Elektrik - Elektronik Mühendisliği Bölümü Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12469/47

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Citation - WoS: 5
Citation - Scopus: 5
Epidemic Models for Phase Transitions: Application To a Physical Gel
(Taylor & Francis Ltd, 2017) Bilge, Ayşe Hümeyra; Pekcan, Önder; Kara, Selim; Öğrenci, Arif Selçuk
Carrageenan gels are characterized by reversible sol-gel and gel-sol transitions under cooling and heating processes and these transitions are approximated by generalized logistic growth curves. We express the transitions of carrageenan-water system as a representative of reversible physical gels in terms of a modified Susceptible-Infected-Susceptible epidemic model as opposed to the Susceptible-Infected-Removed model used to represent the (irreversible) chemical gel formation in the previous work. We locate the gel point T-c of sol-gel and gel-sol transitions and we find that for the sol-gel transition (cooling) T-c > T-sg (transition temperature) i.e. T-c is earlier in time for all carrageenan contents and moves forward in time and gets closer to T-sg as the carrageenan content increases. For the gel-sol transition (heating) T-c is relatively closer to T-gs
Citation - WoS: 14
Citation - Scopus: 13
Mathematical Characterization of Thermo-Reversible Phase Transitions of Agarose Gels
(Taylor & Francis Inc, 2018) Öğrenci, Arif Selçuk; Pekcan, Önder; Kara, Selim; Bilge, Ayşe Hümeyra
The thermal phase transition temperatures of high (HMP) and low melting point (LMP) agarose gels were investigated by using UV-vis spectroscopy techniques. Transmitted light intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol-gel) processes. It was observed that the transition temperatures T-m defined as the location of the maximum of the first derivative of the sigmoidal transition paths obtained from the UV-vis technique slightly increased by increasing the agarose concentration in both the HMP and LMP samples. Here we express the phase transitions of the agar-water system as a representative of reversible physical gels in terms of a modified Susceptible-Infected-Susceptible epidemic model whose solutions are the well-known 5-point sigmoidal curves. The gel point is hard to determine experimentally and various computational techniques are used for its characterization. Based on previous work we locate the gel point T-0 of sol-gel and gel-sol transitions in terms of the horizontal shift in the sigmoidal transition curve. For the gel-sol transition (heating) T-0 is greater than T-m i.e. later in time and the difference between T-0 and T-m is reduced as the agarose content increases. For the sol-gel transition (cooling) T-0 is again greater than T-m but it is earlier in time for all agarose contents and moves forward in time and gets closer to T-m as the agarose content increases.
Citation - WoS: 2
Citation - Scopus: 2
Mathematical Models for Phase Transitions in Biogels
(World Scientific Publ Co Pte Ltd, 2019) Bilge, Ayşe Hümeyra; Öğrenci, Arif Selçuk; Pekcan, Önder
It has been shown that reversible and irreversible phase transitions of biogels can be represented by epidemic models. The irreversible chemical sol-gel transitions are modeled by the Susceptible-Exposed-Infected-Removed (SEIR) or Susceptible-Infected-Removed (SIR) epidemic systems whereas reversible physical gels are modeled by a modification of the Susceptible-Infected-Susceptible (SIS) system. Measured sol-gel and gel-sol transition data have been fitted to the solutions of the epidemic models, either by solving the differential equations directly (SIR and SEIR models) or by nonlinear regression (SIS model). The gel point is represented as the "critical point of sigmoid," defined as the limit point of the locations of the extreme values of its derivatives. Then, the parameters of the sigmoidal curve representing the gelation process are used to predict the gel point and its relative position with respect to the transition point, that is, the maximum of the first derivative with respect to time. For chemical gels, the gel point is always located before the maximum of the first derivative and moves backward in time as the strength of the activation increases. For physical gels, the critical point for the sol-gel transition occurs before the maximum of the first derivative with respect to time, that is, it is located at the right of this maximum with respect to temperature. For gel-sol transitions, the critical point is close to the transition point; the critical point occurs after the maximum of the first derivative for low concentrations whereas the critical point occurs after the maximum of the first derivative for higher concentrations.

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