile, in the flow regime of low$Pe^{\prime }$, the flow corresponds to the Hele-Shaw regime introduced by Fernandezet al.(J. Fluid Mech., vol. 451, 2002, pp. 239–260).