urface force. This resistance in surface is caused

urface tension measurements were carried out for all the samples as well as polymer melt mixtures of different surfactant concentration to elucidate the effect of surface tension on the lifetime of the liquid thin films as well as to calculate the elasticity of the films using modified Frankel experiment.We used Teclis Tracker tensiometer (Figure 2&3) for our surface tension measurements which measures the variation in surface tension or interface tension over time. Surface tension is nothing but a property of liquid surface that allows it to hinders the application of any external force. This resistance in surface is caused due to adherence of similar particles to each other on the surface.Figure 2:Teclis Tracker tensiometer                         Figure 3: Schematic of tensiometerSingle bubble method is adopted by the tensiometer for the surface tension calculation where the interfacial tension is calculated form the study of the shape of the bubble which is constantly recorded with a high resolution camera and the Laplace equation is fitted to the imagery to obtain the surface tension. Syringe used for bubble formation is motor controlled to maintain precision and produce bubble of pre-determined volume.

It also allows for sinusoidal variation in volume of the bubble for the calculation of elasticity modulus. It is very much useful is determining the characteristics of the surfactant as well as its effectiveness and behaviour on the surface. Dynamic contact angle measurement can be used to carry out calculations like surface elasticity and rigidity.For our experiment the sample was kept under controlled environment and the temperature was maintained at 25°C other parameters were: Initial bubble volume 3.5ml ; Final volume 3ml ; drop formation speed 20.

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 The experiment was allowed to run until the equilibrium surface tension was achieved for all the samples. Although the time taken to achieve equilibrium for pure samples is 1-3 hours, the surfactant mixtures with low surfactant concentration takes about 2-3 days to achieve equilibrium.Spectrometry Oceans Optics Flame spectrometer with light source having both deuterium and halogen bulbs are used to measure the drainage of the free standing liquid thin films, Spectroscopy is desired as it is a non-destructive testing method to determine the thickness of the films as free standing films are very fragile and tend to break even due to slightest of turbulence in the surrounding environment.Figure 4: General Spectrometry setupThe thickness can be determined at a particular time from a single spectrophotometric recording using either transmittance or reflectance mode. For our experiment we will concentrate on transmittance mode.In transmittance mode when light from the source strikes the film at an angle of 0° the intensity of light transmitted through the thin film creates an undulated interference spectrum as shown is figure 5. This interference spectrum gives us the transmittance vs wavelength data at a particular instance which can be interpreted to deduce the thickness of the film at that point in time.

Figure 5: Interference spectrum obtained  from spectrometry  Interpreting interference spectrum for thicknessTo extract thickness from the interference spectrum we use the equation mentioned belowt   =       Mab?a?b      2(?a-?b)(n12-sin2?0)1/2  t : Film thickness?a & ?b : Maxima and Minima of the wavelengthMab: No. of fringes separating Maxima and Miniman1 : Refractive index of the polymer filmsin2?0 : Angle of incidenceAs we are using transmittance mode the angle of incidence is 0°. To determine no. of fringes for example if two adjacent maxima are chosen then Mab is 1 however if we select one minima and adjacent maxima then Mab is 0.5.

Mab = 3 ?b?aFigure 6: illustration for thickness calculation from interference spectrumThe above interference spectrum is for surfactant mixture and as it indicates we have selected two minima’s and the peaks between these two minima’s gives us Mab (no. of peaks) which is then utilized in the calculation of film thickness.This interference spectrum was obtained from the thinning zone of the film at a film of length of 7mm.Experimental setup and procedure7645321Figure 7:Experimental setup for drainage experimentAbove is the schematic diagram of the experimental setup for drainage measurement.

(1,7) Spectrometer probe support.(2,6) transmitting and receiving spectrometer probes.(3) support for film frame connected to a microcontroller for pulling speed variation and film length control.(4) Pure polymer/ Surfactant solution tank.(5) 3d printed polymer frame gridded with three nylon fishing lines.

Experimental procedurePolymer was kept in the melt tank and the frame is lowered into the tank till the fish lines are completely dipped in the polymer melt whose coordinates are then marked as zero. The frame is then pulled up from the tank with controlled pulling velocity till the desired film length is achieved.


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