Judul: Analisis Kestabilan Model Populasi Pertumbuhan Virus Chikungunya dalam Tubuh Manusia dengan Waktu Tunda.
Abstrak: Dalam tulisan ini dipelajari pertumbuhan virus Chikungunya dalam tubuh manusia. Populasi yang terlibat dalam model ini terdiri dari host cells yang rentan terhadap penyakit, terinfeksi penyakit, virus Chikungunya, dan sel imun dalam tubuh manusia. Model infeksi ini dipengaruhi oleh dua waktu tunda yaitu waktu tunda pematangan virus dan waktu tunda respon aktivasi imun. Dalam model ini dihasilkan dua titik tetap E0 (titik tetap bebas penyakit) dan E1 (titik tetap endemik). Dengan menggunakan The Next Generation Matrix dihasilkan bilangan reproduksi dasar (ℛ0), jika ℛ0 < 1 maka akan stabil di sekitar E0 sedangkan jika ℛ0 > 1 maka akan stabil di sekitar E1. Pada model ini dianalisis juga mengenai perubahan nilai parameter tingkat kontak efektif antara host cells rentan dengan virus Chikungunya yang berpengaruh terhadap nilai ℛ0. Dengan hanya mengubah nilai waktu tunda pematangan virus tidak berpengaruh terhadap kestabilan sistem, dengan mengubah nilai waktu tunda respon aktivasi imun akan mengubah kestabilan sistem dengan munculnya limit cycle dan terjadinya bifurkasi Hopf.
Keyword: bifurkasi Hopf, bilangan reproduksi dasar, infeksi virus Chikungunya, the next generation matrix, antioksidan

Judul: Analisis Model Virus Chikungunya dengan Dua Cara Transmisi dan Imunitas Humoral
Abstrak: Chikungunya merupakan penyakit yang menginfeksi sendi dan otot yang disebarkan oleh nyamuk Aedes Aegepty dan Aedes Albopictus. Virus chikungunya dapat menginfeksi sel rentan melalui dua cara, yaitu sel rentan langsung terinfeksi oleh virus atau sel terinfeksi lainnya. Tujuan penelitian ini yaitu menganalisis kestabilan titik tetap dan sensitivitas parameter dalam model, dan melakukan simulasi numerik. Dalam analisis ini dihasilkan dua titik tetap, yaitu titik tetap bebas penyakit dan titik tetap endemik. Hasil analisis sensitivitas dan simulasi numerik menyatakan bahwa untuk mencapai kondisi bebas penyakit diperlukan tindakan menurunkan laju transmisi sel rentan oleh sel virus dan sel terinfeksi lainnya, dan meningkatkan laju produksi antibodi., Chikungunya is a disease that infects joints and muscles spread by Aedes aegypti and Aedes albopictus mosquitoes. Chikungunya virus can infect susceptible cells in two ways: susceptible cells directly infected by virus or by other infected cells. The purpose of this study is to analyze the stability of the fixed point and the sensitivity of the parameters, and perform numerical simulations. In this analysis, two fixed points were produced, the disease-free and the endemic fixed points. The results of sensitivity analysis and numerical simulations indicate that to achieve stability of the disease-free point, it is necessary to reduce the rate of transmission of susceptible cells both by virus and by infected cells, also to increase the rate of antibody production.
Keyword: Basic reproduction number, Chikungunya virus, Sensitivity analysis

Judul: Production and formulation of methyl ester sulfonate-based surfactant from palm olein for enhanced oil recovery application
Abstrak: The remaining oil in a reservoir that cannot be produced using the natural driving force (primary recovery) is about 50 to 80% of the initial oil volumes (IOIP). A secondary recovery technique such as waterflooding can only increase the recovery as much as 5 to 20% of the IOIP. Therefore, a method known as Enhanced Oil Recovery (EOR) has to be implemented in the reservoir in the effort to recover much more oil. One of the EOR methods is surfactant injection. The injection of surfactant solution is aimed to significantly reduce the capillary forces in the reservoir by lowering the oil-water interfacial tension within the pore spaces. Once the capillary force is diminished, the reservoir oil is much easier to be displaced and moved toward the production wells. This is an excellent opportunity to develop a type of vegetable oil-based surfactants. One of potential surfactant types to develop is methyl ester sulphonate surfactant (MES). This study was aimed at producing MES surfactant and formula-based methyl ester sulfonate surfactant from palm olein to be applied for in the process of improving oil recovery using carbonate formation fluids. Results of the study showed that the best condition was achieved in a sulphonation process done in 3-4 hours with the flowrate of dry air of 1.8 kg/hour and a purification performed without the addition of methanol in a pH of 8. The best MES surfactant-based formula for EOR applications on the formation of carbonate was the one with the composition of 0.3% MES surfactant, 0.3% Na2CO3, and 15,000 ppm salinity. MES surfactant produced from this formula had more heat resistance, higher salinity, and higher hardness than the commercial surfactants did. Results of the performance test of surfactant-based formula showed that the MES surfactant formula was compatible with formation water and injection water. The formed phase was a lower phase which was relatively stable on a heating up to day 77 (reservoir temperature of 70 and 112 °C) with a range of interfacial tension of 10-2 dynes/cm. The adsorption reached 152.86 μg active matter/g core and the incremental oil recovery using synthetic core of 8-19% and using native core of 9.1%. Surfactant solution coreflooding test with bottom-up flow direction resulted in a greater oil recovery (16%) than that with top-down flow direction. This was caused by the influence of gravity.
Keyword: Palm Olein