MENISKUS DAN KAPILARITAS

MENISKUS

Pengertian Meniskus

Pengertian meniskus adalah sifat kelengkungan zat cair pada sebuah tabung reaksi. Jadi zat cair tersebut dimasukan ke dalam tabung reaksi maka akan terbentuk pada bagian atas permukaannya apakah cekung ataukah cembung.

  1. Meniskus Cekung

Contoh meniskus cekung adalah apabila air berada di dalam tabung reaksi maka pada permukaannya akan berbentuk cekung. Air juga akan membasahi dinding kaca.
Peristiwa meniskus cekung ini terjadi apabila gaya kohesi lebih kecil daripada gaya adhesinya. Jadi gaya adhesinya yang lebih besar antara air dan kaca sehingga air tersebut membasahi dinding kaca.

  1. Meniskus Cembung

Contoh meniskus cembung adalah apabila air raksa berada di dalam tabung reaksi maka pada permukaannya akan berbentuk cembung. Air raksa tidak membasahi dinding kaca
Peristiwa meniskus cembung ini terjadi apabila gaya kohesi air raksa lebih besar daripada gaya adhesinya. Jadi gaya adhesi antara kaca tabung reaksi dengan air raksa lebih kecil dibandingkan dengan gaya kohesi air raksanya.
Catatan : Gaya kohesi adalah gaya tarik-menarik antara partikel-partikel yang sejenis. Gaya adhesi adalah gaya tarik-menarik antara partikel-partikel yang berbeda sejenis.

Gejala Meniskus

Jika pada lengkungan air dan raksa kita tarik garis lurus, maka garis itu akan membentuk sudut θ terhadap dinding vertikal tabung kaca. Sudut θ tersebut dinamakan sudut kontak. Oleh karena itu, sudut kontak adalah sudut yang dibentuk antara permukaan zat cair dengan permukaan dinding pada titik persentuhan zat cair dengan dinding.

Gaya Kohesi Dan Adhesi Pada Gejala Meniskus

Untuk menjelaskan memahami peristiwa tersebut, kita harus mengingat kembali konsep gaya adhesi dan gaya kohesi. Akibat adanya gaya kohesi antara partikel air (FA) lebih besar daripada gaya adhesi antara partikel air dengan partikel kaca (Fk), maka resultan kedua gaya (FR) arahnya keluar. Agar tercapai keadaan yang seimbang, permukaan air yang menempel pada dinding kaca harus melengkung ke atas.

Kelengkungan permukaan suatu zat cair di dalam tabung disebut meniskus. Karena bentuknya cekung maka meniskus air dalam bejana kaca dinamakan meniskus cekung. Sudut yang dibentuk oleh kelengkungan air terhadap garis vertikal dinamakan sudut kontak θ. Besarnya sudut kontak untuk meniskus cekung lebih kecil dari 90°.

Bagaimana dengan bentuk kelengkungan permukaan raksa dalam tabung? Gaya kohesi antara partikel-partikel raksa (FA) lebih kecil daripada gaya adhesi antara partikel raksa dengan partikel kaca (Fk), sehingga resultan kedua gaya (FR) mengarah ke dalam. Agar tercapai keseimbangan, maka permukaan raksa yang menempel pada dinding kaca harus tegak lurus terhadap gaya resultan FR. Akibatnya permukaan raksa yang menempel pada tabung kaca melengkung ke bawah dan disebut sebagai meniskus cembung. Besarnya sudut kontak untuk meniskus cembung ini lebih besar dari 90o.

Terdapat hubungan antara kemampuan membasahi air dengan tegangan permukaan air. Makin kecil nilai tegangan permukaan air, makin besar kemampuan air untuk membasahi benda. Makin tinggi suhu air, makin kecil tegangan permukaan. Artinya makin baik air tersebut untuk membasahi benda. Itulah sebabnya mencuci dengan air panas dan air sabun hasilnya lebih bersih daripada menggunakan air biasa, hal ini terjadi karena adanya gejala meniskus.

KAPILARITAS

Pengertian Kapilaritas

Dalam fisika kapilaritas diartikan sebagai gejala naiknya zat cair melalui celah sempit atau pipa rambut. Celah sempit atau pipa rambut disebut sebagai pipa kapiler. kapilaritas disebabkan oleh adanya gaya adhesi dan gaya kohesi antara zat cair dengan dinding pipa kapiler sehingga jika pembuluh kaca masuk ke dalam zat cair menyebabkan permukanan zat cair menjadi tidak rata atau tidak sama.

Pengaruh gaya adesi dan kohesi terhadap kapilaritas

Zat cair akan naik ke dalam pipa kapiler apabila zat cair membasahi tabung yaitu ketika gaya adhesi zat cair lebih besar daripada gaya kohesi. Hal ini disebabkan gaya tegangan permukaan sepanjang dinding tabung bekerja ke arah atas. Ketinggian maksimum terjadi pada saat gaya tegangan permukaan setara atau sama dengan berat zat cair yang berada dalam pipa kapiler. Permukaan zat cair akan turun apabila zat cair tidak membasahi tabung yaitu pada saat gaya kohesi lebih besar daripada gaya adesi. Ketika permukaan zat cair naik di dalam pipa kapiler sudut kontak yang terbentuk kurang dari 90 ⁰ dan ketika permukaan zat cair turun di dalam pipa kapiler maka sudut kontak yang terbentuk lebih dari 90⁰. Sudut kontak merupakan sudut yang terbentuk oleh lengkungan. Kohesi merupakan gaya tarik menarik antara molekul-molekul dalam zat sejenis. Adesi merupakan gaya tarik menarik antara molekul-molekul zat yang tidak sejenis.

Kapilaritas Pada Air

Kita dapat mengamati bahwa tinggi permukaan air dalam pipa kapiler lebih tinggi daripada tinggi air dalam bejana. Hal ini berarti permukaan air naik dalam pipa kapiler. Jika diameter pipa kapiler makin kecil, tinggi permukaan air dalam pipa kapiler makin tinggi.

Pasti kita pernah menyalakan kompor ketika akan memasak. Pernahkah kita berpikir mengapa minyak tanah yang ada di bawah dapat bergerak naik sehingga api kompor menyala? Minyak tanah naik bergerak melalui sumbunya yang terbuat dari kain yang berpori-pori kecil. Begitu juga dari kegiatan tadi, ketika air berada pada pipa kapiler ternyata air naik pada bagian pipa yang terkecil. Peristiwa ini dinamakan kapilaritas. Yaitu peristiwa naiknya zat cair pada pembuluh atau celah kecil atau pori-pori kecil. Air pada pembuluh atau celah kecil akan lebih tinggi dari yang lainnya itu, akibat adhesi (partikel air dan partikel gelas) lebih besar dari kohesinya (partikel air).

Kapilaritas Pada Air Raksa

Lain lagi dengan raksa. Raksa pada pembuluh atau celah kecil akan lebih rendah dari yang lebih besar lainnya, akibat kohesi antar partikel raksa lebih besar dari pada adhesi partikel raksa dan partikel gelas.

Contoh Kapilaritas dalam Kehidpuan Sehari-Hari

  1. Menetesnya air pada ujung kain ataupun ujung kertas.
  2. Naiknya minyak tanah melalui sumbu pada kompor minyak tanah
  3. Naiknya minyak pada sumbu lampu minyak.
  4. Meresapnya air melalui dinding.
  5. Naiknya air dan zat hara melalui akar pada tumbuhan hijau
  6. Menyebarnya tinta di atas permukaan kertas
  7. Air yang menggenag dapat diserap oleh kain pel maupun spons.

Manfaat Kapilaritas

Peristiwa kapilaritas memberikan beberapa manfaat bagi makhluk hidup, diantaranya :

  1. Pada manusia

Hemoglobin akan mengambil oksigen dari paru paru dan akan dilepaskan pada saat sel darah merah (eritrosit) melewati pembuluh kapiler

  1. Pada ikan

Filamen pada pembuluh darah insang mengandung pembuluh kapiler untuk memudahkan proses pertukaran oksigen dan karbondioksida.

  1. Pada tumbuhan

Rambut akar dan batang tumbuhan terdiri dari pembuluh kapiler sehingga air dan zat hara dari dalam tanah akan naik menuju batang, dahan, dan ranting tumbuhan.

Kerugian Kapilaritas

  1. Dinding rumah menjadi retak karena pori pori dinding menyerap air dalam tanah
  2. Lumut yang menempel pada dinding,
  3. Cat pada dinding rumah menjadi rusak karena air yang diserap oleh dinding

Demikianlah pembahasan tentang Meniskus dan Kapilaritas, semoga bermanfaat… 🙂

35 Responses to MENISKUS DAN KAPILARITAS

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